OF THE FIGURE AND MOTION OF THE EARTH.
The earth, as we have observed, was long considered as a large circular plane, spreading out on all sides to an indefinite distance ; but it is now ascertained that it is of a spherical figure, nearly resembling that of a globe. The evidence for this fact is decisive, without having recourse to scientific principles, by consideringthat the celebrated navigators, Magellan, Sir Francis Drake, Lord Anson, and captain Cook, have all, at different times, sailed round the earth. They set out from Eu ropean ports, and, by steering their course westward, arrived at length at the very place from whence they departed, which could not have happened, had the earth been of any other than a spherical or a globular figure. This form is also apparent, from the circular appearance of the sea itself, and the circumstances which attend large objects, when seen at a distance on its surface. For when a ship goes out to sea, we first lose sight of the hull, or body of the vessel, see fig. 4 ; afterwards that of the rigging ; and at last can discern old; the top of the mast, which is evidently owing to the convexity of the water between the eye and the object ; for, otherwise, the largest and most conspicuous part would be visible the longest. Another proof is taken from the shadow of the earth upon the face of the moon, during the time of a lunar eclipse ; for the moon, having no light but what i receives from the sun, and the earth being interposed between them, the moon must either wholly, or in part, become obscure. And since in every eclipse of this kind, which is not total, the obscure part always appears to be bounded by a circular line, the earth itself, for that reason, must be spherical ; it being evident that none but a spherical body can, in all situations, cast a circular shadow.
It is not ascertained who was the first person that asserted the figure of the earth to be spherical, but the opinion is of very great antiquity. For, when Babylon was taken by Alexander dm Great, it was known that the philosophers in that city had been long in the habit of calculating eclipses, which they could not have ac complished without a knowledge of the true figure of the earth. Tholes, who flourished six centuries before tire birth of Christ, predicted, according to the testimony of Herodotus, an eclipse of the sun. Hence it should seem, that in those early days, the globular figure of the earth had been, by the learned, investi gated and credited. This being known, its magnitude would also soon be disco vered: the solution of this apparently difficult problem engaged the attention of many great men about the same peri od ; and though the measures which they have given are wide of the truth, and even very different from one another, yet this may be imputed to the inaccuracy of their instruments, and the want of ma thematical knowledge, rather than to the impracticability in the thing itself With out, however, entering upon this subject, we may observe, that the universe in gene ral, as well as the solar system in particu lar, are in some measure connected with the motion of the globe diet we inhabit. By the universe may be understood, the whole frame of nature to the utmost ex tent of the creation, and by the solar sys tem is meant, that portion ofit which com prehends the sun, planets, satellites, and comets. Of this system the sun is suppos ed to be in the centre, round which there are eleven planets continually revolving.
If we can form a notion of the manner in which the earth moves, we shall easily conceive the motions of all the rest of the planets, and by that means obtain a complete idea of the order and economy of the whole system. And in order to this, nothing more is necessary than to consider the common appearances of the heavens, which are constantly presented to our view, and attend to the conse quences. For since it is well known that the sun and stars appear to move daily from east to west, and to return nearly to the same places in the heavens again in 24 hours, it follows, that they must really move, as they appear to do, or else that we ourselves must be moved, and attri bute our motion to them : it being a self evident principle, that if two things change their situation with respect to each other, one of them, at least, must have moved. But if this change be ow
ing to the revolution ofthe stars, we must suppose them to be endowed with a mo tion so exceedingly swift, as to exceed all conception ; since it is now known, by calculations founded on the surest ob servations, that their distances from us are so immense, and the orbits they have to.run round so prodigiously great, that the nearest of them would move at least one hundred thousand miles in a minute. Now as nature never does that in a com plicated and laborious manner, which may be-done in a more simple and easy one, it is certainly more agreeable to rea son, as well as to the power and wisdom of the Creator, that these effects should be produced by the motion of the earth ; especially as such a motion will best ac count for all the celestial appearances; and at the same time preserve that beau tiful simplicity and harmony, which is found to prevail in every other part of the creation. And this argument will ap pear still more forcible, if we compare the vast bulk of the celestial bodies with the bulk of the earth. For it is now well known, that the sun is above a million of times larger than the earth ; and from the best modern observations it appears, that many of the stars are at least equally large. It is much more probable, there fore, that the earth revolves round its axis, with an easy natural motion, once in 24 hours, than that those immense bo dies should be carried from one place to another with such incredible swiftness. Nor is it any objection to this rotation of the earth, that we are unable to perceive it. For as the motion of a ship at sea, when she sails swiftly over the smooth surface of the water, is almost impercep tible to the passengers and company on board, much more so must it be with such a large body as the earth, which has no impediments or obstacles of any kind to meet with in its way, or to disturb its mo tion. And in a manner equally easy may another objection be removed, which has frequently been brought against this doc trine. It has been asserted, that if the earth moved, a stone dropped from the top of a tower, or any other high build ing, would not fall just at the bottom of it, as the building must have advanced con siderably forward during the time of the fall. But this is evidently a mistake ; for it is well known, by repeated experi ments, that if a body be projected from another body in motion, it will always partake of the motion of that other body. Thus, a stone dropped from the top of a mast, while the ship is under sail, is not left by the vessel, but falls exactly at the foot of the mast. And if a bottle of water be hung up in the cabin, with its neck downwards, it will empty itself drop by drop, into another bottle placed exactly underneath it, though the ship shall have run many feet whilst each drop was in the air. This motion of the earth round its axis, which, from the instances already given, has been sufficiently proved, is called its diurnal or daily motion, and is that which occasions the regular return of day and night, and all the celestial ap pearances before mentioned. But there is also another motion of the earth, called its annual or yearly motion, which occa sions the various vicissitudes of the sea sons, summer, winter, spring, and autumn. And the proofs of this second motion may be easily gathered from celestial appear ances, in nearly the same manner as the former. For as that luminary seems to move round the earth from east to west, in the space of a day, which is really owing to the diurnal revolution of the earth upon its axis, in a contrary direc tion ; so likewise he seems to have an annual motion in the heavens, and to rise and set continually in different parts of theM ; which is certainly occasioned by the daily motion of the earth in its orbit, or path round the sun, which it completes in the space of a year.