Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/235

 not move, and so he shall not be bound to shew how that in descending it continueth all the way vertically over one point, for that it will not do any such thing.

Very well; But if grave bodies, and light can have no principle, either internal or external of moving circularly, than neither can the terrestrial Globe move with a circular motion: and thus you have the intent of the Author.

I did not say, that the Earth had no principle, either interne, or externe to the motion of gyration, but I say, that I do not know which of the two it hath; and yet my not knowing it hath not a power to deprive it of the same; but if this Author can tell by what principle other mundane bodies are moved round, of whose motion there is no doubt; I say, that that which maketh the Earth to move, is a vertue, like to that, by which Mars and Jupiter are moved, and wherewith he believes that the starry Sphere it self also doth move; and if he will but assure me, who is the mover of one of these moveables, I will undertake to be able to tell him who maketh the Earth to move. Nay more; I will undertake to do the same, if he can but tell me, who moveth the parts of the Earth downwards.

The cause of this is most manifest, and every one knows that it is gravity.

You are out, Simplicius, you should say, that every one knowes, that it is called Gravity: but I do not question you about the name, but the essence of the thing, of which essence you know not a tittle more than you know the essence of the mover of the stars in gyration; unlesse it be the name that hath been put to this, and made familiar, and domestical, by the many experiences which we see thereof every hour in the day,: but not as if we really understand any more, what principle or vertue that is which moveth a stone downwards, than we know who moveth it upwards, when it is separated from the projicient, or who moveth the Moon round, except (as I have said) onely the name, which more particularly and properly we have assigned to the motion of descent, namely, Gravity; whereas for the cause of circular motion, in more general termes, we assign the Vertue impressed, and call the same an Intelligence, either assisting, or informing; and to infinite other motions we ascribe Nature for their cause.

It is my opinion, that this Author asketh far lesse than that, to which you deny to make answer; for he doth not ask what is nominally and particularly the principle that moveth grave and light bodies circularly, but whatsoever it be, he desireth to know, whether you think it intrinsecal, or extrinsecal: For howbeit, v.gr. I do not know, what kind of thing that gravity is, by which the Earth descendeth; yet I know that it is an intern