Micrographia, by Robert Hooke

Observ. xxvii. Of the Beard of a wilde Oat, and the use that may be made of it for exhibiting always to the Eye the temperature of the Air, as to driness and moisture.

This Beard of a wild Oat, is a body of a very curious structure, though to the naked Eye it appears very slight, and inconsiderable, it being only a small black or brown Beard or Bristle, which grows out of the side of the inner Husk that covers the Grain of a wild Oat; the whole length of it, when put in Water, so that it may extend it self to its full length, is not above an Inch and a half, and for the most part somewhat shorter, but when the Grain is ripe, and very dry, which is usualy in the Moneths of July, and August, this Beard is bent somewhat below the middle, namely, about 2/5 from the bottom of it, almost to a right Angle, and the under part of it is wreath'd lik a With; the substance of it is very brittle when dry, and it will very easily be broken from the husk on which it grows.

If you take one of these Grains, and wet the Beard in Water, you will presently see the small bended top to turn and move round, as if it were sensible; and by degrees, if it be continued wet enough, the joint or knee will streighten it self; and if it be suffer'd to dry again, it will by degrees move round another way, and at length bend again into its former posture.

If it be view'd with an ordinary single Microscope, it will appear like a small wreath'd Sprig, with two clefts; and if wet as before, and then look'd on with this Microscope, it will appear to unwreath it self, and by degrees, to streighten its knee, and the two clefts will become streight, and almost on opposite sides of the small cylindrical body.

If it be continued to be look'd on a little longer with a Microscope, it will within a little while begin to wreath it self again, and soon after return to its former posture, bending it self again neer the middle, into a kind of knee or angle.

Schem. 15.
Fig. 2.
Several of those bodies I examin'd with larger Microscopes, and there found them much of the make of those two long wreath'd cylinders delineated in the second Figure of the 15. Scheme, which two cylinders represent the wreathed part broken into two pieces, whereof the end AB is to be suppos'd to have join'd to the end CD, so that EACF does represent the whole wreath'd part of the Beard, and EG a small piece of the upper part of the Beard which is beyond the knee, which as I had not room to insert, so was it not very considerable, either for its form, or any known property; but the under or wreathed part is notable for both: As to its form, it appear'd, if it were look'd on side-ways, almost like a Willow, or a small tapering rod of Hazel, the lower or bigger half of which onely, is twisted round several times, in some three, in others more, in others less, according to the bigness and maturity of the Grain on which it grew, and according to the driness and moisture of the ambient Air, as I shall shew more at large by and by.

The whole outward Superficies of this Cylindrical body is curiously adorned or fluted with little channels, and interjacent ridges, or little protuberances between them, which run the whole length of the Beard, and are streight where the Beard is not twisted, and wreath'd where it is, just after the same manner: each of those sides is beset pretty thick with small Brides or Thorns, somewhat in form resembling that of Porcupines Quills, such as aaaaa in the Figure; all whose points are directed like so many Turn-pikes towards the small end or top of the Beard, which is the reason, why, if you endeavour to draw the Beard between your fingers the contrary way, you will find it to stick, and grate, as it were, against the skin.

The proportion of these small conical bodies aaaaa to that whereon they grow, the Figure will sufficiently shew, as also their manner of growing, their thickness, and neerness to each other, as, that towards the root or bottom of the Beard, they are more thin, and much shorter, insomuch that there is usually left between the top of the one, and the bottom of that next above it, more then the length of one of them, and that towards the top of the Beard they grow more thick and close (though there be fewer ridges) so that the root, and almost half the upper are hid by the tops of those next below them.

I could not perceive any transverse pores, unless the whole wreath'd part were separated and cleft, in those little channels, by the wreathing into so many little strings as there were ridges, which was very difficult to determine; but there were in the wreathed part two very conspicuous channels or clefts, which were continued from the bottom F to the elbow bow EH or all along the part which was wreath'd, which seem'd to divide the wreath'd Cylinder into two parts, a bigger and a less; the bigger was that which was at the convex side of the knee, namely, on the side A, and was wreath'd by OOOOO; this, as it seem'd the broader, so did it also the longer, the other PPPPP, which was usually purs'd or wrinckled in the bending of the knee, as about E, seem'd both the shorter and narrower, so that at first I thought the wreathing and unwreathing of the Beard might have been caus'd by the shrinking or swelling of that part; but upon further examination, I sound that the clefts, KK, LL, were stuft up with a kind of Spongie substance, which, for the most part, was very conspicuous neer the knee, as in the cleft KK, when the Beard was dry; upon the discovery of which, I began to think, that it was upon the swelling of this porous pith upon the access of moisture or water that the Beard, being made longer in the midst, was streightned, and by the shrinking or subsiding of the parts of that Spongie substance together, when the water or moisture was exhal'd or dried, the pith or middle parts growing shorter, the whole became twisted.

But this I cannot be positive in, for upon cutting the wreath'd part in many places transversly, I was not so well satisfy'd with the shape and manner of the pores of the pith; for looking on these transverse Sections with a very good Microscope, I found that the ends of Schem. 15.
Fig. 3.
those transverse Sections appear'd much of the manner of the third Figure of the 15. scheme ABCFE, and the middle of pith CC, seem'd very full of pores indeed, but all of them seem'd to run the long-ways.

This Figure plainly enough shews in what manner those clefts, K and L divided the wreath'd Cylinder into two unequal parts, and also of what kind of substance the whole body consists; for by cutting the same Beard in many places, with transverse Sections, I found much the same appearance with this express'd; so that those pores seem to run, as in most other such Cany bodies, the whole length of it.

The clefts of this body KK, and LL, seem'd (as is also express'd in the Figure) to wind very oddly in the inner part of the wreath, and in some parts of them, they seem'd stuffed, as it were, with that Spongie substance, which I just now described.

This so oddly constituted Vegetable substance, is first (that I have met with) taken notice of by Baptista Porta, in his Natural Magick, as a thing known to children and Juglers, and it has been call'd by some of those last named persons, the better to cover their cheat, the Legg of an Arabian Spider, or the Legg of an inchanted Egyptian fly, and has been used by them to make a small Index, Cross, or the like, to move round upon the wetting of it with a drop of Water, and muttering certain words.

But the use that has been made of it, for the discovery of the various constitutions of the Air, as to driness and moistness, is incomparably beyond any other, for this it does to admiration: The manner of contriving it so, as to perform this great effect, is onely thus:

Provide a good large Box of Ivory, about four Inches over, and of what depth you shall judge convenient (according to your intention of making use of one, two, three, or more of these small Beards, ordered in the manner which I shall by and by describe) let all the sides of this Box be turned of Basket-work (which here in London is easily enough procur'd) full of holes, in the manner almost of a Lettice, the bigger, or more the holes are, the better, that so the Air may have the more free passage to the inclosed Beard, and may the more easily pass through the Instrument; it will be better yet, though not altogether so handsom, if insteed of the Basket-work on the sides of the Box, the bottom and top of the Box be join'd together onely with three or four small Pillars, after Schem. 15.
Fig. 4.
the manner represented in the 4. Figure of the 15. Scheme. Or, if you intend to make use of many of these small Beards join'd together, you may have a small long Case of Ivory, whose sides are turn'd of Basket-work, full of holes, which may be screw'd on to the underside of a broad Plate of Ivory, on the other side of which is to be made the divided Ring or Circle, to which divisions the pointing of the Hand or Index, which is moved by the conjoin'd Beard, may shew all the Minute variations of the Air.

There may be multitudes of other ways for contriving this small Instrument, so as to produce this effect, which any one may, according to his peculiar use, and the exigency of his present occasion, easily enough contrive and take, on which I shall not therefore insist. The whole manner of making any one of them is thus: Having your Box or frame AABB, fitly adapted for the free passage of the Air through it, in the midst of the bottom BBB, you must have a very small hole C, into which the lower end of the Beard is to be fix'd, the upper end of which Beard ab, is to pass through a small hole of a Plate, or top AA, if you make use onely of a single one, and on the top of it e, is to be fix'd a small and very light Index fg, made of a very thin sliver of a Reed or Cane; but if you make use of two or more Beards, they must be fix'd and bound together, either with a very fine piece of Silk, or with a very small touch of hard Wax, or Glew, which is better, and the Index fg, is to be fix'd on the top of the second, third, or fourth in the same manner as on the single one.

Now, because that in every of these contrivances, the Index fg, will with some temperatures of Air, move two, three, or more times round, which without some other contrivance then this, will be difficult to distinguish, therefore I thought of this Expedient: The Index or Hand fg, being rais'd a pretty way above the surface of the Plate AA, fix in at a little distance from the middle of it a small Pin h, so as almost to touch the surface of the Plate AA, and then in any convenient place of the surface of the Plate, fix a small Pin, on which put on a small piece of Paper, or thin Past-board, Vellom, or Parchment, made of a convenient cize, and shap'd in the manner of that in the Figure express'd by ik, so that having a convenient number of teeth every turn or return of the Pin h, may move this small indented Circle, a tooth forward or backwards, by which means the teeth of the Circle, being mark'd, it will be thereby very easie to know certainly, how much variation any change of weather will make upon the small wreath'd body. In the making of this Secundary Circle of Vellom, or the like, great care is to be had, that it be made exceeding light, and to move very easily, for otherwise a small variation will spoil the whole operation. The Box may be made of Brass, Silver, Iron, or any other substance, if care be taken to make it open enough, to let the Air have a sufficiently free access to the Beard. The Index also may be various ways contrived, so as to shew both the number of the revolutions it makes, and the Minute divisions of each revolution.

I have made several trials and Instruments for discovering the driness and moisture of the Air with this little wreath'd body, and find it to vary exceeding sensibly with the least change in the constitution of the Air, as to driness and moisture, so that with one breathing upon it, I have made it untwist a whole bout, and the Index or Hand has shew'd or pointed to various divisions on the upper Face or Ring of the Instrument, according as it was carried neerer and neerer to the fire, or as the heat of the Sun increased upon it.

Other trials I have made with Gut-strings, but find them nothing neer so sensible, though they also may be so contriv'd as to exhibit the changes of the Air, as to driness and moisture, both by their stretching and shrinking in length, and also by their wreathing and unwreathing themselves; but these are nothing neer so exact or so tender, for their varying property will in a little time change very much. But there are several other Vegetable substances that are much more sensible then even this Beard of a wilde Oat; such I have found the Beard of the seed of Musk-grass, or Geranium moschatum, and those of other kinds of Cranes-bil seeds, and the like. But always the smaller the wreathing substance be, the more sensible is it of the mutations of the Air, a conjecture at the reason of which I shall by and by add.

The lower end of this wreath'd Cylinder being stuck upright in a little soft Wax, so that the bended part or Index of it lay horizontal, I have observ'd it always with moisture to unwreath it self from the East (For instance) by the South to the West, and so by the North to the East again, moving with the Sun (as we commonly say) and with heat and drouth to re-twist; and wreath it self the contrary way, namely, from the East, (for instance) by the North to the West, and so onwards.

The cause of all which Phænomena, seems to be the differing texture of the parts of these bodies, each of them (especially the Beard of a wilde Oat, and of Mosk-grass seed) seeming to have two kind of substances, one that is very porous, loose, and spongie, into which the watry steams of the Air may be very easily forced, which will be thereby swell'd and extended in its dimensions, just as we may observe all kind of Vegetable substance upon steeping in water to swell and grow bigger and longer. And a second that is more hard and close, into which the water can very little, or not at all penetrate, this therefore retaining always very neer the same dimensions, and the other stretching and shrinking, according as there is more or less moisture or water in its pores, by reason of the make and shape of the parts, the whole body must necessarily unwreath and wreath it self.

And upon this Principle, it is very easie to make several sorts of contrivances that should thus wreath and unwreath themselves, either by heat and cold, or by driness and moisture, or by any greater or less force, from whatever cause it proceed, whether from gravity or weight, or from wind which is motion of the Air, or from some springing body, or the like.

This, had I time, I should enlarge much more upon; for it seems to me to be the very first footstep of Sensation, and Animate motion, the most plain, simple, and obvious contrivance that Nature has made use of to produce a motion; next to that of Rarefaction and Condensation by heat and cold. And were this Principle very well examin'd, I am very apt to think, it would afford us a very great help to find out the Mechanism of the Muscles, which indeed, as farr as I have hitherto been able to examine, seems to me not so very perplex as one might imagine, especially upon the examination which I made of the Muscles of Crabs, Lobsters, and several sorts of large Shell-fish, and comparing my Observations on them, with the circumstances I observ'd in the muscles of terrestrial Animals.

Now, as in this Instance of the Beard of a wilde Oat, we see there is nothing else requisite to make it wreath and unwreath it self, and to streighten and bend its knee, then onely a little breath of moist or dry Air, or a small atome almost of water or liquor, and a little heat to make it again evaporate, for, by holding this Beard, plac'd and fix'd as I before directed, neer a Fire, and dipping the tip of a small shred of Paper in well rectify'd spirit of Wine, and then touching the wreath'd Cylindrical part, you may perceive it to untwist it self; and presently again, upon the avolation of the spirit, by the great heat, it will re-twist it self, and thus will it move forward and backwards as oft as you repeat the touching it with the spirit of Wine; so may, perhaps, the shrinking and relaxing of the muscles be by the influx and evaporation of some kind of liquor or juice. But of this Enquiry I shall add more elsewhere.

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