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to have been that of a globe, which is the shape it would naturally assume from the action of the centripetal force, the opposite action of the centrifugal force must evidently change it into an oblate spheroid, or a body nearly resembling a turnip or an orange. And this is the true figure of the earth, as determined by Sir Isaac Newton, who found, by mathematical calculations, that the polar diameter of the earth is to the equatorial, as 229 is to 230; or that the regions of the equator are elevated about thirty-five miles more than those at the poles'. This great mathematical truth is farther confirmed by analogy; for it is easy to perceive, by a good telescope, that the planet Jupiter is flattened about his poles, in nearly the same manner as has been asserted of our earth; but as his rotation upon his axis is performed with a far greater rapidity than that of the earth, the alteration in his figure is found to be much more considerable; as would naturally follow from such a motion. The relation of his diameters, according to Newton, is nearly as 12 to 13, and the difference between his equatorial and polar diameters is about 6230 miles
I am now to consider the motion of the earth; and in this discussion nothing more is necessary, than to observe the common appearances of the heavens. The Sun and stars, it is well known, seem to move daily from east to west, and to return nearly to the same place in the heavens in twenty-four hours. It follows, therefore, that they must really move as they appear to do, or else that we ourselves must be moved; it being a self-evident principle, that if the situation of two
This was upon the supposition that the matter of which the earth is composed is homogeneous: according to more recent and accurate hypotheses, confirmed by actual admeasurements, the ratio of the earth's axes is stated to be about that of 323 to 324.
bodies with respect to each other be changed, one of them, at least, must be moved.
But if this change be owing to the revolution of the stars, we must suppose them to be endued with a motion so amazingly swift as to exceed all conception. Their distances from us are so im→ mense, and the orbits they have to run round so prodigiously great, that the nearest of them would move at least 100,000 miles in a minute. Now, as Nature never does that in a complicated and laborious manner, which may be done in a more simple and easy way, it is more consonant to reason, that these effects should be produced by the motion of the earth; especially, as such a motion will best account for all the celestial appearances, and, at the same time, preserve that beautiful simplicity and harmony, which are found to prevail in every other part of the creation. And it is much more probable, that the earth revolves round its axis, with an easy natural motion, once in twenty-four hours, than that such stupendous bodies as the Sun and stars (the former above a million times bigger than the earth) should be carried from one place to another with such incredible velocity.
It is no objection to the 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 imperceptible to the company on board; much more so must it be with such a vast body as the earth, that can meet with no obstacles to disturb its motion. Milton obviates this objection, by representing the earth advancing in her silent course,
With inoffensive pace that spinning sleeps
A balloon, turning upon its axis as it floats through the atmosphere, affords a sensible representation of the earth in its annual progress round the Sun, which is productive of the various vicissitudes of the seasons, winter, spring, summer, and autumn.
It has likewise been asserted, that if the earth moved, a stone dropped from the top of a tower, or any other high building, would not fall just at the bottom of it, as the building must have advanced considerably forward during the time of the fall. But it is well known, 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 thus the atmosphere, as Milton has observed above, is likewise borne along with the earth.
This motion of the earth round its axis is called its diurnal motion, and is that which occasions the regular return of day and night, and all the celestial appearances before mentioned.-I could here descant with pleasure on the various beneficial consequences attending this daily revolution; but, as this would lead my present disquisition too far, I shall conclude with the sentiment of the Roman orator: Diei noctisque vicissitudo conservat animantes, tribuens aliud agendi tempus, aliud quiescendi. Sic undique omne ratione concluditur, mente, concilioque divino omnia in hoc mundo ad salutem omnium, conservationemque admirabiliter administrari.' And this reference of all things to the benevolent agency of the Supreme Being, is beautifully expressed by Mallet;
Thee, Nature's God!
First Source of all things lovely, all things good!-
Obedient to thy nod, alternate night
Thine these noble works
Great universal Ruler!
Thy virtual energy the frame of things
O, unprofuse magnificence divine!
MY late inquiries into the Solar System, the Plurality of Worlds, and the Figure and Motion of the Earth, naturally lead me to some considerations on Gravity, which is the most antient and universal property of matter, and that, principally, by which the beauty, order, and harmony of the universe are invariably maintained. The illustrious Newton has pursued his speculations on this subject with such success, that, the most simple nature of gravity being supposed, he has esta
blished the true system of the world beyond all controversy, and explained clearly the most considerable phenomena of Nature. And his opinion of the nature and properties of gravity is
Every single particle, of all bodies whatever, gravitates to every single particle of all bodies whatever: that is, they are impelled toward each other by the force of gravity.
This gravitating force is universal as to the extent of it; that is, all bodies whatever, so far as we know, wherever they are placed, not only on the earth, but also in the heavens, whether in the Moon or planets, the Sun, or any other place, are endued with this power.
This force is also universal as to the kinds of bodies; that is, all bodies, whatever their figure, form, or texture be; whether they be simple or compound, solid or fluid; whether they be great or small, in motion or at rest; are endued with this power.
This force is also universal as to time; that is, all other conditions being the same, it never increases, nor diminishes.
The quantity of this gravity, at equal distances, is always exactly in proportion to the quantity of matter in the gravitating bodies: for instance, if a cubic foot of gold has a thousand pound weight upon the superficies of the earth, two cubic feet will have two thousand pound weight upon the same superficies; and if the earth contained but half the quantity of matter that it does now, the same cubic foot of gold, that has now a thousand pound weight upon the superficies of the earth, would have five hundred only.
The gravity of given bodies is greater or less, according to the distance of those bodies from each other; for example, a stone which, near the