Of all the solutions of the Ebbing and Flowing of the Sea, the most simple and ingenious, though afterwards found by observation to be inadequate, is that of Descartes, who supposes a vortex of subtile matter, of an elliptic form, to invest our globe, and compress it on all sides. The moon, according to this philosopher, is immerged in this elliptic vortex, and when at its greatest elongation from the earth, it makes less impression upon the circumambient ethereal matter; but when it comes to the narrowest part of the ellipse, gives such an impulse to the atmosphere, as puts the whole ocean in agitation. He supports his system by this remark, that the ebbing and flowing of the sea generally coincides with the irregularity of the moon’s course.
The opinion of Kepler and Newton is more conformable to observation, and is founded on this hypothesis—that the moon attracts the waters of the sea, diminishing the weight of those parts of it over whose zenith it comes, and increasing the weight of the collateral parts, so that the parts directly opposite to the moon, and under it in the same hemisphere, must become more elevated than the rest. According to this system, the action of the sun concurs with that of the moon, in occasioning the tides; which are higher or lower respectively, according to the situation of those two luminaries, which, when in conjunction, act in concert, raising the tides to the greatest height; and when in opposition, produce nearly the same effect, in swelling the waters of the opposite hemispheres; but when in quadrature, suspend each other’s force, so as to act only by the difference of their powers; and thus the tides vary, according to the different positions of the sun and moon.
The Cartesian method of solution has been indicated by Pytheas Massiliensis, who observes, that the tides, in their increase and decrease, follow the irregular course of the moon; and by Seleucus of Erythrea, the mathematician, who ascribing to the earth a rotation about its axis, imputes the cause of tides to the activity of the earth’s vortex, in conjunction with that of the moon.
Pliny’s account has more affinity to that of sir Isaac Newton. The great naturalist of the ancients maintained, that “the sun and moon had a reciprocal share in causing the tides:” and after a course of observations for many years, he remarked, that “the moon acted most forcibly upon the waters when it was nearest to the earth; but that the effect was not immediately perceived by us, but at such an interval as may well take place between the action of celestial causes, and the discernible result of them on earth.” He remarked also, that the waters, which are naturally inert, do not swell up immediately upon the conjunction of the sun and moon; but having gradually admitted the impulse, and begun to raise themselves, continue in that elevation, even after the conjunction is over.
There are few things which have more engaged the attention of naturalists, and with less success, than the wonderful properties of the Loadstone. Almost all have agreed in affirming that there are corpuscles of a peculiar form and energy that continually circulate around and through the loadstone, and that a vortex of the same matter circulates around and through the earth. Upon these suppositions Descartes and others have advanced, that the loadstone has two poles similar to those of the earth; and that the magnetic matter which issues at one of the poles, and circulates around to enter at the other, occasions that impulse which brings iron to the loadstone, whose small corpuscles have an analogy to the pores of iron, fitting them to lay hold of it, but not of other bodies.
All this the ancients had said before. The impulsive force which joins iron to the loadstone, and other things to Amber, was known to Plato; though he would not call it attraction, as allowing no such cause in nature. This philosopher called the magnet the stone of Hercules, because it subdued iron, which conquers every thing.
Descartes’s idea of his explanation was doubtless derived from Lucretius, who admitted, that there was a “vortex of corpuscles, or magnetic matter, which, continually circulating around the loadstone, repelled the intervening air betwixt itself and the iron. The air thus repelled, the intervening space became a vacuum; and the iron, finding no resistance, approached with an impulsive force, pushed on by the air behind it.”
Plutarch likewise is of the same opinion. He says, that “amber attracts none of those things that are brought to it, any more than the loadstone, but emits a matter, which reflects the circumambient air, and thereby forms a void. The expelled air puts in motion the air before it, which making a circle, returns to the void space, driving before it, towards the loadstone, the iron which it meets in its way.” He then proposes a difficulty, to wit, “why the vortex which circulates around the loadstone does not make its way to wood or stone, as well as iron?” He answers, like Descartes, that “the pores of iron have an analogy to the particles of the vortex circulating about the loadstone, which yields them such access as they can find in no other bodies, whose pores are differently formed.”
Certain authors report, that the properties of the loadstone, particularly its tendency towards the north pole, enabled the ancients to undertake long voyages; and they pretend, that the Egyptians, Phœnicians, and Carthaginians, employed the compass to guide them in their naval excursions; though afterwards they lost the use of it, just as they did of dying purple,[462] and of embroidering, and of composing bricks, and a cement able to resist the force of all weathers; arts, without all doubt, formerly well known to them. Pineda and Kircher affirm likewise, that Solomon knew the use of the compass, and that his subjects steered their course by it in sailing to the land of Ophir. There is also a passage of Plautus[463] produced, wherein it is alleged he speaks of the compass. There is not however a single passage in the ancients that directly supports these pretensions.[464]