Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/32

 is its motion. Having confirmed these motions, he proceeds saying, that of natural bodies some being simple, and some composed of them (and he calleth simple bodies those, that have a principle of motion from nature, as the Fire and Earth) it follows that simple motions belong to simple bodies, and mixt to the compound; yet in such sort, that the compounded incline to the part predominant in the composition.

Pray you hold a little Salviatus, for I find so many doubts to spring up on all sides in this discourse, that I shall be constrained, either to communicate them if I would attentively hearken to what you shall add, or to take off my attention from the things spoken, if I would remember objections.

I will very willingly stay, for that I also run the same hazard, and am ready at every step to lose my self whilst I sail between Rocks, and boisterous Waves, that make me, as they say, to lose my Compass; therefore before I make them more, propound your difficulties.

You and Aristotle together would at first take me a little out of the sensible World, to tell me of the Architecture, wherewith it ought to be fabricated; and very appositly begin to tell me, that a natural body is by nature moveable, nature being (as elsewhere it is defined) the principle of motion. But here I am somewhat doubtfull why Aristotle said not that of natural bodies, some are moveable by nature, and others immoveable, for that in the definition, nature is said to be the principle of Motion, and Rest; for if natural bodies have all a principle of motion, either he might have omitted the mention of Rest, in the definition of nature: or not have introduced such a definition in this place. Next, as to the declaration of what Aristotle intends by simple motions, and how by Spaces he determines them, calling those simple, that are made by simple lines, which are onely the right, and circular, I entertain it willingly; nor do I desire to tenter the instance of the Helix, about the Cylinder; which in that it is in every part like to it self, might seemingly be numbred among simple lines. But herein I cannot concurre, that he should so restrain simple motions (whilst he seems to go about to repeat the same definition in other words) as to call one of them the motion about the medium, the others Sursum & Deorsum, namely upwards and downward; which terms are not to be used, out of the World fabricated, but imply it not onely made, but already inhabited by us; for if the right motion be simple, by the simplicity of the right line, and if the simple motion be natural, it is made on every side, to wit, upwards, downwards, backwards, forwards, to the right, to the left, and if any other way can be imagined, provided it be straight, it shall agree to any simple natural body; or