Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/236

 principle, seeing that if it be not hindered, it moveth spontaneously: and on the contrary, I know that the principle which moveth it upwards, is external; although that I do not know, what thing that vertue is, impressed on it by the projicient.

Into how many questions must we excurre, if we would decide all the difficulties, which successively have dependance one upon another! You call that an external (and you also call it a preternatural and violent) principle, which moveth the grave project upwards; but its possible that it may be no lesse interne and natural, than that which moveth it downwards; it may peradventure be called external and violent, so long as the moveable is joyned to the projicient; but being separated, what external thing remaineth for a mover of the arrow, or ball? In summe, it must necessarliy be granted, that that vertue which carrieth such a moveable upwards, is no lesse interne, than that which moveth it downwards; and I think the motion of grave bodies ascending by the impetus conceived, to be altogether as natural, as the motion of descent depending on gravity.

I will never grant this; for the motion of descent hath its principle internal, natural, and perpetual, and the motion of ascent hath its principle externe, violent, and finite.

If you refuse to grant me, that the principles of the motions of grave bodies downwards and upwards, are equally internal and natural; what would you do, if I should say, that they may also be the same in number?

I leave it to you to judge.

But I desire you your self to be the Judge: Therefore tell me, Do you believe that in the same natural body, there may reside interne principles, that are contrary to one another?

I do verily believe there cannot.

What do you think to be the natural inclination of Earth, of Lead, of Gold, and in sum, of the most ponderous matters; that is, to what motion do you believe that their interne principle draweth them?

To that towards the centre of things grave, that is, to the centre of the Universe, and of the Earth, whither, if they be not hindered, it will carry them.

So that, if the Terrestrial Globe were bored thorow, and a Well made that should passe through the centre of it, a Cannon bullet being let fall into the same, as being moved by a natural and intrinsick principle, would passe to the centre; and it would make all this motion spontaneously, and by intrinsick principle, is it not so?

So I verily believe.

But when it is arrived at the centre, do you think that