Definition V:
Of this ſort is gravity, by which bodies tend to the centre of the Earth; Magnetiſm, by which iron tends to the loadſtone; and that force, whatever it is, by which the planets are continually drawn aſide from the rectilinear motions, which otherwiſe they would purſue, and made to revolve in curvilinear orbits. A ſtone, whirled about in a ſling, endeavours to recede from the hand that turns it; and by that endeavour, diſtends the sling, and that with ſo much the greater force, as it is revolv'd with the greater velocity, and as ſoon as ever it is let go, flies away. That force which oppoſes itſelf to this endeavour, and by which the ſling perpetually draws back the ſtone towards the hand, and retains it in its orbit, becauſe 'tis directed to the hand as the centre of the orbit, I call the Centripetal force. And the ſame thing is to be underſtood of all bodies, revolv'd in any orbits. They all endeavour to recede from the centres of their orbits; and were it not for the opposition of a contrary force which reſtrains them to, and detains them in their orbits, which I therefore call Centripetal, would fly off in right lines, with an uniform motion. A projectile, if it was not for the force of gravity, would not deviate towards the Earth, but would go off from it in a right line, and that with an uniform motion, if the reſiſtance of the air was taken away. 'Tis by its gravity, that it is drawn aſide perpetually from its rectilinear courſe, and made to deviate towards the Earth, more or leſs, accord-
