CHAP. XI.
On the essential magnetick activity sphærically
effused.
iscourse hath often been held concerning
the * poles of the earth and of the
stone, and concerning the æquinoctial zone; whilst lately we have been speaking about the declining of magneticks toward
the earth and toward the terrella, and the causes of it. But while by various and complicated devices we have laboured
long and hard to arrive at the cause of this declination, we have by good fortune found out a new and admirable (beyond
the marvels of all virtues magnetical) science of the orbes themselves. For such is the power of magnetick globes, that
it is diffused and extended into orbes outside the body itself, the form being carried beyond the limits of the corporeal
substance; and a mind diligently versed in this study of nature will find the definite causes of the motions and
revolutions. The same powers of a terrella exist also within the whole orbe of its power; and these orbes at any distance
from the body of the terrella have in themselves, in proportion to their diameter and the magnitude of their
circumference, their own limits of influences, or points wherein magnetick bodies rotate; but they do not look toward the
same part of the terrella or the same point at any distance from the same (unless they be on the axis of the orbes and of
the terrella); but they always tend to those points of their own orbes, which are distant by similar arcs from the common
axis of the orbes. As, for example, in the following diagram, we show the body of a terrella, with its poles and æquator;
and also a versorium on three other concentrick orbes around the terrella at some distance from it. In these orbes (as in
all those which we may imagine without end) the magnetick body or versorium conforms to its own orbe in which it is
located, and to its diameter and poles and æquator, not to those of the terrella; and it is by them and according to the
magnitude of their orbes that the magnetick body is governed, rotated, and directed, in any arc of that orbe, both while
the centre of the magnetick body stands still, and also while it moves along. And yet we do not mean that the magnetick
forms and orbes exist in air or water or in any medium that is not magnetical; as if the air or the water were
susceptible of them, or were induced by them; for the forms are only effused and really subsist when magnetick substances
are there; whence a magnetick body is laid hold of within the forces and limits of the orbes; and within the orbes
magneticks dispose magneticks and incite them, as if the orbes of virtue were solid and
material loadstones. For the magnetick force does not pass through the whole medium or really exist as in a continuous
body; so the orbes are magnetick, and yet not real orbes nor existent by themselves.
Diagram of motions in magnetick orbes.
A B is the axis of the terrella and of the orbes, C D the æquator. On all the orbes, as on the terrella, at the equator the versorium arranges itself along the plane of the horizon; on the axis it everywhere looks perpendicularly toward the centre; in the intermediate spaces E looks toward D; and G looks toward H, not toward F, as the versorium L does on the surface of the terrella. But as is the relation of L to F on the surface of the terella, so is that of G to H on its orbe and of E to D on its orbe; also all the rotations on the orbes toward the termini of the orbes are such as they are on the surface of the terrella, or toward the termini of its surface. But if in the more remote orbes this fails somewhat at times, it happens on account of the sluggishness of the stone, or on account of the feebler forces due to the too great distance of the orbes from the terrella.
Demonstration.
Set upon the instrumental diagram described farther back [chap. 3] a plate or stiff circle of brass or tin, on which may be described the magnetick orbes, as in the diagram above; and in the middle let a hole be made according to the size of the terrella, so that the plate may lie evenly on the wood about the middle of the terrella on a meridional circle. Then let a small versorium of the length of a barley-corn be placed on any orbe; upon which, when it is moved to various positions on the same circle, it will always pay regard to the dimensions of that orbe, not to those of the stone; as is shown in the diagram of the effused magnetick forms.
While some assign occult and hidden virtues of substances, others a property of matter, as the causes of the wonderful magnetical effects; we have discovered the primary substantive form of globes, not from a conjectural shadow of the truth of reasons variously controverted; but we have laid hold of the true efficient cause, as from many other demonstrations, so also from this most certain diagram of magnetick forces effused by the form. Though this (the form) has not been brought under any of our senses, and on that account is the less perceived by the intellect, it now appears manifest and conspicuous even to the eyes through this essential activity which proceeds from it as light from a lamp. And here it must be noted that a magnetick needle, moved on the top of the earth or of a terrella or of the effused orbes, makes two complete rotations in one circuit of its centre, like some epicycle about its orbit.