Others, and among them Dr. Vossius, assert the Cause of the Descent of heavy Bodies, to be the Diurnal Rotation of the Earth upon its Axis, without considering, that according to the Doctrine of Motion fortified with Demonstration, all Bodies moved in Circulo, would recede from the Center of their Motion; whereby the contrary to Gravity would follow, and all loose Bodies would be cast into the Air in a Tangent to the Parallel of Latitude, without the intervention of some other Principle to keep them fast, such as is that of Gravity. Besides, the Effect of this Principle is throughout the whole Surface of the Globe found nearly equal; and certain Experiments have proved it rather less near the Æquinoctial, than towards the Poles; which could not be by any means, if the Diurnal Rotation of the Earth upon its Axis were the Cause of Gravity; for where the Motion was swiftest, the Effect would be most considerable.
Others assign the Pressure of the Atmosphere, to be the Cause of this Tendency towards the Center of the Earth; but unhappily they have mistaken the Cause for the Effect; it being from undoubted Principles plain, that the Atmosphere has no other Pressure but what it derives from its Gravity; and that the Weight of the upper Parts of the Air, pressing on the lower Parts thereof, do so far bend the Springs of that Elastick Body, as to give it a Force equal to the Weight that compress'd it, having of it self no force at all: And supposing it had, it will be very hard to explain the Modus, how that Pressure should occasion the Descent of a Body circumscribed by it, and pressed equally above and below, without some other Force to draw, or thrust it downwards. But to demonstrate the contrary of this Opinion, an Experiment was long since shewn before the Royal Society, whereby it appeared, that the Atmosphere was so far from being the Cause of Gravity, that the Effects thereof were much more vigorous, where the Pressure of the Atmosphere was taken off; for a long Glass-Receiver having a light Down-feather included, being evacuated of Air, the Feather, which in the Air would hardly sink, did in vacuo descend with nearly the same Velocity, as if it had been a Stone.
Some think to illustrate this Descent of Heavy Bodies, by comparing it with the Vertue of the Loadstone; but setting aside the difference there is in the manner of their Attractions, the Loadstone drawing only in and about its Poles, and the Earth near equally in all Parts of its Surface, this Comparison avails no more than to explain ignotum per æque ignotum.
Others assign a certain Sympæthetical Attraction between the Earth and its Parts, whereby they have, as it were, a desire to be united, to be the Cause we enquire after: But this is so far from explaining the Modus, that it is little more, than to tell us in other Terms, that Heavy Bodies descend, because they descend.
This, I say, not that I can pretend to substitute any Solution of this Important Philosophical Problem, that shall more happily explicate the Appearances of Gravity; only it may be serviceable to those with whom the Credit of great Authors sways much, and who too readily assent in Verba Magistri, to let them see that their Books are not always infallible: Besides, the detection of Errors is the first and surest Step towards the discovery of Truth.
Though the efficient Cause of Gravity be so obscure, yet the final Cause thereof is clear enough; for it is by this single Principle, that the Earth and all the Cœlestial Bodies are kept from Dissolution; the least of their Particles not being suffer'd to recede far from their Surfaces, without being immediately brought down again by Virtue of this Natural Tendency; which, for their Preservation, the Infinite Wisdom of their Creator has ordained to be towards each of their Centers; nor can the Globes of the Sun and Planets otherwise be destroy'd, but by taking from them this Power of keeping their Parts united.
The Affections or Properties of Gravity, and its manner of acting upon Bodies falling, have been in a great measure discovered, and most of them made out by Mathematical Demonstration in this our Century, by the accurate diligence of Galilæus, Torricellius, Hugenius, and others, and now lately by our worthy Countryman, Mr. Isaac Newton, which Properties it may be very material here to enumerate, that they may serve for a Foundation to all those that shall be willing to spend their Thoughts in search of the true Cause of this Descent of Bodies.
The first Property is, That by this Principle of Gravitation, all Bodies do descend towards a Point, which either is, or else is very near to the Center of Magnitude of the Earth and Sea, about which the Sea forms it self exactly into a Spherical Surface, and the Prominences of the Land, considering the Bulk of the whole, differ but insensibly therefrom.
Secondly, That this Point or Center of Gravitation, is fix'd within the Earth, or at least has been so, ever since we have any Authentick History: For a Consequence of its Change, though never so little, would be the over-flowing of the low Lands on that side of the Globe towards which it approached, and the leaving new Islands bare on the opposite side, from which it receded; but for this Two Thousand Years it appears, that the low Islands of the Mediterranean Sea (near to which the ancientest Writers liv'd) have continued much at the same height above the Water, as they now are found; and no Inundations or Recesses of the Sea arguing any such Change, are recorded in History; excepting the Universal Deluge, which can no better way be accounted for, than by supposing this Center of Gravitation removed for a time, towards the middle of the then inhabited Parts of the World; and a change of its Place, but the Two Thousandth Part of the Radius of this Globe, were sufficient to bury the Tops of the highest Hills under Water.
Thirdly, That in all Parts of the Surface of the Earth, or rather in all Points equidistant from its Center, the Force of Gravity is nearly equal; so that the length of the Pendulum vibrating Seconds of Time, is found in all Parts of the World to be very near the same. 'Tis true at St. Helena, in the Latitude of 16 Degrees South, I found that the Pendulum of my Clock, which vibrated Seconds, needed to be made shorter than it had been in England, by a very sensible Space (but which at that time I neglected to observe accurately) before it would keep time; and since the like Observations have been made by the French Observers, near the Æquinoctial: Yet I dare not affirm, that in mine it proceeded from any other Cause, than the great Height of my Place of Observation above the Surface of the Sea, whereby the Gravity being diminished, the length of the Pendulum vibrating Seconds, is proportionably short'ned.