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