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LETTER II.

OF THE FIGURE AND MOTION OF THE EARTH.

IN In my last letter I have shown you the excellence and advantage of astronomy in general. I shall now come to particulars; and endeavour to lead you, by gentle and easy steps, from the first principles of the science, to those parts which are the most interesting and useful. And as the earth we inhabit is constantly subject to our observation, and is that with which we are the best acquainted, a description of its form and magnitude will naturally excite your curiosity and attention.

This vast body was long considered as a large circular plane, spreading out on all sides to an infinite distance: and the heavens, above it, in which the sun, moon and stars appear to move daily from east to west, were imagined to be at no great distance from it, and to have been created solely for the use and ornament of our earth. Of this notion are the vulgar, and those who are ignorant of astronomy, to this day. But it is now well known to mathematicians and philosophers, that the earth is of a round or spherical figure, nearly resembling that of a globe.

The truth of this doctrine, without having recourse to scientific principles, will appear sufficiently evident from the voyages of those celebrated navigators Magellan, Sir Francis Drake, Lord Anson, Cook, &c. who all set out, at different

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times, to sail round the world; and, by steering their course continually westward, arrived, at length, at the place they departed from; which could never have happened, had the earth been of any other than a spherical or globular figure.

This form is also obvious, from the circular appearance of the sea itself, and the circumstances which attend large objects when seen at a distance on its surface. Thus, when a ship leaves the shore, we first lose sight of the hull, or body of the vessel; afterwards of the rigging; and at last discern only the top of the mast; which is evidently owing to the convexity of the water between the eye and the object; or otherwise, the largest and most conspicuous part would have been visible the longest, as is manifest from experience.

Another proof, which is of no less force than either of the former, is taken from the shadow of the earth, upon the face of the moon, in the time of a lunar eclipse. For as the moon has no light but what it receives from the sun, and the earth being, at this time, 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 dark part always appears to be bounded by a circular line, the earth itself, for that reason, must be spherical; because it is evident, that none but a spherical body can, in all situations, cast a circular shadow.

Nor are the little unevennesses on the earth's surface, arising from hills and valleys, any material objection to its being considered as a round body;

since the highest mountains we are acquainted with, bear a less proportion to the whole bulk of the earth, than the small risings on the coat of an orange bear to that fruit; or a grain of sand, to an artificial globe of a foot in diameter. And accordingly we find, that these trifling protuberances occasion no irregularities in the shadow of the earth, during the time of a lunar eclipse; but that the circumference of it always appears to be even and regular, as if cast by a body perfectly globular,

A number of other proofs might be given to the same purpose; but these are the most popular, and such as I apprehend must entirely convince every impartial enquirer, whose object is truth, and whose mind is unclouded by superstition, or an obstinate attachment to early notions and vulgar prejudices. I mention this the more particularly, because, notwithstanding the clearest arguments and most decisive demonstrations, there have been some who have violently opposed this doctrine, and even represented it as dangerous to religion and morals. Thus, several of the ancient fathers went so far out of their province, as to pronounce it heretical for any person to declare there were Antipodes, or people who live opposite to us upon the globe.

They took their objection from some passages of Scripture, which they either ill understood, or strangely perverted from their true meaning. For it is evident, that the sacred writers speak every where according to the common appearance of

things; and were not so solicitous to instruct us in philosophy and astronomy, as to make us good men, by laying before us a plain rule of faith and conduct. Thus, when Joshua speaks of the sun and moon standing still while the Jews avenged themselves of their enemies; and Job describes the earth as being supported by pillars, they used the popular language of the times, without concerning themselves with the strict philosophical propriety of the terms they employed: the one being an historical relation; and the other a dramatical composition, in which such figures of speech have been always allowable.

It is not known who was the first that asserted the figure of the earth to be spherical; but the doctrine is undoubtedly very ancient. For at the taking of Babylon by Alexander the Great, eclipses were found to have been set down and computed for many centuries before that time; which, without a knowledge of the globular figure of the earth, could not have been done. Thales the Milesian, who lived about six hundred years before Christ, must likewise have been sufficiently acquainted with this subject; since, according to the testimony of Herodotus, he predicted an eclipse of the sun. "After the war, he observes, had been carried on for six years between the Medes and Lydians, as they were going out to battle, the day became presently as dark as the night; which change, though it had been predicted by Thales to the Ionians, so far terrified both parties, that they

became equally desirous of a peace, which was immediately concluded.” (a)

It appears, therefore, that, in those early times, the true figure of the earth was not unknown in the world. But it is also equally certain, that this knowledge was confined to a few; and that even some of the greatest mathematicians themselves were ignorant of it. Thus Heraclitus, a Greek philosopher, who flourished about five hundred years before Christ, supposed it to have the shape of a skiff or canoe; Anaximander, his contemporary, imagined it to be cylindrical; and Aristotle, the great oracle of antiquity, gave it a form equally remote from the truth.

These, and many other absurd opinions of the same kind, which are attributed to the ancients, plainly show the confusion and uncertainty which, at that time, attended the subject. But it is the glory of modern philosophy, that this doctrine is now set in so clear a light, and the knowledge of it so generally diffused, that it is almost impossible it should ever again be forgotten; or that the ravages of ignorant barbarians, should involve it in doubt and obscurity. We are now certain, that

(a) This eclipse has given rise to many discussions, and various opinions have been advanced as to the time in which it happened; but from a paper, lately published in the Philosophical Transactions, by F. Baily, Esq., it appears to have taken place Sept. 30th, six hundred and ten years before Christ; at which time, therefore, the knowledge of astronomy must have been very considerably advanced.

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