Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/249

 and you had no other object visible and stable, wherewith to make comparisons to that, you could not perceive its motion?

No, nor the stone it self; for if I would see it, when it is at the highest, I must raise up my head, and as it descendeth, I must hold it lower and lower, and in a word, must continually move either that, or my eyes, following the motion of the said stone.

You have now rightly answered: you know then that the stone lyeth still, when without moving your eye, you alwayes see it before you; and you know that it moveth, when for the keeping it in sight, you must move the organ of sight, the eye. So then when ever without moving your eye, you continually behold an object in the self same aspect, you do always judge it immoveable.

I think it must needs be so.

Now fancy your self to be in a ship, and to have fixed your eye on the point of the Sail-yard: Do you think, that because the ship moveth very fast, you must move your eye, to keep your sight alwayes upon the point of the Sail-yard, and to follow its motion?

I am certain, that I should need to make no change at all; and that not only in the sight; but if I had aimed a Musket at it, I should never have need, let the ship move how it will, to stir it an hairs breadth to keep it full upon the same.

And this happens because the motion, which the Ship conferreth on the Sail-yard, it conferreth also upon you, and upon your eye; so that you need not stir it a jot to behold the top of the Sail-yard: and consequently, it will seem to you immoveaableimmoveable [sic]. Now this Discourse being applied to the revolution of the Earth, and to the stone placed in the top of the Tower, in which you cannot discern any motion, because that you have that motion which is necessary for the following of it, in common with it from the Earth; so that you need not move your eye. When again there is conferred upon it the motion of descent, which is its particular motion, and not yours, and that it is intermixed with the circular, that part of the circular which is common to the stone, and to the eye, continueth to be imperceptible, and the right onely is perceived, for that to the perception of it, you must follow it with your eye, looking lower and lower. I wish for the undeceiving of this Philosopher, that I could advise him, that some time or other going by water, he would carry along with him a Vessel of reasonable depth full of water, and prepare a ball of wax, or other matter that would descend very slowly to the bottome, so that in a minute of an hour, it would scarce sink a yard; and that rowing the boat as fast as could be, so that in a minute of an hour