Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/152

 hard, as suppose of brass; what think you it would do being let go? do not you believe (as for my part I do) that it would lie still?

If that superficies were inclining?

Yes; for so I have already supposed.

I cannot conceive how it should lie still: nay, I am confident that it would move towards the declivity with much propensness.

Take good heed what you say, Simplicius, for I am confident that it would lie still in what ever place you should lay it.

So long as you make use of such suppositions, Salvictus, I shall cease to wonder if you inferr most absurd conclusions.

Are you assured, then, that it would freely move towards the declivity?

Who doubts it?

And this you verily believe, not because I told you so, (for I endeavoured to perswade you to think the contrary) but of your self, and upon your natural judgment.

Now I see what you would be at; you spoke not this as really believing the same; but to try me, and to wrest matter out of my own mouth wherewith to condemn me.

You are in the right. And how long would that Ball move, and with what velocity? But take notice that I instanced in a Ball exactly round, and a plain exquisitely polished, that all external and accidental impediments might be taken away. And so would I have you remove all obstructions caused by the Airs resistance to division, and all other casual obstacles, if any other there can be.

I very well understand your meaning, and as to your demand, I answer, that the Ball would continue to move in infinitum, if the inclination of the plain should so long last, and continually with an accelerating motion; for such is the nature of ponderous moveables, that vires acquirant eundo: and the greater the declivity was, the greater the velocity would be.

But if one should require that that Ball should move upwards on that same superficies, do you believe that it would so do?

Not spontaneously; but being drawn, or violently thrown, it may.

And in case it were thrust forward by the impression of some violent impetus from without, what and how great would its motion be?

The motion would go continually decreasing and re-