Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/165

 The top will run reeling along the floor towards that part whither its upper parts encline it.

And why not whither the contrary parts tend, namely, those which touch the ground?

Because those upon the ground happen to be impeded by the roughness of the touch, that is, by the floors unevenness; but the superiour, which are in the tenuous and flexible air, are hindred very little, if at all; and therefore the top will obey their inclination.

So that that taction, if I may so say, of the neither parts on the floor, is the cause that they stay, and onely the upper parts spring the top forward.

And therefore, if the top should fall upon the ice, or other very smooth superficies, it would not so well run forward, but might peradventure continue to revolve in it self, (or sleep) without acquiring any progressive motion.

It is an easie thing for it so to do; but yet nevertheless, it would not so speedily come to sleep, as when it falleth on a superficies somewhat rugged. But tell me, Simplicius, when the top-turning round about it self, in that manner, is let fall, why doth it not move forwards in the air, as it doth afterwards when it is upon the ground?

Because having air above it, and beneath, neither those parts, nor these have any where to touch, and not having more occasion to go forward than backward, it falls perpendicularly.

So then the onely reeling about its self, without other impetus, can drive the top forward, being arrived at the ground, very nimbly. Now proceed we to what remains. That lash, which the driver tyeth to his Top-stick, and with which, winding it about the top, he sets it (i. e. makes it go) what effect hath it on the said top?

It constrains it to turn round upon its toe, that so it may free it self from the Top-lash.

So then, when the top arriveth at the ground, it cometh all the way turning about its self, by means of the lash. Hath it not reason then to move in it self more swiftly upon the ground, than it did whilst it was in the air?

Yes doubtless; for in the air it had no other impulse than that of the arm of the projicient; and if it had also the reeling, this (as hath been said) in the air drives it not forward at all▪ but arriving at the floor, to the motion of the arm is added the progression of the reeling, whereby the velocity is redoubled. And I know already very well, that the top skipping from the ground, its velocity will deminish, because the help of its circulation is wanting; and returning to the Earth will get it again, and by that