Simpl.So it is, in my opinion.
Sagr.Now imagine the cylinder to be erected, and that the Earth doth revolve about with a diurnal motion, carrying the piece along with it, tell me what shall be the motion of the ball within the cylinder, having given fire?
Simpl.It shall be a streight and perpendicular motion, the cylinder being erected perpendicularly.
Sagr.Consider well what you say: for I believe that it will not be perpendicular. It would indeed be perpendicular, if the Earth stood still, for so the ball would have no other motion but that proceeding from the fire.The revolution of the Earth supposed, the ball in the piece erected perpendicularly, doth not move by a perpendicular, but an inclined line. But in case the Earth turns round, the ball that is in the piece, hath likewise a diurnal motion, so that there being added to the same the impulse of the fire, it moveth from the breech of the piece to the muzzle with two motions, from the composition whereof it cometh to passe that the motion made by the centre of the balls gravity is an inclining line. And for your clearer understanding the same, let the piece AC [in Fig. 2.] be erected, and in it the ball B; it is manifest, that the piece standing immoveable, and fire being given to it, the ball will make its way out by the mouth A, and with its centre, passing thorow the the piece, shall have described the perpendicular line BA, and it shall pursue that rectitude when it is out of the piece, moving toward the Zenith. But in case the Earth should move round, and consequently carry the piece along with it, in the time that the ball driven out of the piece shall move along the cylinder, the piece being carried by the Earth, shall passe into the situation DE, and the ball B, in going off, would be at the cornish D, and the motion of the bals centre, would have been according to the line BD, no longer perpendicular, but inclining towards the East; and the ball (as hath been concluded) being to continue its motion through the air, according to the direction of the motion made in the piece, the said motion shall continue on according to the inclination of the line BD, and so shall no longer be perpendicular, but inclined towards the East, to which part the piece doth also move; whereupon the ball may follow the motion of the Earth, and of the piece. Now Simplicius, you see it demonstrated, that the Range which you took to be perpendicular, is not so.
Simpl.I do not very well understand this business; do you, Salviatus?
Salv.I apprehend it in part; but I have a certain kind of scruple, which I wish I knew how to express. It seems to me, that according to what hath been said, if the Piece be erected perpendicular, and the Earth do move, the ball would not be to fall, as Aristotle and Tycho will have it, far from the Piece towards theWest,
