Chap. i. PHILOSOPHY.

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the line A D is always to be determined after this manner.Let AE be the space, which the body would have movedthrough in the line A B during any certain portion of time;provided that body, when at A, had received no second im-pulse. Suppose likewise, that AF is the part of the line AC,through which the body would have moved during an equalportion of time, if it had been at rest in A, when it receivedthe impulse in the direction A C: then if from E be drawna line parallel to, or equidistant from A C, and from F an-other line parallel to AB, those two lines will meet in theline A D.

6 . The third and last of these laws of motion is, thatwhen any body acts upon another, the action of that bodyupon the other is equalled by the contrary reaction of thatother body upon the first.

7. These laws of motion are abundantly confirmed bythis, that all the deductions made from them, in relation tothe motion of bodies, how complicated soever, are sound toagree perfectly with observation. This shall be shewn moreat large in the next chapter. But before we proceed to sodiffusive a proof; I chuse here to point out those appearan-ces of bodies, whereby the laws of motion are first suggestedto us.

8. Daily observation makes it appear to us, that anybody, which we once fee at rest, never puts it self into fresh

motion ;