Note a — BOOK II.

Although the Terraqueous Globe be of an orbicular Figure, yet it is not strictly so, 1. On account of its Hills and Vallies. But there are inconsiderable to the Earth’s Semidiameter; for they are but as the Dust upon a common Globe. But, 2. Our modern Astronomers assign a much greater Variation from a globous Form, namely, that of a prolate Sphæroid, making the Polar about 34 Miles shorter than the Equatorial Diameter. The Cause of which they make to be the centrifugal Force of the diurnal Rotation of the Globe.

This Figure they imagine is in Jupiter, his Polar being to his Equatorial Diameter, as 39⅗ to 40⅗. But whether it be so or no, I confess I could never perceive, although I have often viewed that Planet through very good, and long Glasses, particularly a tolerable good one of 72 Feet in my Hands: And although by Reason of cloudy Weather, and (at present) Jupiter’s Proximity to the Sun, I have not been of late able to take a review of that Planet; yet Saturn (so far as his Ring would permit,) and Mars appear perfectly round thro’ Mr. Huygens’s long Glass of 126 Feet, which by Will he bequeathed, with its whole Apparatus, to our R. S. by whose Favour it is now in my Hands. And moreover, I believe it difficult, next to impossible, to measure the two Diameters to a 40ᵗʰ Part, by reason of the smallness of Jupiter’s apparent Diameter, and by reason he is moving all the time of measuring him.

As to what is alledged from lengthening the Pendulums of Clocks, to make them keep the same Time under the Equator, as they do in our Climes; I have shewn from the like Variations in the Air-Pump, that this may arise from the rarity of the Air there, more than here. V. Phil. Trans. Nᵒ. 294. But if the Degrees of a Meridian grow larger, the more we go towards the Line, (as Mr. Cassini affirms they do, by an 800ᵗʰ Part in every Degree, in Phil. Trans. Nᵒ. 278.) then there is great reason to conclude in behalf of this Sphæroidal Form.

The natural Cause of this Sphericity of our Globe, is (according to Sir Isaac Newton’s Principles) that Attraction, which the infinite Creator hath stamp’d on all the Matter of the Universe, whereby all Bodies, and all the Parts of Bodies mutually attract themselves and one another. By which means, as all the Parts of Bodies tend naturally to their Center, so they all betake themselves to a globous Figure, unless some other more prevalent Cause interpose. Thus Drops of Quick-silver put on a spherical Form, the Parts thereof strongly attracting one another. So Drops of Water have the same Form, when falling in the Air; but are Hemispherical only when they lie on a hard Body, by reason their Gravity doth so far over-power their self-attracting Power, as to take off one half of their Sphericity. This Figure is commonly attributed to the Pressure of the circumambient Air: But that this can’t be the cause, is manifest from the Air-Pump; the case being the very same in an exhausted Receiver, as in the open Air, and not any the least Alteration of the Figure that I could perceive, in all the Trials I have made.