Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/37

 Now this acceleration of motion is never made, but when the moveable in moving acquireth it; nor is its acquist other than an approaching to the place desired, to wit, whither its natural inclination attracts it, and thither it tendeth by the shortest way; namely, by a right line. We may upon good grounds therefore say, That Nature, to confer upon a moveable first constituted in rest a determinate velocity, useth to make it move according to a certain time and space with a right motion. This presupposed, let us imagine God to have created the Orb v. g. of Jupiter, on which he had determined to confer such a certain velocity, which it ought afterwards to retain perpetually uniform; we may with Plato say, that he gave it at the beginning a right and accelerate motion, and that it afterwards being arrived to that intended degree of velocity, he converted its right, into a circular motion, the velocity of which came afterwards naturally to be uniform.

I hearken to this Discourse with great delight; and I believe the content I take therein will be greater, when you have satisfied me in a doubt: that is, (which I do not very well comprehend) how it of necessity ensues, that a moveable departing from its rest, and entring into a motion to which it had a natural inclination, it passeth thorow all the precedent degrees of tardity, comprehended between any assigned degree of velocity, and the state of rest, which degrees are infinite? so that Nature was not able to confer them upon the body of Jupiter, his circular motion being instantly created with such and such velocity.

I neither did, nor dare say, that it was impossible for God or Nature to confer that velocity which you speak of, immediately; but this I say, that de facto she did not do it; so that the doing it would be a work extra-natural, and by consequence miraculous

Then you believe, that a stone leaving its rest, and entring into its natural motion towards the centre of the Earth, passeth thorow all the degrees of tardity inferiour to any degree of velocity?

I do believe it, nay am certain of it; and so certain, that I am able to make you also very well satisfied with the truth thereof.

Though by all this daies discourse I should gain no more but such a knowledge, I should think my time very well bestowed.

By what I collect from our discourse, a great part of your scruple lieth in that it should in a time, and that very short, pass thorow those infinite degrees of tardity precedent to any velocity, acquired by the moveable in that time: and therefore before we go any farther, I will seek to remove this difficulty, which