Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/195

 offer upon some other day: but I would not have Sagredus offended at this digression.

I am rather very much pleased with it, for that I remember that when I studied Logick, I could never comprehend that so much cry'd up and most potent demonstration of Aristotle.

Let us go on therefore; and let Simplicius, tell me what that motion is which the stone maketh that is held fast in the slit of the sling, when the boy swings it about to throw it a great way?

The motion of the stone, so long as it is in the slit, is circular, that is, moveth by the arch of a circle, whose stedfast centre is the knitting of the shoulder, and its semi-diameter the arm and stick.

And when the stone leaveth the sling, what is its motion? Doth it continue to follow its former circle, or doth it go by another line?

It will continue no longer to swing round, for then it would not go farther from the arm of the projicient, whereas we see it go a great way off.

With what motion doth it move then?

Give me a little time to think thereof; For I have never considered it before.

Hark hither, Sagredus; this is the Quoddam reminisci in a subject well understood. You have paused a great while, Simplicius.

As far as I can see, the motion received in going out of the sling, can be no other than by a right line; nay, it must necessarily be so, if we speak of the pure adventitious impetus. I was a little puzled to see it make an arch, but because that arch bended all the way upwards, and no other way, I conceive that that incurvation cometh from the gravity of the stone, which naturally draweth it downwards. The impressed impetus, I say, without respecting the natural, is by a right line.

But by what right line? Because infinite, and towards every side may be produced from the slit of the sling, and from the point of the stones separation from the sling.

It moveth by that line which goeth directly from the motion which the stone made in the sling.

The motion of the stone whilst it was in the slit, you have affirmed already to be circular; now circularity opposeth directness, there not being in the circular line any part that is direct or streight.

I mean not that the projected motion is direct in respect of the whole circle, but in reference to that ultimate point, where the circular motion determineth. I know what I would