Page:Discourse Concerning the Natation of Bodies.djvu/8

And so I say, that Lead is more grave in specie than Tinn, because if you take of them two equall Masses, that of the Lead weigheth more.

Definition IV.

But I call that Body more grave absolutely than this, if that Weigh more than this without any respect had to the Masses.

And thus a great piece of Wood is said to weigh more than a little lump of Lead, though the Lead be in specie more heavy than the Wood. And the same is to be understood of the less grave in specie, and the less grave absolutely.

These Termes defined, I take from the Mechanicks two Principles: the first is, that

Axiome. I.

Weights absolutely equall, moved with equall Velocity, are of equall Force and Moment in their operations.

Definition V.

Moment, amongst Mechanicians, signifieth that Vertue, that Force, or that Efficacy, with which the Mover moves, and the Moveable resists.

Which Vertue dependes not only on the simple Gravity, but on the Velocity of the Motion, and on the diverse Inclinations of the Spaces along which the Motion is made: For a descending Weight makes a greater Impetus in a Space much declining, than in one less declining; and in summe, what ever is the occasion of such Vertue, it ever retaines the name of Moment; nor in my Judgement, is this sence new in our Idiome, for, if I mistake not, I think we often say; This is a weighty businesse, but the other is of small moment: and we consider lighter matters and let pass those of Moment; a Metaphor, I suppose, taken from the Mechanicks.

As for example, two weights equall in absolute Gravity, being put into a Ballance of equall Arms, they stand in Equilibrium, neither one going down, nor the other up: because the equality of the Distances of both, from the Centre on which the Ballance is supported, and about which it moves, causeth that those weights, the said Ballance moving, shall in the same Time move equall Spaces, that is, shall move with equall Velocity, so that there is no reason for whichthis