IET. VII.

And discoveries.

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US

tion ; then, as soon as it began to move, gravity•would aft upon it, and impel it towards thecenter of the earth. But as a body impelledby two forces will follow the direction of neither,the moon, so circumstanced, would neither pro-ceed directly forwards, nor fall directly down-wards, but keep a middle course, and moveround the earth in a curvilinear orbit.

This idea will be more fully illustrated, byattending to the motion of a soell, or any otherprojectile. A ball soot out of the mouth of acannon, in an horizontal direction, does not fallto the ground till it has proceeded to a con-siderable distance; and if it be dischargedfrom the top of a high mountain, it will fly stillfarther before it comes to the earth. Increasethe force, and the distance will be augmentedaccordingly. And thus, in imagination, at least,we can suppose the ball to be discharged withsuch a velocity, that it will never come to theground, but return again to the place fromwhence it set out; and so proceed on again, andcirculate continually round the earth, in themanner of a little moon.

Newton did not stop here: he began togeneralise the problem, and by means of hismathematics, soon came to this importantconclusion. A body which moves in a curve,tound a fixed point, by virtue of a force directedso that point, describes equal areas in. equalI 2 times.