Sir I s a a c N e vv t o n’s Book I.

3 2

motion; but continues always in the fame place, till removedby some power applied to it.

9. A g a i n, whenever a body is once in motion, it continuesin that motion some time after the moving power has quittedit, and it is left to it self. Now if the body continue to movebut a single moment, aster the moving power has left it, therecan no reason be assigned, why it should ever stop withoutsome external force. For it is plain, that this continuance ofthe motion is caused only by die body’s having already mov-ed, the sole operation of the power upon the body being dieputting it in motion; therefore that motion continued will e-qually be the cause of its farther motion, and so on withoutend. The only doubt that can remain, is, whether this motioncommunicated continues intire, after the power, that causedit, ceases to act; or whether it does not gradually languish anddecrease. And this suspicion cannot be removed by a transi-ent and flight observation on bodies, but will be fully clearedup by those more accurate proofs of the laws of motion,which are to be considered in the next chapter.

10. Lastly, bodies in motion appear to affect a straightcourse without any deviation, unless when disturbed by someadventitious power acting upon them. If a body be thrownperpendicularly upwards or downwards, it appears to continuein the fame straight line during the whole time of its motion.If a body be thrown in any other direction, it is found to de-viate from the line, in which it began to move, more and

more