Page:The Mathematical Principles of Natural Philosophy - 1729 - Volume 1.djvu/23

Rh afely when we come to conider it in the heavenly bodies, that lie at o vat a ditance from us. It is now agreed by all philoophers that all circumterretrial bodies gravitate towards the Earth. That no bodies really light are to be found, is now confirmed by manifold experience. That which is relative levity, is not true levity, but apparent only; and aries from the preponderating gravity of the contiguous bodies.

Moreover, as all bodies gravitate towards the Earth, o does the Earth again towards bodies. That the action of gravity is mutual, and equal on both ides, is thus proved. Let the mas of the Earth be ditinguihed into any two parts whatever, either equal, or any how unequal; now if the weights of the parts towards each other were not mutually equal, the leer weight would give way to the greater, and the two parts joined together would move on ad infinitum in a right line towards that part to which the greater weight tends; altogether againt experience. Therefore we mut ay that the weights of the parts are contituted in equilibrio; that is, that the action of gravity is mutual and equal on both ides.

The weights of bodies, at equal ditances from the centre of the Earth, are as the quantities of matter in the bodies. This is collected from the equal acceleration of all bodies that fall from a tate of ret by the force of their weights; for the forces by which unequal bodies are equally accelerated mut be proportional to the quantities of the matter to be moved. Now that all bodies are in falling equally accelerated appears from hence, that when the reitance of the air is taken away, as it is under an exhauted receiver, bodies falling Rh