Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/179

 from the piece; and the departing from the state of rest, cannot be, unlesse the immobility of the Terrestrial Globe be presupposed, which is the conclusion of that was in dispute; Therefore, I reply, that those who make the Earth moveable, answer, that the piece, and the ball that is in it, partake of the same motion with the Earth; nay that they have this together with her from nature; and that therefore the ball departs in no other manner from its quiescence, but conjoyned with its motion about the centre, the which by its projection upwards, is neither taken away, nor hindered; and in this manner following, the universal motion of the Earth towards the East, it alwayes keepeth perpendicular over the said piece, as well in its rise as in its return. And the same you see to ensue, in making the experiment in a ship with a bullet shot upwards perpendicularly with a Crosse-bow, which returneth to the same place whether the ship doth move, or stand still.

This satisfieth very well to all; but because that I have seen that Simplicius taketh pleasure with certain subtilties to puzzle his companions, I will demand of him whether, supposing for this time that the Earth standeth still, and the piece erected upon it perpendicularly, directed to our Zenith, he do at all question that to be the true perpendicular shot, and that the ball in departing, and in its return is to go by the same right line, still supposing all external and accidental impediments to be removed?

I understand that the matter ought to succeed exactly in that manner.

But if the piece were placed, not perpendicularly, but inclining towards some place, what would the motion of the ball be? Would it go haply, as in the other shot, by the perpendicular line, and return again by the same?

It would not so do; but issuing out of the piece, it would pursue its motion by a right line which prolongeth the erect perpendicularity of the concave cylinder of the piece, unlesse so far as its own weight would make it decline from that erection towards the Earth.

So that the mounture of the cylinder is the regulator of the motion of the ball, nor doth it, or would it move out of that line, if its own gravity did not make it decline downwards. And therefore placing the cylinder perpendicularly, and shooting the ball upwards, it returneth by the same right line downwards; because the motion of the ball dependent on its gravity is downward, by the same perpendicular. The journey therefore of the ball out of the piece, continueth or prolongeth the rectitude or perpendicularity of that small part of the said journey, which it made within the said piece; is it not so?