From my earliest youth I felt an ardent desire to travel into distant regions, seldom visited by Europeans. This desire is characteristic of a period of our existence when appears an unlimited horizon, and when we find an irresistible attraction in the impetuous agitations of the mind, and the image of positive danger. Though educated in a country which has no direct communication with either the East or the West Indies, living amidst mountains remote from coasts, and celebrated for their numerous mines, I felt an increasing passion for the sea and distant expeditions. Objects with which we are acquainted only by the animated narratives of travellers have a peculiar charm; imagination wanders with delight over that which is vague and undefined; and the pleasures we are deprived of seem to possess a fascinating power, compared with which all we daily feel in the narrow circle of sedentary life appears insipid. The taste for herborisation, the study of geology, rapid excursions to Holland, England, and France, with the celebrated Mr. George Forster, who had the happiness to accompany captain Cook in his second expedition round the globe, contributed to give a determined direction to the plan of travels which I had formed at eighteen years of age. No longer deluded by the agitation of a wandering life, I was anxious to contemplate nature in all her variety of wild and stupendous scenery; and the hope of collecting some facts useful to the advancement of science, incessantly impelled my wishes towards the luxuriant regions of the torrid zone. As personal circumstances then prevented me from executing the projects by which I was so powerfully influenced, I had leisure to prepare myself during six years for the observations I proposed to make on the New Continent, as well as to visit different parts of Europe, and to explore the lofty chain of the Alps, the structure of which I might afterwards compare with that of the Andes of Quito and of Peru.
I had traversed a part of Italy in 1795, but had not been able to visit the volcanic regions of Naples and Sicily; and I regretted leaving Europe without having seen Vesuvius, Stromboli, and Etna. I felt, that in order to form a proper judgment of many geological phenomena, especially of the nature of the rocks of trap-formation, it was necessary to examine the phenomena presented by burning volcanoes. I determined therefore to return to Italy in the month of November, 1797. I made a long stay at Vienna, where the fine collections of exotic plants, and the friendship of Messrs. de Jacquin, and Joseph van der Schott, were highly useful to my preparatory studies. I travelled with M. Leopold von Buch, through several cantons of Salzburg and Styria, countries alike interesting to the landscape-painter and the geologist; but just when I was about to cross the Tyrolese Alps, the war then raging in Italy obliged me to abandon the project of going to Naples.
A short time before, a gentleman passionately fond of the fine arts, and who had visited the coasts of Greece and Illyria to inspect their monuments, made me a proposal to accompany him in an expedition to Upper Egypt. This expedition was to occupy only eight months. Provided with astronomical instruments and able draughtsmen, we were to ascend the Nile as far as Assouan, after minutely examining the positions of the Said, between Tentyris and the cataracts. Though my views had not hitherto been fixed on any region but the tropics, I could not resist the temptation of visiting countries so celebrated in the annals of human civilization. I therefore accepted this proposition, but with the express condition, that on our return to Alexandria I should be at liberty to continue my journey through Syria and Palestine. The studies which I entered upon with a view to this new project, I afterwards found useful, when I examined the relations between the barbarous monuments of Mexico, and those belonging to the nations of the old world. I thought myself on the point of embarking for Egypt, when political events forced me to abandon a plan which promised me so much satisfaction.
An expedition of discovery in the South Sea, under the direction of captain Baudin, was then preparing in France. The plan was great, bold, and worthy of being executed by a more enlightened commander. The purpose of this expedition was to visit the Spanish possessions of South America, from the mouth of the river Plata to the kingdom of Quito and the isthmus of Panama. After visiting the archipelago of the Pacific, and exploring the coasts of New Holland, from Van Diemen’s Land to that of Nuyts, both vessels were to stop at Madagascar, and return by the Cape of Good Hope. I was in Paris when the preparations for this voyage were begun. I had but little confidence in the personal character of captain Baudin, who had given cause of discontent to the court of Vienna, when he was commissioned to conduct to Brazil one of my friends, the young botanist, Van der Schott; but as I could not hope, with my own resources, to make a voyage of such extent, and view so fine a portion of the globe, I determined to take the chances of this expedition. I obtained permission to embark, with the instruments I had collected, in one of the vessels destined for the South Sea, and I reserved to myself the liberty of leaving captain Baudin whenever I thought proper. M. Michaux, who had already visited Persia and a part of North America, and M. Bonpland, with whom I then formed the friendship that still unites us, were appointed to accompany this expedition as naturalists.
I had flattered myself during several months with the idea of sharing the labours directed to so great and honourable an object when the war which broke out in Germany and Italy, determined the French government to withdraw the funds granted for their voyage of discovery, and adjourn it to an indefinite period. Deeply mortified at finding the plans I had formed during many years of my life overthrown in a single day, I sought at any risk the speediest means of quitting Europe, and engaging in some enterprise which might console me for my disappointment.
I became acquainted with a Swedish consul, named Skioldebrand, who having been appointed by his court to carry presents to the dey of Algiers, was passing through Paris, to embark at Marseilles. This estimable man had resided a long time on the coast of Africa; and being highly respected by the government of Algiers, he could easily procure me permission to visit that part of the chain of the Atlas which had not been the object of the important researches of M. Desfontaines. He despatched every year a vessel for Tunis, where the pilgrims embarked for Mecca, and he promised to convey me by the same medium to Egypt. I eagerly seized so favourable an opportunity, and thought myself on the point of executing a plan which I had formed previously to my arrival in France. No mineralogist had yet examined that lofty chain of mountains which, in the empire of Morocco, rises to the limits of the perpetual snow. I flattered myself, that, after executing some operations in the alpine regions of Barbary, I should receive in Egypt from those illustrious men who had for some months formed the Institute of Cairo, the same kind attentions with which I had been honoured during my abode in Paris. I hastily completed my collection of instruments, and purchased works relating to the countries I was going to visit. I parted from a brother who, by his advice and example, had hitherto exercised a great influence on the direction of my thoughts. He approved the motives which determined me to quit Europe; a secret voice assured us that we should meet again; and that hope, which did not prove delusive, assuaged the pain of a long separation. I left Paris with the intention of embarking for Algiers and Egypt; but by one of those vicissitudes which sway the affairs of this life, I returned to my brother from the river Amazon and Peru, without having touched the continent of Africa.
The Swedish frigate which was to convey M. Skioldebrand to Algiers, was expected at Marseilles toward the end of October. M. Bonpland and myself repaired thither with great celerity, for during our journey we were tormented with the fear of being too late, and missing our passage.
M. Skioldebrand was no less impatient than ourselves to reach his place of destination. Several times a day we climbed the mountain of Notre Dame de la Garde, which commands an extensive view of the Mediterranean. Every sail we descried in the horizon excited in us the most eager emotion; but after two months of anxiety and vain expectation, we learned by the public papers, that the Swedish frigate which was to convey us, had suffered greatly in a storm on the coast of Portugal, and had been forced to enter the port of Cadiz, to refit. This news was confirmed by private letters, assuring us that the Jaramas, which was the name of the frigate, would not reach Marseilles before the spring.
We felt no inclination to prolong our stay in Provence till that period. The country, and especially the climate, were delightful, but the aspect of the sea reminded us of the failure of our projects. In an excursion we made to Hyeres and Toulon, we found in the latter port the frigate la Boudeuse, which had been commanded by M. de Bougainville, in his voyage round the world. She was then fitting out for Corsica. M. de Bougainville had honoured me with particular kindness during my stay in Paris, when I was preparing to accompany the expedition of captain Baudin. I cannot describe the impression made upon my mind by the sight of the vessel which had carried Commerson to the islands of the South Sea. In some conditions of the mind, a painful emotion blends itself with all our feelings.
We still persisted in the intention of visiting the African coast, and were nearly becoming the victims of our perseverance. A small vessel of Ragusa, on the point of setting sail for Tunis, was at that time in the port of Marseilles; we thought the opportunity favourable for reaching Egypt and Syria, and we agreed with the captain for our passage. The vessel was to sail the following day; but a circumstance trivial in itself happily prevented our departure. The live-stock intended to serve us for food during our passage, was kept in the great cabin. We desired that some changes should be made, which were indispensable for the safety of our instruments; and during this interval we learnt at Marseilles, that the government of Tunis persecuted the French residing in Barbary, and that every person coming from a French port was thrown into a dungeon. Having escaped this imminent danger, we were compelled to suspend the execution of our projects. We resolved to pass the winter in Spain, in hopes of embarking the next spring, either at Carthagena, or at Cadiz, if the political situation of the East permitted.
We crossed Catalonia and the kingdom of Valencia, on our way to Madrid. We visited the ruins of Tarragona and those of ancient Saguntum; and from Barcelona we made an excursion to Montserrat, the lofty peaks of which are inhabited by hermits, and where the contrast between luxuriant vegetation and masses of naked and arid rocks, forms a landscape of a peculiar character. I employed myself in ascertaining by astronomical observations the position of several points important for the geography of Spain, and determined by means of the barometer the height of the central plain. I likewise made several observations on the inclination of the needle, and on the intensity of the magnetic forces.
On my arrival at Madrid I had reason to congratulate myself on the resolution I had formed of visiting the Peninsula. Baron de Forell, minister from the court of Saxony, treated me with a degree of kindness, of which I soon felt the value. He was well versed in mineralogy, and was full of zeal for every undertaking that promoted the progress of knowledge. He observed to me, that under the administration of an enlightened minister, Don Mariano Luis de Urquijo, I might hope to obtain permission to visit, at my own expense, the interior of Spanish America. After the disappointments I had suffered, I did not hesitate a moment to adopt this idea.
I was presented at the court of Aranjuez in March 1799 and the king received me graciously. I explained to him the motives which led me to undertake a voyage to the new world and the Philippine Islands, and I presented a memoir on the subject to the secretary of state. Senor de Urquijo supported my demand, and overcame every obstacle. I obtained two passports, one from the first secretary of state, the other from the council of the Indies. Never had so extensive a permission been granted to any traveller, and never had any foreigner been honoured with more confidence on the part of the Spanish government.
Many considerations might have induced us to prolong our abode in Spain. The abbe Cavanilles, no less remarkable for the variety of his attainments than his acute intelligence; M. Nee, who, together with M. Haenke, had, as botanist, made part of the expedition of Malaspina, and who had formed one of the greatest herbals ever seen in Europe; Don Casimir Ortega, the abbe Pourret, and the learned authors of the Flora of Peru, Messrs. Ruiz and Pavon, all opened to us without reserve their rich collections. We examined part of the plants of Mexico, discovered by Messrs. Sesse, Mocino, and Cervantes, whose drawings had been sent to the Museum of Natural History of Madrid. This great establishment, the direction of which was confided to Senor Clavijo, author of an elegant translation of the works of Buffon, offered us, it is true, no geological representation of the Cordilleras, but M. Proust, so well known by the great accuracy of his chemical labours, and a distinguished mineralogist, M. Hergen, gave us curious details on several mineral substances of America. It would have been useful to us to have employed a longer time in studying the productions of the countries which were to be the objects of our research, but our impatience to take advantage of the permission given us by the court was too great to suffer us to delay our departure. For a year past, I had experienced so many disappointments, that I could scarcely persuade myself that my most ardent wishes would be at length fulfilled.
We left Madrid about the middle of May, crossed a part of Old Castile, the kingdoms of Leon and Galicia, and reached Corunna, whence we were to embark for Cuba. The winter having been protracted and severe, we enjoyed during the journey that mild temperature of the spring, which in so southern a latitude usually occurs during March and April. The snow still covered the lofty granitic tops of the Guadarama; but in the deep valleys of Galicia, which resemble the most picturesque spots of Switzerland and the Tyrol, cistuses loaded with flowers; and arborescent heaths clothed every rock. We quitted without regret the elevated plain of the two Castiles, which is everywhere devoid of vegetation, and where the severity of the winter’s cold is followed by the overwhelming heat of summer. From the few observations I personally made, the interior of Spain forms a vast plain, elevated three hundred toises (five hundred and eighty-four metres) above the level of the ocean, is covered with secondary formations, grit-stone, gypsum, sal-gem, and the calcareous stone of Jura. The climate of the Castiles is much colder than that of Toulon and Genoa; its mean temperature scarcely rises to 15° of the centigrade thermometer.
We are astonished to find that, in the latitude of Calabria, Thessaly, and Asia Minor, orange-trees do not flourish in the open air. The central elevated plain is encircled by a low and narrow zone, where the chamaerops, the date-tree, the sugar-cane, the banana, and a number of plants common to Spain and the north of Africa, vegetate on several spots, without suffering from the rigours of winter. From the 36th to 40th degrees of latitude, the medium temperature of this zone is from 17 to 20°; and by a concurrence of circumstances, which it would be too long to explain, this favoured region has become the principal seat of industry and intellectual improvement.
When, in the kingdom of Valencia, we ascend from the shore of the Mediterranean towards the lofty plains of La Mancha and the Castiles, we seem to discern, far inland, from the lengthened declivities, the ancient coast of the Peninsula. This curious phenomenon recalls the traditions of the Samothracians, and other historical testimonies, according to which it is supposed that the irruption of the waters through the Dardanelles, augmenting the basin of the Mediterranean, rent and overflowed the southern part of Europe. If we admit that these traditions owe their origin, not to mere geological reveries, but to the remembrance of some ancient catastrophe, we may conceive the central elevated plain of Spain resisting the efforts of these great inundations, till the draining of the waters, by the straits formed between the pillars of Hercules, brought the Mediterranean progressively to its present level, lower Egypt emerging above its surface on the one side, and the fertile plains of Tarragona, Valencia, and Murcia, on the other. Everything that relates to the formation of that sea,* which has had so powerful an influence on the first civilization of mankind, is highly interesting. We might suppose, that Spain, forming a promontory amidst the waves, was indebted for its preservation to the height of its land; but in order to give weight to these theoretic ideas, we must clear up the doubts that have arisen respecting the rupture of so many transverse dikes; — we must discuss the probability of the Mediterranean having been formerly divided into several separate basins, of which Sicily and the island of Candia appear to mark the ancient limits. We will not here risk the solution of these problems, but will satisfy ourselves in fixing attention on the striking contrast in the configuration of the land in the eastern and western extremities of Europe. Between the Baltic and the Black Sea, the ground is at present scarcely fifty toises above the level of the ocean, while the plain of La Mancha, if placed between the sources of the Niemen and the Borysthenes, would figure as a group of mountains of considerable height. If the causes, which may have changed the surface of our planet, be an interesting speculation, investigations of the phenomena, such as they offer themselves to the measures and observations of the naturalist, lead to far greater certainty.
[* Some of the ancient geographers believed that the Mediterranean, swelled by the waters of the Euxine, the Palus Maeotis, the Caspian Sea, and the Sea of Aral, had broken the pillars of Hercules; others admitted that the irruption was made by the waters of the ocean. In the first of these hypotheses, the height of the land between the Black Sea and the Baltic, and between the ports of Cette and Bordeaux, determine the limit which the accumulation of the waters may have reached before the junction of the Black Sea, the Mediterranean, and the Atlantic, as well to the north of the Dardanelles, as to the east of this strip of land which formerly joined Europe to Mauritania, and of which, in the time of Strabo, certain vestiges remained in the Islands of Juno and the Moon.]
From Astorga to Corunna, especially from Lugo, the mountains rise gradually. The secondary formations gently disappear, and are succeeded by the transition rocks, which indicate the proximity of primitive strata. We found considerable mountains composed of that ancient grey stone which the mineralogists of the school of Freyberg name grauwakke, and grauwakkenschiefer. I do not know whether this formation, which is not frequent in the south of Europe, has hitherto been discovered in other parts of Spain. Angular fragments of Lydian stone, scattered along the valleys, seemed to indicate that the transition schist is the basis of the strata of greywacke. Near Corunna even granitic ridges stretch as far as Cape Ortegal. These granites, which seem formerly to have been contiguous to those of Britanny and Cornwall, are perhaps the wrecks of a chain of mountains destroyed and sunk in the waves. Large and beautiful crystals of feldspar characterise this rock. Common tin ore is sometimes discovered there, but working the mines is a laborious and unprofitable operation for the inhabitants of Galicia.
The first secretary of state had recommended us very particularly to brigadier Don Raphael Clavijo, who was employed in forming new dock-yards at Corunna. He advised us to embark on board the sloop Pizarro,* which was to sail in company with the Alcudia, the packet-boat of the month of May, which, on account of the blockade, had been detained three weeks in the port. Senor Clavijo ordered the necessary arrangements to be made on board the sloop for placing our instruments, and the captain of the Pizarro received orders to stop at Teneriffe, as long as we should judge necessary to enable us to visit the port of Orotava, and ascend the peak.
[* According to the Spanish nomenclature, the Pizarro was a light frigate (fragata lijera).]
We had yet ten days to wait before we embarked. During this interval, we employed ourselves in preparing the plants we had collected in the beautiful valleys of Galicia, which no naturalist had yet visited: we examined the fuci and the mollusca which the north-west winds had cast with great profusion at the foot of the steep rock, on which the lighthouse of the Tower of Hercules is built. This edifice, called also the Iron Tower, was repaired in 1788. It is ninety-two feet high, its walls are four feet and a half thick, and its construction clearly proves that it was built by the Romans. An inscription discovered near its foundation, a copy of which M. Laborde obligingly gave me, informs us, that this pharos was constructed by Caius Sevius Lupus, architect of the city of Aqua Flavia (Chaves), and that it was dedicated to Mars. Why is the Iron Tower called in the country by the name of Hercules? Was it built by the Romans on the ruins of a Greek or Phoenician edifice? Strabo, indeed, affirms that Galicia, the country of the Callaeci, had been peopled by Greek colonies. According to an extract from the geography of Spain, by Asclepiades the Myrlaean, an ancient tradition stated that the companions of Hercules had settled in these countries.
The ports of Ferrol and Corunna both communicate with one bay, so that a vessel driven by bad weather towards the coast may anchor in either, according to the wind. This advantage is invaluable where the sea is almost always tempestuous, as between capes Ortegal and Finisterre, which are the promontories Trileucum and Artabrum of ancient geography. A narrow passage, flanked by perpendicular rocks of granite, leads to the extensive basin of Ferrol. No port in Europe has so extraordinary an anchorage, from its very inland position. The narrow and tortuous passage by which vessels enter this port, has been opened, either by the irruption of the waves, or by the reiterated shocks of very violent earthquakes. In the New World, on the coasts of New Andalusia, the Laguna del Obispo (Bishop’s lake) is formed exactly like the port of Ferrol. The most curious geological phenomena are often repeated at immense distances on the surface of continents; and naturalists who have examined different parts of the globe, are struck with the extreme resemblance observed in the rents on coasts, in the sinuosities of the valleys, in the aspect of the mountains, and in their distribution by groups. The accidental concurrence of the same causes must have everywhere produced the same effects; and amidst the variety of nature, an analogy of structure and form is observed in the arrangement of inanimate matter, as well as in the internal organization of plants and of animals.
Crossing from Corunna to Ferrol, over a shallow, near the White Signal, in the bay, which according to D’Anville is the Portus Magnus of the ancients, we made several experiments by means of a valved thermometrical sounding lead, on the temperature of the ocean, and on the decrement of caloric in the successive strata of water. The thermometer on the bank, and near the surface, was from 12.5 to 13.3° centigrades, while in deep water it constantly marked 15 or 15.3°, the air being at 12.8°. The celebrated Franklin and Mr. Jonathan Williams* were the first to invite the attention of naturalists to the phenomena of the temperature of the Atlantic over shoals, and in that zone of tepid and flowing waters which runs from the gulf of Mexico to the banks of Newfoundland and the northern coasts of Europe. The observation, that the proximity of a sand-bank is indicated by a rapid descent of the temperature of the sea at its surface, is not only interesting to the naturalist, but may become also very important for the safety of navigators. The use of the thermometer ought certainly not to lead us to neglect the use of the lead; but experiments sufficiently prove, that variations of temperature, sensible to the most imperfect instruments, indicate danger long before the vessel reaches the shoals. In such cases, the frigidity of the water may induce the pilot to heave the lead in places where he thought himself in the most perfect safety. The waters which cover the shoals owe in a great measure the diminution of their temperature to their mixture with the lower strata of water, which rise towards the surface on the edge of the banks.
[* Author of a work entitled “Thermometrical Navigation,” published at Philadelphia.]
The moment of leaving Europe for the first time is attended with a solemn feeling. We in vain summon to our minds the frequency of the communication between the two worlds; we in vain reflect on the great facility with which, from the improved state of navigation, we traverse the Atlantic, which compared to the Pacific is but a larger arm of the sea; the sentiment we feel when we first undertake so distant a voyage is not the less accompanied by a deep emotion, unlike any other impression we have hitherto felt. Separated from the objects of our dearest affections, entering in some sort on a new state of existence, we are forced to fall back on our own thoughts, and we feel within ourselves a dreariness we have never known before. Among the letters which, at the time of our embarking, I wrote to friends in France and Germany, one had a considerable influence on the direction of our travels, and on our succeeding operations. When I left Paris with the intention of visiting the coast of Africa, the expedition for discoveries in the Pacific seemed to be adjourned for several years. I had agreed with captain Baudin, that if, contrary to his expectation, his voyage took place at an earlier period, and intelligence of it should reach me in time, I would endeavour to return from Algiers to a port in France or Spain, to join the expedition. I renewed this promise on leaving Europe, and wrote to M. Baudin, that if the government persisted in sending him by Cape Horn, I would endeavour to meet him either at Monte Video, Chile, or Lima, or wherever he should touch in the Spanish colonies. In consequence of this engagement, I changed the plan of my journey, on reading in the American papers, in 1801, that the French expedition had sailed from Havre, to circumnavigate the globe from east to west. I hired a small vessel from Batabano, in the island of Cuba, to Portobello, and thence crossed the isthmus to the coast of the Pacific; this mistake of a journalist led M. Bonpland and myself to travel eight hundred leagues through a country we had no intention to visit. It was only at Quito, that a letter from M. Delambre, perpetual secretary of the first class of the Institute, informed us, that captain Baudin went by the Cape of Good Hope, without touching on the eastern or western coasts of America.
We spent two days at Corunna, after our instruments were embarked. A thick fog, which covered the horizon, at length indicated the change of weather we so anxiously desired. On the 4th of June, in the evening, the wind turned to north-east, a point which, on the coast of Galicia, is considered very constant during the summer. The Pizarro prepared to sail on the 5th, though we had intelligence that only a few hours previously an English squadron had been seen from the watch-tower of Sisarga, appearing to stand towards the mouth of the Tagus. Those who saw our ship weigh anchor asserted that we should be captured in three days, and that, forced to follow the fate of the vessel, we should be carried to Lisbon. This prognostic gave us the more uneasiness, as we had known some Mexicans at Madrid, who, in order to return to Vera Cruz, had embarked three times at Cadiz, and having been each time taken at the entrance of the port, were at length obliged to return to Spain through Portugal.
The Pizarro set sail at two in the afternoon. As the long and narrow passage by which a ship sails from the port of Corunna opens towards the north, and the wind was contrary, we made eight short tacks, three of which were useless. A fresh tack was made, but very slowly, and we were for some moments in danger at the foot of fort St. Amarro, the current having driven us very near the rock, on which the sea breaks with considerable violence. We remained with our eyes fixed on the castle of St. Antonio, where the unfortunate Malaspina was then a captive in a state prison. On the point of leaving Europe to visit the countries which this illustrious traveller had visited with so much advantage, I could have wished to have fixed my thoughts on some object less affecting.
At half-past six we passed the Tower of Hercules, which is the lighthouse of Corunna, as already mentioned, and where, from a very remote time, a coal-fire has been kept up for the direction of vessels. The light of this fire is in no way proportionate to the noble construction of so vast an edifice, being so feeble that ships cannot perceive it till they are in danger of striking on the shore. Towards the close of day the wind increased and the sea ran high. We directed our course to north-west, in order to avoid the English frigates, which we supposed were cruising off these coasts. About nine we spied the light of a fishing-hut at Sisarga, which was the last object we beheld in the west of Europe.
On the 7th we were in the latitude of Cape Finisterre. The group of granitic rocks, which forms part of this promontory, like that of Torianes and Monte de Corcubion, bears the name of the Sierra de Torinona. Cape Finisterre is lower than the neighbouring lands, but the Torinona is visible at seventeen leagues’ distance, which proves that the elevation of its highest summit is not less than 300 toises (582 metres). Spanish navigators affirm that on these coasts the magnetic variation differs extremely from that observed at sea. M. Bory, it is true, in the voyage of the sloop Amaranth, found in 1751, that the variation of the needle determined at the Cape was four degrees less than could have been conjectured from the observations made at the same period along the coasts. In the same manner as the granite of Galicia contains tin disseminated in its mass, that of Cape Finisterre probably contains micaceous iron. In the mountains of the Upper Palatinate there are granitic rocks in which crystals of micaceous iron take the place of common mica.
On the 8th, at sunset, we descried from the mast-head an English convoy sailing along the coast, and steering towards south-east. In order to avoid it we altered our course during the night. From this moment no light was permitted in the great cabin, to prevent our being seen at a distance. This precaution, which was at the time prescribed in the regulations of the packet-ships of the Spanish navy, was extremely irksome to us during the voyages we made in the course of the five following years. We were constantly obliged to make use of dark-lanterns to examine the temperature of the water, or to read the divisions on the limb of the astronomical instruments. In the torrid zone, where twilight lasts but a few minutes, our operations ceased almost at six in the evening. This state of things was so much the more vexatious to me as from the nature of my constitution I never was subject to sea-sickness, and feel an extreme ardour for study during the whole time I am at sea.
On the 9th of June, in latitude 39° 50′, and longitude 16° 10′ west of the meridian of the observatory of Paris, we began to feel the effects of the great current which from the Azores flows towards the straits of Gibraltar and the Canary Islands. This current is commonly attributed to that tendency towards the east, which the straits of Gibraltar give to the waters of the Atlantic Ocean. M. de Fleurieu observes that the Mediterranean, losing by evaporation more water than the rivers can supply, causes a movement in the neighbouring ocean, and that the influence of the straits is felt at the distance of six hundred leagues. Without derogating from the respect I entertain for the opinion of that celebrated navigator, I may be permitted to consider this important object in a far more general point of view.
When we cast our eyes over the Atlantic, or that deep valley which divides the western coasts of Europe and Africa from the eastern coasts of the new world, we distinguish a contrary direction in the motion of the waters. Within the tropics, especially from the coast of Senegal to the Caribbean Sea, the general current, that which was earliest known to mariners, flows constantly from east to west. This is called the equinoctial current. Its mean rapidity, corresponding to different latitudes, is nearly the same in the Atlantic and in the Pacific, and may be estimated at nine or ten miles in twenty-four hours, consequently from 0.59 to 0.65 of a foot every second! In those latitudes the waters run towards the west with a velocity equal to a fourth of the rapidity of the greater part of the larger rivers of Europe. The movement of the ocean in a direction contrary to that of the rotation of the globe, is probably connected with this last phenomenon only as far as the rotation converts into trade winds* the polar winds, which, in the low regions of the atmosphere bring back the cold air of the high latitudes toward the equator. To the general impulsion which these trade-winds give the surface of the sea, we must attribute the equinoctial current, the force and rapidity of which are not sensibly modified by the local variations of the atmosphere.
[* The limits of the trade winds were, for the first time, determined by Dampier in 1666.]
In the channel which the Atlantic has dug between Guiana and Guinea, on the meridian of 20 or 23°, and from the 8th or 9th to the 2nd or 3rd degrees of northern latitude, where the trade-winds are often interrupted by winds blowing from the south and south-south-west, the equinoctial current is more inconstant in its direction. Towards the coasts of Africa, vessels are drawn in the direction of south-east; whilst towards the Bay of All Saints and Cape St. Augustin, the coasts of which are dreaded by navigators sailing towards the mouth of the Plata, the general motion of the waters is masked by a particular current (the effects of which extend from Cape St. Roche to the Isle of Trinidad) running north-west with a mean velocity of a foot and a half every second.
The equinoctial current is felt, though feebly, even beyond the tropic of Cancer, in the 26th and 28th degrees of latitude. In the vast basin of the Atlantic, at six or seven hundred leagues from the coasts of Africa, vessels from Europe bound to the West Indies, find their sailing accelerated before they reach the torrid zone. More to the north, in 28 and 35°, between the parallels of Teneriffe and Ceuta, in 46 and 48° of longitude, no constant motion is observed: there, a zone of 140 leagues in breadth separates the equinoctial current (the tendency of which is towards the west) from that great mass of water which runs eastward, and is distinguished for its extraordinary high temperature. To this mass of waters, known by the name of the Gulf-stream,* the attention of naturalists was directed in 1776 by the curious observations of Franklin and Sir Charles Blagden.
[* Sir Francis Drake observed this extraordinary movement of the waters, but he was unacquainted with their high temperature.]
The equinoctial current drives the waters of the Atlantic towards the coasts inhabited by the Mosquito Indians, and towards the shores of Honduras. The New Continent, stretching from south to north, forms a sort of dyke to this current. The waters are carried at first north-west, and passing into the Gulf of Mexico through the strait formed by Cape Catoche and Cape St. Antonio, follow the bendings of the Mexican coast, from Vera Cruz to the mouth of the Rio del Norte, and thence to the mouths of the Mississippi, and the shoals west of the southern extremity of Florida. Having made this vast circuit west, north, east, and south, the current takes a new direction northward, and throws itself with impetuosity into the Gulf of Florida. At the end of the Gulf of Florida, in the parallel of Cape Cannaveral, the Gulf-stream, or current of Florida, runs north-east. Its rapidity resembles that of a torrent, and is sometimes five miles an hour. The pilot may judge, with some certainty, of the proximity of his approach to New York, Philadelphia, or Charlestown when he reaches the edge of the stream; for the elevated temperature of the waters, their saltness, indigo-blue colour, and the shoals of seaweed which cover their surface, as well as the heat of the surrounding atmosphere, all indicate the Gulf-stream. Its rapidity diminishes towards the north, at the same time that its breadth increases and the waters become cool. Between Cayo Biscaino and the bank of Bahama the breadth is only 15 leagues, whilst in the latitude of 28 1/2°, it is 17, and in the parallel of Charlestown, opposite Cape Henlopen, from 40 to 50 leagues. The rapidity of the current is from three to five miles an hour where the stream is narrowest, and is only one mile as it advances towards the north. The waters of the Mexican Gulf; forcibly drawn to north-east, preserve their warm temperature to such a point, that in 40 and 41° of latitude I found them at 22.5° (18° R.) when, out of the current, the heat of the ocean at its surface was scarcely 17.5° (14° R.). In the parallel of New York and Oporto, the temperature of the Gulf-stream is consequently equal to that of the seas of the tropics in the 18th degree of latitude, as, for instance, in the parallel of Porto Rico and the islands of Cape Verd.
To the east of the port of Boston, and on the meridian of Halifax, in latitude 41° 25′, and longitude 67°, the current is near 80 leagues broad. From this point it turns suddenly to the east, so that its western edge, as it bends, becomes the western limit of the running waters, skirting the extremity of the great bank of Newfoundland, which M. Volney ingeniously calls the bar of the mouth of this enormous sea-river. The cold waters of this bank, which according to my experiments are at a temperature of 8.7 or 10° (7 or 8° R.) present a striking contrast with the waters of the torrid zone, driven northward by the Gulf-stream, the temperature of which is from 21 to 22.5° (17 to 18° R.). in these latitudes, the caloric is distributed in a singular manner throughout the ocean; the waters of the bank are 9.4° colder than the neighbouring sea; and this sea is 3° colder than the current. These zones can have no equilibrium of temperature, having a source of heat, or a cause of refrigeration, which is peculiar to each, and the influence of which is permanent.
From the bank of Newfoundland, or from the 52nd degree of longitude to the Azores, the Gulf-stream continues its course to east and east-south-east. The waters are still acted upon by the impulsion they received near a thousand leagues distance, in the straits of Florida, between the island of Cuba and the shoals of Tortoise Island. This distance is double the length of the course of the river Amazon, from Jaen or the straits of Manseriche to Grand Para. On the meridian of the islands of Corvo and Flores, the most western of the group of the Azores, the breadth of the current is 160 leagues. When vessels, on their return from South America to Europe, endeavour to make these two islands to rectify their longitude, they are always sensible of the motion of the waters to south-east. At the 33rd degree of latitude the equinoctial current of the tropics is in the near vicinity of the Gulf-stream. In this part of the ocean, we may in a single day pass from waters that flow towards the west, into those which run to the south-east or east-south-east.
From the Azores, the current of Florida turns towards the straits of Gibraltar, the isle of Madeira, and the group of the Canary Islands. The opening of the Pillars of Hercules has no doubt accelerated the motion of the waters towards the east. We may in this point of view assert, that the strait, by which the Mediterranean communicates with the Atlantic, produces its effects at a great distance; but it is probable also, that, without the existence of this strait, vessels sailing to Teneriffe would be driven south-east by a cause which we must seek on the coasts of the New World. Every motion is the cause of another motion in the vast basin of the seas as well as in the aerial ocean. Tracing the currents to their most distant sources, and reflecting on their variable celerity, sometimes decreasing as between the gulf of Florida and the bank of Newfoundland; at other times augmenting, as in the neighbourhood of the straits of Gibraltar, and near the Canary Islands, we cannot doubt but the same cause which impels the waters to make the circuitous sweep of the gulf of Mexico, agitates them also near the island of Madeira.
On the south of that island, we may follow the current, in its direction south-east and south-south-east towards the coast of Africa, between Cape Cantin and Cape Bojador. In those latitudes a vessel becalmed is running on the coast, while, according to the uncorrected reckoning, it was supposed to be a good distance out at sea. Were the motion of the waters caused by the opening at the straits of Gibraltar, why, on the south of those straits, should it not follow an opposite direction? On the contrary, in the 25th and 26th degrees of latitude, the current flows at first direct south, and then south-west. Cape Blanc, which, after Cape Verd, is the most salient promontory, seems to have an influence on this direction, and in this parallel the waters, of which we have followed the course from the coasts of Honduras to those of Africa, mingle with the great current of the tropics to resume their tour from east to west. Several hundred leagues westward of the Canary Islands, the motion peculiar to the equinoctial waters is felt in the temperate zone from the 28th and 29th degrees of north latitude; but on the meridian of the island of Ferro, vessels sail southward as far as the tropic of Cancer, before they find themselves, by their reckoning, eastward of their right course.*
[* See Humboldt’s Cosmos volume 1 page 312 Bohn’s edition.]
We have just seen that between the parallels of 11 and 43°, the waters of the Atlantic are driven by the currents in a continual whirlpool. Supposing that a molecule of water returns to the same place from which it departed, we can estimate, from our present knowledge of the swiftness of currents, that this circuit of 3800 leagues is not terminated in less than two years and ten months. A boat, which may be supposed to receive no impulsion from the winds, would require thirteen months to go from the Canary Islands to the coast of Caracas, ten months to make the tour of the gulf of Mexico and reach Tortoise Shoals opposite the port of the Havannah, while forty or fifty days might be sufficient to carry it from the straits of Florida to the bank of Newfoundland. It would be difficult to fix the rapidity of the retrograde current from this bank to the shores of Africa; estimating the mean velocity of the waters at seven or eight miles in twenty-four hours, we may allow ten or eleven months for this last distance. Such are the effects of the slow but regular motion which agitates the waters of the Atlantic. Those of the river Amazon take nearly forty-five days to flow from Tomependa to Grand Para.
A short time before my arrival at Teneriffe, the sea had left in the road of Santa Cruz the trunk of a cedrela odorata covered with the bark. This American tree vegetates within the tropics, or in the neighbouring regions. It had no doubt been torn up on the coast of the continent, or of that of Honduras. The nature of the wood, and the lichens which covered its bark, bore evidence that this trunk had not belonged to these submarine forests which ancient revolutions of the globe have deposited in the polar regions. If the cedrela, instead of having been cast on the strand of Teneriffe, had been carried farther south, It would probably have made the whole tour of the Atlantic, and returned to its native soil with the general current of the tropics. This conjecture is supported by a fact of more ancient date, recorded in the history of the Canaries by the abbe Viera. In 1770, a small vessel laden with corn, and bound from the island of Lancerota, to Santa Cruz, in Teneriffe, was driven out to sea, while none of the crew were on board. The motion of the waters from east to west, carried it to America, where it went on shore at La Guayra, near Caracas.
Whilst the art of navigation was yet in its infancy, the Gulf-stream suggested to the mind of Christopher Columbus certain indications of the existence of western regions. Two corpses, the features of which indicated a race of unknown men, were cast ashore on the Azores, towards the end of the 15th century. Nearly at the same period, the brother-inlaw of Columbus, Peter Correa, governor of Porto Santo, found on the strand of that island pieces of bamboo of extraordinary size, brought thither by the western currents. The dead bodies and the bamboos attracted the attention of the Genoese navigator, who conjectured that both came from a continent situate towards the west. We now know that in the torrid zone the trade-winds and the current of the tropics are in opposition to every motion of the waves in the direction of the earth’s rotation. The productions of the new world cannot reach the old but by the very high latitudes, and in following the direction of the current of Florida. The fruits of several trees of the Antilles are often washed ashore on the coasts of the islands of Ferro and Gomera. Before the discovery of America, the Canarians considered these fruits as coming from the enchanted isle of St. Borondon, which according to the reveries of pilots, and certain legends, was situated towards the west in an unknown part of the ocean, buried, as was supposed, in eternal mists.
My chief view in tracing a sketch of the currents of the Atlantic is to prove that the motion of the waters towards the south-east, from Cape St. Vincent to the Canary Islands, is the effect of the general motion to which the surface of the ocean is subjected at its western extremity. We shall give but a very succinct account of the arm of the Gulf-stream, which in the 45th and 50th degrees of latitude, near the bank called the Bonnet Flamand, runs from south-west to north-east towards the coasts of Europe. This partial current becomes very strong at those times when the west winds are of long continuance: and, like that which flows along the isles of Ferro and Gomera, it deposits every year on the western coasts of Ireland and Norway the fruit of trees which belong to the torrid zone of America. On the shores of the Hebrides, we collect seeds of Mimosa scandens, of Dolichos urens, of Guilandina bonduc, and several other plants of Jamaica, the isle of Cuba, and of the neighbouring continent. The current carries thither also barrels of French wine, well preserved, the remains of the cargoes of vessels wrecked in the West Indian seas. To these examples of the distant migration of the vegetable world, others no less striking may be added. The wreck of an English vessel, the Tilbury, burnt near Jamaica, was found on the coast of Scotland. On these same coasts are sometimes found various kinds of tortoises, that inhabit the waters of the Antilles. When the western winds are of long duration, a current is formed in the high latitudes, which runs directly towards east-south-east, from the coasts of Greenland and Labrador, as far as the north of Scotland. Wallace relates, that twice (in 1682 and 1684), American savages of the race of the Esquimaux, driven out to sea in their leathern canoes, during a storm, and left to the guidance of the currents, reached the Orkneys. This last example is the more worthy of attention, as it proves at the same time how, at a period when the art of navigation was yet in its infancy, the motion of the waters of the ocean may have contributed to disseminate the different races of men over the face of the globe.
In reflecting on the causes of the Atlantic currents, we find that they are much more numerous than is generally believed; for the waters of the sea may be put in motion by an external impulse, by difference of heat and saltness, by the periodical melting of the polar ice, or by the inequality of evaporation, in different latitudes. Sometimes several of these causes concur to one and the same effect, and sometimes they produce several contrary effects. Winds that are light, but which, like the trade-winds, are continually acting on the whole of a zone, cause a real movement of transition, which we do not observe in the heaviest tempests, because these last are circumscribed within a small space. When, in a great mass of water, the particles at the surface acquire a different specific gravity, a superficial current is formed, which takes its direction towards the point where the water is coldest, or where it is most saturated with muriate of soda, sulphate of lime, and muriate or sulphate of magnesia. In the seas of the tropics we find, that at great depths the thermometer marks 7 or 8 centesimal degrees. Such is the result of the numerous experiments of commodore Ellis and of M. Peron. The temperature of the air in those latitudes being never below 19 or 20°, it is not at the surface that the waters can have acquired a degree of cold so near the point of congelation, and of the maximum of the density of water. The existence of this cold stratum in the low latitudes is an evident proof of the existence of an under-current, which runs from the poles towards the equator: it also proves that the saline substances which alter the specific gravity of the water, are distributed in the ocean, so as not to annihilate the effect produced by the differences of temperature.
Considering the velocity of the molecules, which, on account of the rotatory motion of the globe, vary with the parallels, we may be tempted to admit that every current, in the direction from south to north, tends at the same time eastward, while the waters which run from the pole towards the equator, have a tendency to deviate westward. We may also be led to think that these tendencies diminish to a certain point the speed of the tropical current, in the same manner as they change the direction of the polar current, which in July and August, is regularly perceived during the melting of the ice, on the parallel of the bank of Newfoundland, and farther north. Very old nautical observations, which I have had occasion to confirm by comparing the longitude given by the chronometer with that which the pilots obtained by their reckoning, are, however, contrary to these theoretical ideas. In both hemispheres, the polar currents, when they are perceived, decline a little to the east; and it would seem that the cause of this phenomenon should be sought in the constancy of the westerly winds which prevail in the high latitudes. Besides, the particles of water do not move with the same rapidity as the particles of air; and the currents of the ocean, which we consider as most rapid, have only a swiftness of eight or nine feet a second; it is consequently very probable, that the water, in passing through different parallels, gradually acquires a velocity correspondent to those parallels, and that the rotation of the earth does not change the direction of the currents.
The variable pressure on the surface of the sea, caused by the changes in the weight of the air, is another cause of motion which deserves particular attention. It is well known, that the barometric variations do not in general take place at the same moment in two distant points, which are on the same level. If in one of these points the barometer stands a few lines lower than in the other, the water will rise where it finds the least pressure of air, and this local intumescence will continue, till, from the effect of the wind, the equilibrium of the air is restored. M. Vaucher thinks that the tides in the lake of Geneva, known by the name of the seiches, arise from the same cause. We know not whether it be the same, when the movement of progression, which must not be confounded with the oscillation of the waves, is the effect of an external impulse. M. de Fleurieu, in his narrative of the voyage of the Isis, cites several facts, which render it probable that the sea is not so still at the bottom as naturalists generally suppose. Without entering here into a discussion of this question, we shall only observe that, if the external impulse is constant in its action, like that of the trade-winds, the friction of the particles of water on each other must necessarily propagate the motion of the surface of the ocean even to the lower strata; and in fact this propagation in the Gulf-stream has long been admitted by navigators, who think they discover the effects in the great depth of the sea wherever it is traversed by the current of Florida, even amidst the sand-banks which surround the northern coasts of the United States. This immense river of hot waters, after a course of fifty days, from the 24th to the 45th degree of latitude, or 450 leagues, does not lose, amidst the rigours of winter in the temperate zone, more than 3 or 4° of the temperature it had under the tropics. The greatness of the mass, and the small conductibility of water for heat, prevent a more speedy refrigeration. If, therefore, the Gulf-stream has dug a channel at the bottom of the Atlantic ocean, and if its waters are in motion to considerable depths, they must also in their inferior strata keep up a lower temperature than that observed in the same parallel, in a part of the sea which has neither currents nor deep shoals. These questions can be cleared up only by direct experiments, made by thermometrical soundings.
Sir Erasmus Gower remarks, that, in the passage from England to the Canary islands, the current, which carries vessels towards the south-east, begins at the 39th degree of latitude. During our voyage from Corunna to the coast of South America, the effect of this motion of the waters was perceived farther north. From the 37th to the 30th degree, the deviation was very unequal; the daily average effect was 12 miles, that is, our sloop drove towards the east 75 miles in six days. In crossing the parallel of the straits of Gibraltar, at a distance of 140 leagues, we had occasion to observe, that in those latitudes the maximum of the rapidity does not correspond with the mouth of the straits, but with a more northerly point, which lies on the prolongation of a line passing through the strait and Cape St. Vincent. This line is parallel to the direction which the waters follow from the Azores to Cape Cantin. We should moreover observe (and this fact is not uninteresting to those who examine the nature of fluids), that in this part of the retrograde current, on a breadth of 120 or 140 leagues, the whole mass of water has not the same rapidity, nor does it follow precisely the same direction. When the sea is perfectly calm, there appears at the surface narrow stripes, like small rivulets, in which the waters run with a murmur very sensible to the ear of an experienced pilot. On the 13th of June, in 34° 36′ north latitude, we found ourselves in the midst of a great number of these beds of currents. We took their direction with the compass, and some ran north-east, others east-north-east, though the general movement of the ocean, indicated by comparing the reckoning with the chronometrical longitude, continued to be south-east. It is very common to see a mass of motionless waters crossed by threads of water, which run in different directions, and we may daily observe this phenomenon on the surface of lakes; but it is much less frequent to find partial movements, impressed by local causes on small portions of waters in the midst of an oceanic river, which occupies an immense space, and which moves, though slowly, in a constant direction. In the conflict of currents, as in the oscillation of the waves, our imagination is struck by those movements which seem to penetrate each other, and by which the ocean is continually agitated.
We passed Cape St. Vincent, which is of basaltic formation, at the distance of more than eighty leagues. It is not distinctly seen at a greater distance than 15 leagues, but the granitic mountain called the Foya de Monchique, situated near the Cape, is perceptible, as pilots allege, at the distance of 26 leagues. If this assertion be exact, the Foya is 700 toises (1363 metres), and consequently 116 toises (225 metres) higher than Vesuvius.
From Corunna to the 36th degree of latitude we had scarcely seen any organic being, excepting sea-swallows and a few dolphins. We looked in vain for sea-weeds (fuci) and mollusca, when on the 11th of June we were struck with a curious sight which afterwards was frequently renewed in the southern ocean. We entered on a zone where the whole sea was covered with a prodigious quantity of medusas. The vessel was almost becalmed, but the mollusca were borne towards the south-east, with a rapidity four times greater than the current. Their passage lasted near three quarters of an hour. We then perceived but a few scattered individuals, following the crowd at a distance as if tired with their journey. Do these animals come from the bottom of the sea, which is perhaps in these latitudes some thousand fathoms deep? or do they make distant voyages in shoals? We know that the mollusca haunt banks; and if the eight rocks, near the surface, which captain Vobonne mentions having seen in 1732, to the north of Porto Santo, really exist, we may suppose that this innumerable quantity of medusas had been thence detached; for we were but 28 leagues from the reef. We found, beside the Medusa aurita of Baster, and the Medusa pelagica of Bosc with eight tentacula (Pelagia denticulata, Peron), a third species which resembles the Medusa hysocella, and which Vandelli found at the mouth of the Tagus. It is known by its brownish-yellow colour, and by its tentacula, which are longer than the body. Several of these sea-nettles were four inches in diameter: their reflection was almost metallic: their changeable colours of violet and purple formed an agreeable contrast with the azure tint of the ocean.
In the midst of these medusas M. Bonpland observed bundles of Dagysa notata, a mollusc of a singular construction, which Sir Joseph Banks first discovered. These are small gelatinous bags, transparent, cylindrical, sometimes polygonal, thirteen lines long and two or three in diameter. These bags are open at both ends. In one of these openings, we observed a hyaline bladder, marked with a yellow spot. The cylinders lie longitudinally, one against another, like the cells of a bee-hive, and form chaplets from six to eight inches in length. I tried the galvanic electricity on these mollusca, but it produced no contraction. It appears that the genus dagysa, formed at the time of Cook’s first voyage, belongs to the salpas (biphores of Bruguiere), to which M. Cuvier joins the Thalia of Brown, and the Tethys vagina of Tilesius. The salpas journey also by groups, joining in chaplets, as we have observed of the dagysa.
On the morning of the 13th of June, in 34° 33′ latitude, we saw large masses of this last mollusc in its passage, the sea being perfectly calm. We observed during the night, that, of three species of medusas which we collected, none yielded any light but at the moment of a very slight shock. This property does not belong exclusively to the Medusa noctiluca, which Forskael has described in his Fauna Aegyptiaca, and which Gmelin has applied to the Medusa pelagica of Loefling, notwithstanding its red tentacula, and the brownish tuberosities of its body. If we place a very irritable medusa on a pewter plate, and strike against the plate with any sort of metal, the slight vibrations of the plate are sufficient to make this animal emit light. Sometimes, in galvanising the medusa, the phosphorescence appears at the moment that the chain closes, though the exciters are not in immediate contact with the organs of the animal. The fingers with which we touch it remain luminous for two or three minutes, as is observed in breaking the shell of the pholades. If we rub wood with the body of a medusa, and the part rubbed ceases shining, the phosphorescence returns if we pass a dry hand over the wood. When the light is extinguished a second time, it can no longer be reproduced, though the place rubbed be still humid and viscous. In what manner ought we to consider the effect of the friction, or that of the shock? This is a question of difficult solution. Is it a slight augmentation of temperature which favours the phosphorescence? or does the light return, because the surface is renewed, by putting the animal parts proper to disengage the phosphoric hydrogen in contact with the oxygen of the atmospheric air? I have proved by experiments published in 1797, that the shining of wood is extinguished in hydrogen gas, and in pure azotic gas, and that its light reappears whenever we mix with it the smallest bubble of oxygen gas. These facts, to which several others may be added, tend to explain the causes of the phosphorescence of the sea, and of that peculiar influence which the shock of the waves exercises on the production of light.
When we were between the island of Madeira and the coast of Africa, we had slight breezes and dead calms, very favourable for the magnetic observations, which occupied me during this passage. We were never weary of admiring the beauty of the nights; nothing can be compared to the transparency and serenity of an African sky. We were struck with the innumerable quantity of falling stars, which appeared at every instant. The farther progress we made towards the south, the more frequent was this phenomenon, especially near the Canaries. I have observed during my travels, that these igneous meteors are in general more common and luminous in some regions of the globe than in others; but I have never beheld them so multiplied as in the vicinity of the volcanoes of the province of Quito, and in that part of the Pacific ocean which bathes the volcanic coasts of Guatimala. The influence which place, climate, and season appear to exercise on the falling stars, distinguishes this class of meteors from those to which we trace stones that drop from the sky (aerolites), and which probably exist beyond the boundaries of our atmosphere. According to the observations of Messrs. Benzenberg and Brandes, many of the falling stars seen in Europe have been only thirty thousand toises high. One was even measured which did not exceed fourteen thousand toises, or five nautical leagues. These measures, which can give no result but by approximation, deserve well to be repeated. In warm climates, especially within the tropics, falling stars leave a tail behind them, which remains luminous 12 or 15 seconds: at other times they seem to burst into sparks, and they are generally lower than those in the north of Europe. We perceive them only in a serene and azure sky; they have perhaps never been below a cloud. Falling stars often follow the same direction for several hours, which direction is that of the wind. In the bay of Naples, M. Gay–Lussac and myself observed luminous phenomena very analogous to those which fixed my attention during a long abode at Mexico and Quito. These meteors are perhaps modified by the nature of the soil and the air, like certain effects of the looming or mirage, and of the terrestrial refraction peculiar to the coasts of Calabria and Sicily.
When we were forty leagues east of the island of Madeira, a swallow* perched on the topsail-yard. It was so fatigued, that it suffered itself to be easily taken. It was remarkable that a bird, in that season, and in calm weather, should fly so far. In the expedition of d’Entrecasteaux, a common swallow was seen 60 leagues distant from Cape Blanco; but this was towards the end of October, and M. Labillardiere thought it had newly arrived from Europe. We crossed these latitudes in June, at a period when the seas had not for a long time been agitated by tempests. I mention this last circumstance, because small birds and even butterflies, are sometimes forced out to sea by the impetuosity of the winds, as we observed in the Pacific ocean, when we were on the western coast of Mexico.
[* Hirundo rustica, Linn.]
The Pizarro had orders to touch at the isle of Lancerota, one of the seven great Canary Islands; and at five in the afternoon of the 16th of June, that island appeared so distinctly in view that I was able to take the angle of altitude of a conic mountain, which towered majestically over the other summits, and which we thought was the great volcano which had occasioned such devastation on the night of the 1st of September, 1730.
The current drew us toward the coast more rapidly than we wished. As we advanced, we discovered at first the island of Forteventura, famous for its numerous camels;* and a short time after we saw the small island of Lobos in the channel which separates Forteventura from Lancerota. We spent part of the night on deck. The moon illumined the volcanic summits of Lancerota, the flanks of which, covered with ashes, reflected a silver light. Antares threw out its resplendent rays near the lunar disk, which was but a few degrees above the horizon. The night was beautifully serene and cool. Though we were but a little distance from the African coast, and on the limit of the torrid zone, the centigrade thermometer rose no higher than 18°. The phosphorescence of the ocean seemed to augment the mass of light diffused through the air. After midnight, great black clouds rising behind the volcano shrouded at intervals the moon and the beautiful constellation of the Scorpion. We beheld lights carried to and fro on shore, which were probably those of fishermen preparing for their labours. We had been occasionally employed, during our passage, in reading the old voyages of the Spaniards, and these moving lights recalled to our fancy those which Pedro Gutierrez, page of Queen Isabella, saw in the isle of Guanahani, on the memorable night of the discovery of the New World.
[* These camels, which serve for labour, and sometimes for food, did not exist till the Bethencourts made the conquest of the Canaries. In the sixteenth century, asses were so abundant in the island of Forteventura, that they became wild and were hunted. Several thousands were killed to save the harvest. The horses of Forteventura are of singular beauty, and of the Barbary race. —“Noticias de la Historia General de las Islas Canarias” por Don Jose de Viera, tome 2 page 436.]
On the 17th, in the morning, the horizon was foggy, and the sky slightly covered with vapour. The outlines of the mountains of Lancerota appeared stronger: the humidity, increasing the transparency of the air, seemed at the same time to have brought the objects nearer our view. This phenomenon is well known to all who have made hygrometrical observations in places whence the chain of the Higher Alps or of the Andes is seen. We passed through the channel which divides the isle of Alegranza from Montana Clara, taking soundings the whole way; and we examined the archipelago of small islands situated northward of Lancerota. In the midst of this archipelago, which is seldom visited by vessels bound for Teneriffe, we were singularly struck with the configuration of the coasts. We thought ourselves transported to the Euganean mountains in the Vicentin, or the banks of the Rhine near Bonn. The form of organized beings varies according to the climate, and it is that extreme variety which renders the study of the geography of plants and animals so attractive; but rocks, more ancient perhaps than the causes which have produced the difference of the climate on the globe, are the same in both hemispheres. The porphyries containing vitreous feldspar and hornblende, the phonolite, the greenstone, the amygdaloids, and the basalt, have forms almost as invariable as simple crystallized substances. In the Canary Islands, and in the mountains of Auvergne, in the Mittelgebirge in Bohemia, in Mexico, and on the banks of the Ganges, the formation of trap is indicated by a symmetrical disposition of the mountains, by truncated cones, sometimes insulated, sometimes grouped, and by elevated plains, both extremities of which are crowned by a conical rising.
The whole western part of Lancerota, of which we had a near view, bears the appearance of a country recently convulsed by volcanic eruptions. Everything is black, parched, and stripped of vegetable mould. We distinguished, with our glasses, stratified basalt in thin and steeply-sloping strata. Several hills resembled the Monte Novo, near Naples, or those hillocks of scoria and ashes which the opening earth threw up in a single night at the foot of the volcano of Jorullo, in Mexico. In fact, the abbe Viera relates, that in 1730, more than half the island changed its appearance. The great volcano, which we have just mentioned, and which the inhabitants call the volcano of Temanfaya, spread desolation over a most fertile and highly cultivated region: nine villages were entirely destroyed by the lavas. This catastrophe had been preceded by a tremendous earthquake, and for several years shocks equally violent were felt. This last phenomenon is so much the more singular, as it seldom happens after an eruption, when the elastic vapours have found vent by the crater, after the ejection of the melted matter. The summit of the great volcano is a rounded hill, but not entirely conic. From the angles of altitude which I took at different distances, its absolute elevation did not appear to exceed three hundred toises. The neighbouring hills, and those of Alegranza and Isla Clara, were scarcely above one hundred or one hundred and twenty toises. We may be surprised at the small elevation of these summits, which, viewed from the sea, wear so majestic a form; but nothing is more uncertain than our judgment on the greatness of angles, which are subtended by objects close to the horizon. From illusions of this sort it arose, that before the measures of Messrs. de Churruca and Galleano, at Cape Pilar, navigators considered the mountains of the straits of Magellan, and those of Terra del Fuego, to be extremely elevated.
The island of Lancerota bore formerly the name of Titeroigotra. On the arrival of the Spaniards, its inhabitants were distinguished from the other Canarians by marks of greater civilization. Their houses were built with freestone, while the Guanches of Teneriffe dwelt in caverns. At Lancerota, a very singular custom prevailed at that time, of which we find no example except among the people of Thibet. A woman had several husbands, who alternately enjoyed the prerogatives due to the head of a family. A husband was considered as such only during a lunar revolution, and whilst his rights were exercised by others, he remained classed among the household domestics. In the fifteenth century the island of Lancerota contained two small distinct states, divided by a wall; a kind of monument which outlives national enmities, and which we find in Scotland, in China, and Peru.
We were forced by the winds to pass between the islands of Alegranza and Montana Clara, and as none on board the sloop had sailed through this passage, we were obliged to be continually sounding. We found from twenty-five to thirty-two fathoms. The lead brought up an organic substance of so singular a structure that we were for a long time doubtful whether it was a zoophyte or a kind of seaweed. The stem, of a brownish colour and three inches long, has circular leaves with lobes, and indented at the edges. The colour of these leaves is a pale green, and they are membranous and streaked like those of the adiantums and Gingko biloba. Their surface is covered with stiff whitish hairs; before their opening they are concave, and enveloped one in the other. We observed no mark of spontaneous motion, no sign of irritability, not even on the application of galvanic electricity. The stem is not woody, but almost of a horny substance, like the stem of the Gorgons. Azote and phosphorus having been abundantly found in several cryptogamous plants, an appeal to chemistry would be useless to determine whether this organized substance belonged to the animal or vegetable kingdom. Its great analogy to several sea-plants, with adiantum leaves, especially the genus caulerpa of M. Lamoureux, of which the Fucus proliter of Forskael is one of the numerous species, engaged us to rank it provisionally among the sea-wracks, and give it the name of Fucus vitifolius. The bristles which cover this plant are found in several other fuci.* The leaf, examined with a microscope at the instant we drew it up from the water, did not present, it is true, those conglobate glands, or those opaque points, which the parts of fructification in the genera of ulva and fucus contain; but how often do we find seaweeds in such a state that we cannot yet distinguish any trace of seeds in their transparent parenchyma.
[* Fucus lycopodioides, and F. hirsutus.]
The vine-leaved fucus presents a physiological phenomenon of the greatest interest. Fixed to a piece of madrepore, this seaweed vegetates at the bottom of the ocean, at the depth of 192 feet, notwithstanding which we found its leaves as green as those of our grasses. According to the experiments of Bouguer, light is weakened after a passage of 180 feet in the ratio of 1 to 1477.8. The seaweed of Alegranza consequently presents a new example of plants which vegetate in great obscurity without becoming white. Several germs, enveloped in the bulbs of the lily tribes, the embryo of the malvaceae, of the rhamnoides, of the pistacea, the viscum, and the citrus, the branches of some subterraneous plants; in short, vegetables transported into mines, where the ambient air contains hydrogen or a great quantity of azote, become green without light. From these facts we are inclined to admit that it is not exclusively by the influence of the solar rays that this carburet of hydrogen is formed in the organs of plants, the presence of which makes the parenchyma appear of a lighter or darker green, according as the carbon predominates in the mixture.
Mr. Turner, who has so well made known the family of the seaweeds, as well as many other celebrated botanists, are of opinion that most of the fuci which we gather on the surface of the ocean, and which, from the 23rd to the 35th degree of latitude and 32nd of longitude, appear to the mariner like a vast inundated meadow, grow primitively at the bottom of the ocean, and float only in their ripened state, when torn up by the motion of the waves. If this opinion be well founded, we must agree that the family of seaweeds offers formidable difficulties to naturalists, who persist in thinking that absence of light always produces whiteness; for how can we admit that so many species of ulvaceae and dictyoteae, with stems and green leaves, which float on the ocean, have vegetated on rocks near the surface of the water?
From some notions which the captain of the Pizarro had collected in an old Portuguese itinerary, he thought himself opposite to a small fort, situated north of Teguisa, the capital of the island of Lancerota. Mistaking a rock of basalt for a castle, he saluted it by hoisting the Spanish flag, and sent a boat with an officer to inquire of the commandant whether any English vessels were cruising in the roads. We were not a little surprised to learn that the land which we had considered as a prolongation of the coast of Lancerota, was the small island of Graciosa, and that for several leagues there was not an inhabited place. We took advantage of the boat to survey the land, which enclosed a large bay.
The small part of the island of Graciosa which we traversed, resembles those promontories of lava seen near Naples, between Portici and Torre del Greco. The rocks are naked, with no marks of vegetation, and scarcely any of vegetable soil. A few crustaceous lichen-like variolariae, leprariae, and urceorariae, were scattered about upon the basalts. The lavas which are not covered with volcanic ashes remain for ages without any appearance of vegetation. On the African soil excessive heat and lengthened drought retard the growth of cryptogamous plants.
The basalts of Graciosa are not in columns, but are divided into strata ten or fifteen inches thick. These strata are inclined at an angle of 80° to the north-west. The compact basalt alternates with the strata of porous basalt and marl. The rock does not contain hornblende, but great crystals of foliated olivine, which have a triple cleavage.* This substance is decomposed with great difficulty. M. Hauy considers it a variety of the pyroxene. The porous basalt, which passes into mandelstein, has oblong cavities from two to eight lines in diameter, lined with chalcedony, enclosing fragments of compact basalt. I did not remark that these cavities had the same direction, or that the porous rock lay on compact strata, as happens in the currents of lava of Etna and Vesuvius. The marl,* which alternates more than a hundred times with the basalts, is yellowish, friable by decomposition, very coherent in the inside, and often divided into irregular prisms, analogous to the basaltic prisms. The sun discolours their surface, as it whitens several schists, by reviving a hydro-carburetted principle, which appears to be combined with the earth. The marl of Graciosa contains a great quantity of chalk, and strongly effervesces with nitric acid, even on points where it is found in contact with the basalt. This fact is the more remarkable, as this substance does not fill the fissures of the rock, but its strata are parallel to those of the basalt; whence we may conclude that both fossils are of the same formation, and have a common origin. The phenomenon of a basaltic rock containing masses of indurated marl split into small columns, is also found in the Mittelgebirge, in Bohemia. Visiting those countries in 1792, in company with Mr. Freiesleben, we even recognized in the marl of the Stiefelberg the imprint of a plant nearly resembling the Cerastium, or the Alsine. Are these strata, contained in the trappean mountains, owing to muddy irruptions, or must we consider them as sediments of water, which alternate with volcanic deposits? This last hypothesis seems so much the less admissible, since, from the researches of Sir James Hall on the influence of pressure in fusions, the existence of carbonic acid in substances contained in basalt presents nothing surprising. Several lavas of Vesuvius present similar phenomena. In Lombardy, between Vicenza and Albano, where the calcareous stone of the Jura contains great masses of basalt, I have seen the latter enter into effervescence with the acids wherever it touches the calcareous rock.
[* Blaettriger olivin.]
We had not time to reach the summit of a hill very remarkable for having its base formed of banks of clay under strata of basalt, like a mountain in Saxony, called the Scheibenbergen Hugel, which is become celebrated on account of the disputes of volcanean and neptunean geologists. These basalts were covered with a mammiform substance, which I vainly sought on the Peak of Teneriffe, and which is known by the names of volcanic glass, glass of Muller, or hyalite: it is the transition from the opal to the chalcedony. We struck off with difficulty some fine specimens, leaving masses that were eight or ten inches square untouched. I never saw in Europe such fine hyalites as I found in the island of Graciosa, and on the rock of porphyry called el Penol de los Banos, on the bank of the lake of Mexico.
Two kinds of sand cover the shore; one is black and basaltic, the other white and quartzose. In a place exposed to the rays of the sun, the first raised the thermometer to 51.2° (41° R.) and the second to 40° (32° R.) The temperature of the air in the shade was 27.7 or 7.5° higher than that of the air over the sea. The quartzose sand contains fragments of feldspar. It is thrown back by the water, and forms, in some sort, on the surface of the rocks, small islets on which seaweed vegetates. Fragments of granite have been observed at Teneriffe; the island of Gomora, from the details furnished me by M. Broussonnet, contains a nucleus of micaceous schist:— the quartz disseminated in the sand, which we found on the shore of Graciosa, is a different substance from the lavas and the trappean porphyries so intimately connected with volcanic productions. From these facts it seems to be evident that in the Canary Islands, as well as on the Andes of Quito, in Auvergne, in Greece, and throughout the greater part of the globe, subterraneous fires have pierced through the rocks of primitive formation. In treating hereafter of the great number of warm springs which we have seen issuing from granite, gneiss, and micaceous schist, we shall have occasion to return to this subject, which is one of the most important of the physical history of the globe.
We re-embarked at sunset, and hoisted sail, but the breeze was too feeble to permit us to continue our course to Teneriffe. The sea was calm; a reddish vapour covered the horizon, and seemed to magnify every object. In this solitude, amidst so many uninhabited islets, we enjoyed for a long time the view of rugged and wild scenery. The black mountains of Graciosa appeared like perpendicular walls five or six hundred feet high. Their shadows, thrown over the surface of the ocean, gave a gloomy aspect to the scenery. Rocks of basalt, emerging from the bosom of the waters, wore the resemblance of the ruins of some vast edifice, and carried our thoughts back to the remote period when submarine volcanoes gave birth to new islands, or rent continents asunder. Every thing which surrounded us seemed to indicate destruction and sterility; but the back-ground of the picture, the coasts of Lancerota presented a more smiling aspect. In a narrow pass between two hills, crowned with scattered tufts of trees, marks of cultivation were visible. The last rays of the sun gilded the corn ready for the sickle. Even the desert is animated wherever we can discover a trace of the industry of man.
We endeavoured to get out of this bay by the pass which separates Alegranza from Montana Clara, and through which we had easily entered to land at the northern point of Graciosa. The wind having fallen, the currents drove us very near a rock, on which the sea broke with violence, and which is noted in the old charts under the name of Hell, or Infierno. As we examined this rock at the distance of two cables’ length, we found that it was a mass of lava three or four toises high, full of cavities, and covered with scoriae resembling coke. We may presume that this rock,* which modern charts call the West Rock (Roca del Oeste), was raised by volcanic fire; and it might heretofore have been much higher; for the new island of the Azores, which rose from the sea at successive periods, in 1638 and 1719, had reached 354 feet when it totally disappeared in 1723, to the depth of 480 feet. This opinion on the origin of the basaltic mass of the Infierno is confirmed by a phenomenon, which was observed about the middle of the last century in these same latitudes. At the time of the eruption of the volcano of Temanfaya, two pyramidal hills of lithoid lava rose from the bottom of the ocean, and gradually united themselves with the island of Lancerota.
[* I must here observe, that this rock is noted on the celebrated Venetian chart of Andrea Bianco, but that the name of Infierno is given, as in the more ancient chart of Picigano, made in 1367, to Teneriffe, without doubt because the Guanches considered the peak as the entrance into hell. In the same latitudes an island made its appearance in 1811.]
As we were prevented by the fall of the wind, and by the currents, from repassing the channel of Alegranza, we resolved on tacking during the night between the island of Clara and the West Rock. This resolution had nearly proved fatal. A calm is very dangerous near this rock, towards which the current drives with considerable force. We began to feel the effects of this current at midnight. The proximity of the stony masses, which rise perpendicularly above the water, deprived us of the little wind which blew: the sloop no longer obeyed the helm, and we dreaded striking every instant. It is difficult to conceive how a mass of basalt, insulated in the vast expanse of the ocean, can cause so considerable a motion of the waters. These phenomena, worthy the attention of naturalists, are well known to mariners; they are extremely to be dreaded in the Pacific ocean, particularly in the small archipelago of the islands of Galapagos. The difference of temperature which exists between the fluid and the mass of rocks does not explain the direction which these currents take; and how can we admit that the water is engulfed at the base of these rocks, (which often are not of volcanic origin) and that this continual engulfing determines the particles of water to fill up the vacuum that takes place.
The wind having freshened a little towards the morning on the 18th, we succeeded in passing the channel. We drew very near the Infierno the second time, and remarked the large crevices, through which the gaseous fluids probably issued, when this basaltic mass was raised. We lost sight of the small islands of Alegranza, Montana Clara, and Graciosa, which appear never to have been inhabited by the Guanches. They are now visited only for the purpose of gathering archil, which production is, however, less sought after, since so many other lichens of the north of Europe have been found to yield materials proper for dyeing. Montana Clara is noted for its beautiful canary-birds. The note of these birds varies with their flocks, like that of our chaffinches, which often differs in two neighbouring districts. Montana Clara yields pasture for goats, a fact which proves that the interior of this islet is less arid than its coasts. The name of Alegranza is synonymous with the Joyous, (La Joyeuse,) which denomination it received from the first conquerors of the Canary Islands, the two Norman barons, Jean de Bethencourt and Gadifer de Salle. This was the first point on which they landed. After remaining several days at Graciosa, a small part of which we examined, they conceived the project of taking possession of the neighbouring island of Lancerota, where they were welcomed by Guadarfia, sovereign of the Guanches, with the same hospitality that Cortez found in the palace of Montezuma. The shepherd king, who had no other riches than his goats, became the victim of base treachery, like the sultan of Mexico.
We sailed along the coasts of Lancerota, of the island of Lobos, and of Forteventura. The second of these islands seems to have anciently formed part of the two others. This geological hypothesis was started in the seventeenth century by the Franciscan, Juan Galindo. That writer supposed that king Juba had named six Canary Islands only, because, in his time, three among them were contiguous. Without admitting the probability of this hypothesis, some learned geographers have imagined they recognized, in the two islands Nivaria and Ombrios, the Canaria and Capraria of the ancients.
The haziness of the horizon prevented us, during the whole of our passage from Lancerota to Teneriffe, from discovering the summit of the peak of Teyde. If the height of this volcano is 1905 toises, as the last trigonometrical measure of Borda indicates, its summit ought to be visible at a distance of 43 leagues, supposing the eye on a level with the ocean, and a refraction equal to 0.079 of distance. It has been doubted whether the peak has ever been seen from the channel which separates Lancerota from Forteventura, and which is distant from the volcano, according to the chart of Varela, 2° 29′, or nearly 50 leagues. This phenomenon appears nevertheless to have been verified by several officers of the Spanish navy. I had in my hand, on board the Pizarro, a journal, in which it was noted, that the peak of Teneriffe had been seen at 135 miles distance, near the southern cape of Lancerota, called Pichiguera. Its summit was discovered under an angle considerable enough to lead the observer, Don Manual Baruti, to conclude that the volcano might have been visible at nine miles farther. It was in September, towards evening, and in very damp weather. Reckoning fifteen feet for the elevation of the eye, I find, that to render an account of this phenomenon, we must suppose a refraction equal to 0.158 of the arch, which is not very extraordinary for the temperate zone. According to the observations of General Roy, the refractions vary in England from one-twentieth to one-third; and if it be true that they reach these extreme limits on the coast of Africa, (which I much doubt,) the peak, in certain circumstances, may be seen on the deck of a vessel as far off as 61 leagues.
Navigators who have much frequented these latitudes, and who can reflect on the physical causes of the phenomena, are surprised that the peaks of Teyde and of the Azores* are sometimes visible at a very great distance, though at other times they are not seen when the distance is much less, and the sky appears serene and the horizon free from fogs. These circumstances are the more worthy of attention because vessels returning to Europe, sometimes wait impatiently for a sight of these mountains, to rectify their longitude; and think themselves much farther off than they really are, when in fine weather these peaks are not perceptible at distances where the angles subtended must be very considerable. The constitution of the atmosphere has a great influence on the visibility of distant objects. It may be admitted, that in general the peak of Teneriffe is seldom seen at a great distance, in the warm and dry months of July and August; and that, on the contrary, it is seen at very extraordinary distances in the months of January and February, when the sky is slightly clouded, and immediately after a heavy rain, or a few hours before it falls. It appears that the transparency of the air is prodigiously increased, as we have already observed, when a certain quantity of water is uniformly diffused through the atmosphere. Independent of these observations, it is not astonishing, that the peak of Teyde should be seldomer visible at a very remote distance, than the summits of the Andes, to which, during so long a time, my observations were directed. This peak, inferior in height to those parts of the chain of Mount Atlas at the foot of which is the city of Morocco, is not, like those points, covered with perpetual snows. The Piton, or Sugar-loaf, which terminates the peak, no doubt reflects a great quantity of light, owing to the whitish colour of the pumice-stone thrown up by the crater; but the height of that little truncated cone does not form a twenty-second part of the total elevation. The flanks of the volcano are covered either with blocks of black and scorified lava, or with a luxuriant vegetation, the masses of which reflect the less light, as the leaves of the trees are separated from each other by shadows of more considerable extent than that of the part enlightened.
[* The height of this peak of the Azores, according to Fleurieu, is 1100 toises; to Ferrer, 1238 toises; and to Tofino, 1260 toises: but these measures are only approximative estimates. The captain of the Pizarro, Don Manuel Cagigal, proved to me, by his journal, that he observed the peak of the Azores at the distance of 37 leagues, when he was sure of his latitude within two minutes. The volcano was seen at 4° south-east, so that the error in longitude must have an almost imperceptible influence in the estimation of the distance. Nevertheless, the angle which the peak of the Azores subtended was so great, that the captain of the Pizarro was of opinion this volcano must be visible at more than 40 or 42 leagues. The distance of 37 leagues supposes an elevation of 1431 toises.]
Hence it results that, setting aside the Piton, the peak of Teyde belongs to that class of mountains, which, according to the expression of Bouger, are seen at considerable distances only in a NEGATIVE MANNER, because they intercept the light which is transmitted to us from the extreme limits of the atmosphere; and we perceive their existence only on account of the difference of intensity subsisting between the aerial light which surrounds them, and that which is reflected by the particles of air placed between the mountains and the eye of the observer. As we withdraw from the isle of Teneriffe, the Piton or Sugar-loaf is seen for a considerable space of time in a POSITIVE MANNER, because it reflects a whitish light, and clearly detaches itself from the sky. But as this cone is only 80 toises high, by 40 in breadth at its summit, it has recently been a question whether, from the diminutiveness of its mass, it can be visible at distances which exceed 40 leagues; and whether it be not probable, that navigators distinguish the peaks as a small cloud above the horizon, only when the base of the Piton begins to be visible on it. If we admit, that the mean breadth of the Sugar-loaf is 100 toises, we find that the little cone, at 40 leagues distance, still subtends, in the horizontal direction, an angle of more than three minutes. This angle is considerable enough to render an object visible; and if the height of the Piton greatly exceeded its base, the angle in the horizontal direction might be still smaller, and the object still continue to make an impression on our visual organs; for micrometrical observations have proved that the limit of vision is but a minute only, when the dimensions of the objects are the same in every direction. We distinguish at a distance, by the eye only, trunks of trees insulated in a vast plain, though the subtended angle be under twenty-five seconds.
As the visibility of an object detaching itself in a brown colour, depends on the quantities of light which the eye meets on two lines, one of which ends at the mountain, and the other extends to the surface of the aerial ocean, it follows that the farther we remove from the object, the smaller the difference becomes between the light of the surrounding atmosphere, and that of the strata of air before the mountain. For this reason, when less elevated summits begin to appear above the horizon, they present themselves at first under a darker hue than those we discern at very great distances. In the same manner, the visibility of mountains seen only in a negative manner, does not depend solely on the state of the lower regions of the air, to which our meteorological observations are limited, but also on the transparency and physical constitution of the air in the most elevated parts; for the image detaches itself better in proportion as the aerial light, which comes from the limits of the atmosphere, has been originally more intense, or has undergone less loss in its passage. This consideration explains to a certain point, why, under a perfectly serene sky, the state of the thermometer and the hygrometer being precisely the same in the air nearest the earth, the peak is sometimes visible, and at other times invisible, to navigators at equal distances. It is even probable, that the chance of perceiving this volcano would not be greater, if the ashy cone, at the summit of which is the mouth of the crater, were equal, as in Vesuvius, to a quarter of the total height. These ashes, being pumice-stone crumbled into dust, do not reflect as much light as the snow of the Andes; and they cause the mountain, seen from afar, to detach itself not in a bright, but in a dark hue. The ashes also contribute, if we may use the expression, to equalize the portions of aerial light, the variable difference of which renders the object more or less distinctly visible. Calcareous mountains, devoid of vegetable earth, summits covered with granitic sand, the high savannahs of the Cordilleras,* which are of a golden yellow, are undoubtedly distinguished at small distances better than objects which are seen in a negative manner; but the theory indicates a certain limit, beyond which these last detach themselves more distinctly from the azure vault of the sky.
[* Los Pajonales, from paja, straw. This is the name given to the region of the gramina, which encircles the zone of the perpetual snows.]
The colossal summits of Quito and Peru, towering above the limit of the perpetual snows, concentre all the peculiarities which must render them visible at very small angles. The circular summit of the peak of Teneriffe is only a hundred toises in diameter. According to the measures I made at Riobamba, in 1803, the dome of the Chimborazo, 153 toises below its summit, consequently in a point which is 1300 toises higher than the peak, is still 673 toises (1312 metres) in breadth. The zone of perpetual snows also forms a fourth of the height of the mountain; and the base of this zone, seen on the coast of the Pacific, fills an extent of 3437 toises (6700 metres). But though Chimborazo is two-thirds higher than the peak, we do not see it, on account of the curve of the globe, at more than 38 miles and a third farther distant. The radiant brilliancy of its snows, when, at the port of Guayaquil, at the close of the rainy season, Chimborazo is discerned on the horizon, may lead us to suppose, that it must be seen at a very great distance in the South Sea. Pilots highly worthy of credit have assured me, that they have seen it from the rock of Muerto, to the south west of the isle of Puna, at a distance of 47 leagues. Whenever it has been seen at a greater distance, the observers, uncertain of their longitude, have not been in a situation to furnish precise data.
Aerial light, projected on mountains, increases the visibility of those which are seen positively; its power diminishes, on the contrary, the visibility of objects which, like the peak of Teneriffe and that of the Azores, detach themselves in a brown tint. Bouguer, relying on theoretical considerations, was of opinion that, according to the constitution of our atmosphere, mountains seen negatively cannot be perceived at distances exceeding 35 leagues. It is important here to observe, that these calculations are contrary to experience. The peak of Teneriffe has been often seen at the distance of 36, 38, and even at 40 leagues. Moreover, in the vicinity of the Sandwich Islands, the summit of Mowna–Roa, at a season when it was without snows, has been seen on the skirt of the horizon, at the distance of 53 leagues. This is the most striking example we have hitherto known of the visibility of a mountain; and it is the more remarkable, that an object seen negatively furnishes this example.
The volcanoes of Teneriffe, and of the Azores, the Sierra Nevada of Santa Martha, the peak of Orizaba, the Silla of Caracas, Mowna–Roa, and Mount St. Elias, insulated in the vast extent of the seas, or placed on the coasts of continents, serve as sea-marks to direct the pilot, when he has no means of determining the position of the vessel by the observation of the stars; everything which has a relation to the visibility of these natural seamarks, is interesting to the safety of navigation.
Last updated Saturday, October 24, 2015 at 23:03