Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/166

 means move again faster, than in the air. It onely rests for me to understand, whether in this second motion on the Earth it move more swiftly, than in the first; for then it would move in infinitum, alwayes accelerating.

I did not absolutely affirm, that this second motion is more swift than the first; but that it may happen so to be sometimes.

This is that, which I apprehend not, and which I desire to know.

And this also you know of your self. Therefore tell me: When you let the top fall out of your hand, without making it turn round (i. e. setting it) what will it do at its coming to the ground?

Nothing, but there lie still.

May it not chance, that in its fall to the ground it may acquire a motion? Think better on it.

Unlesse we let it fall upon some inclining stone, as children do playing at * Chiosa, and that falling side-wayes upon the same, it do acquire the motion of turning round upon its toe, wherewith it afterwards continueth to move progressively on the floor, I know not in what other manner it can do any thing but lie still where it falleth.

You see then that in some case it may acquire a new revolution. When then the top jerked up from the ground, falleth down again, why may it not casually hit upon the declivity of some stone fixed in the floor, and that hath an inclination that way towards which it moveth, and acquiring by that slip a new whirle over and above that conferred by the lash, why may it not redouble its motion, and make it swifter than it was at its first lighting upon the ground?

Now I see that the same may easily happen. And I am thinking that if the top should turn the contrary way, in arriving at the ground, it would work a contrary effect, that is, the motion of the accidental whirl would retard that of the projicient.

And it would sometimes wholly retard and stop it, in case the revolution of the top were very swift. And from hence ariseth the resolution of that slight, which the more skilful Tennis Players use to their advantage; that is, to gull their adversary by cutting (for so is their Phrase) the Ball; which is, to return it with a side Rachet, in such a manner, that it doth thereby acquire a motion by it self contrary to the projected motion, and so by that means, at its coming to the ground, the rebound, which if the ball did not turn in that manner, would be towards the adversary, giving him the usual time to tosse it back again, doth