Page:The Mathematical Principles of Natural Philosophy - 1729 - Volume 1.djvu/48

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Of this ort is gravity, by which bodies tend to the centre of the Earth; Magnetim, by which iron tends to the loadtone; and that force, whatever it is, by which the planets are continually drawn aide from the rectilinear motions, which otherwie they would purue, and made to revolve in curvilinear orbits. A tone, whirled about in a ling, endeavours to recede from the hand that turns it; and by that endeavour, ditends the sling, and that with o much the greater force, as it is revolv'd with the greater velocity, and as oon as ever it is let go, flies away. That force which oppoes itelf to this endeavour, and by which the ling perpetually draws back the tone towards the hand, and retains it in its orbit, becaue 'tis directed to the hand as the centre of the orbit, I call the Centripetal force. And the ame thing is to be undertood 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 retrains 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 reitance of the air was taken away. 'Tis by its gravity, that it is drawn aide perpetually from its rectilinear coure, and made to deviate towards the Earth, more or les, Rh