When the glad ship shoots from the port CAPEL LOFT. Another proof is taken from the shadow of the earth upon the face of the moon in the time of an eclipse: for, as the moon has no light but what it receives from the sun, and the earth (during the eclipse) is interposed between them, the moon must be obscured, either totally, or in part. And since, in every lunar eclipse, which is not total, the obs scure part always appears to be bounded by a circular line, the earth itself, for that reason, must be spherical it being evident, that nothing but a spherical body can, in all situations, cast a circular shadow. : The little unevennesses on the surface of the earth, which are caused by mountains and valleys, afford no objection to its being considered as a round body for the loftiest mountains bear less proportion to the vast magnitude of the earth, than the small risings on the coat of an orange bear to the orange itself; or, indeed, than a grain of sand to an artificial globe of a foot 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. On the contrary, its circumference appears invariably to be as even and regular as if cast by a body perfectly globular. This doctrine of the spherical figure of the earth is, undoubtedly, very antient; for, at the taking of Babylon, by Alexander the Great, eclipses were found to have been computed for many ages before the birth of Christ; which could not have been done without a knowledge of the globular figure of the earth. And Thales, the Milesian, who lived about 600 years before Christ, is said, by Herodotus, to have predicted an eclipse of the sun; which shows that he was sufficiently acquainted with the subject of eclipses in general. But it is also equally certain, that of this spherical figure of the earth, some of the greatest men in those times, were ignorant. Heraclitus imagined it to have the shape of a canoe; Anaximander supposed it to be cylindrical; and Aristotle, the great oracle of antiquity, and even of later ages, gave it the form of a timbrel. The vulgar opinion, that the earth was a flat body, infinitely extended, and covered by the sky, in the form of a dome, continued to be the most universal, even so late as the fifteenth century. But a very little time before the discovery of America, the notion of the earth having a globular form was treated as an impious absurdity, contrary to the opinion of the most venerable fathers of the Church, and to some express declarations of the holy Scriptures'. The dictates of These passages of Scripture are either ill understood, or strangely perverted from their real meaning. For it is evident that the sacred writers speak every where according to the common appearances 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, 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 a historical relation, and the other a dramatical composition, in which such figures of speech have been always allowed; though Job himself, in another place, describes the earth as hung upon nothing.' And even now, the most philosophical persons both in writing and conversation, speak of the progress of the sun, &c. in the popular way, according to the appearance of it, although contrary to the reality. Thus Thomson: 6 And now from Aries rolls the bounteous Sun, reason, however, with the voyage of Christopher Columbus, restored to the earth the spherical figure, which the ancient Egyptians and Chaldeans had given it and it was then generally believed to be a perfect globe, and that the stars made their revolutions round it in circular orbits. Of this opinion were the greatest philosophers of the age. A globe is the most perfect of all geometrical figures; and the manifest simplicity of Nature in most of her operations, seemed to fa vour the idea of the earth having such a form. This simplicity, however, appeared to be a false light, by which its followers were misled.-M. Richer, in a voyage to Cayenne, near the equator, found that the pendulum of his clock no longer made its vibrations so frequently as in the latitude of Paris; and that it was absolutely necessary to shorten it by a little more than the eleventh part of a Paris inch, to make it agree with the times of the stars passing the meridian. Natural philosophy and geometry were then very far from being so much cultivated as they are at present; and who could have believed, says Voltaire, that from an observation so trifling in appearance, such a sublime and philosophical truth could spring? A pendulum, like any other falling body, is acted upon by the force of gravity, or the natural tendency of bodies toward their centre; and, in consequence of Richer's discovery, it was observed, that since the gravity of bodies is by so much the less powerful as those bodies are farther removed from the centre of the earth, the region of the equator must absolutely be much more elevated than that of France; and, consequently, the figure of the earth could not be that of a sphere. The observations made at Cayenne might have F VOL. I. been considered as too local and particular to admit of any satisfactory conclusion; but, as a like alteration was found to take place by subsequent observations in many different situations, we can no longer hesitate to receive it as a general phenomenon, arising from an actual diminution of gravity, in those places where the experiment was performed. 2 Newton and Huygens were the first who perceived the extensive application of which this discovery was capable; and, by pursuing it through all its consequences, they obtained the solution of a problem, which seemed beyond the reach of human abilities. This was no less than the determining the true figure of the earth. : It is a known property of the pendulum, that all its vibrations are made in the same time; and that the time in which each vibration is performed is in proportion to the square root of the length of the rod thus a pendulum of thirty-six Paris inches in length, makes its vibrations in a second; and one of nine inches makes its vibrations in half a the second; so that the shorter the pendulum, more swiftly it moves; and the longer it is, the more slowly it moves; the ratio being always the same as that before mentioned. The time, therefore, in which any pendulum performs its oscillations, depends as well upon the length of that pendulum, as upon the force which impels it toward the surface of the earth. If this force be diminished by any cause whatever, the body, with a less capacity for motion, will employ a longer space of time to move through the same space; and, in order that each vibration may be made in the same time that it was before this diminution took place, the length of the rod must be shortened, by which a new velocity will be given to it, that will be sufficient to supply the de fect; and this was exactly the case in all the experiments made near the equator. Gravity, then, it is evident, is less powerful under the equator than in those parts of the earth that lie near the poles. The limits of my paper will not permit me to enter into such an illustra tion of this, as would be satisfactory to the philo sophical inquirer. I must be content, therefore, to observe, that there are two forces which act in direct opposition to each other. The first, which is called the centrifugal force, is a certain tendency of bodies to fly off from the centre round which they move. This may be made evident, by ob serving the whirling of a mop, or the turning of a stone swiftly round in a sling. The other force is called the centripetal. It is so called, because it is directed toward the centre, and, in the present case, is the same as gravity. It is next to be considered, that the diurnal motion of the earth is performed round an imaginary line, which passes through the two poles; and according to Sir Isaac Newton, as the equator is farther distant from its centre, than any other circle which is parallel to it, it is plain, that those parts of the earth which lie under the equator, will move with a greater velocity than those which are nearer to the poles; and, of course, the equatorial regions will become more elevated than the polar. Now, as every part of the earth gravitates towards the centre, in consequence of the centripetal force; and as this force is found to be about 289 times greater than the centrifugal force, which arises from the rotation of the earth upon its axis, a certain balance will constantly be maintained between them, and the earth will assume such a figure as would naturally result from the difference between these two contrary and opposite forces. Supposing, therefore, the primitive figure of the earth |