Galilei, Galileo - Discourse concerning the natation of bodies

Galilei, Galileo, Discourse concerning the natation of bodies, 1663

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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.

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 Princi
ples: the firſt is, that

AXIOME. I.

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

Moment, amongſt Mechanicians, ſigrifieth 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 mat
ters 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, nei
ther one going down, nor the other up: becauſe the equality of the
Diſtances of both, from the Centre on which the Ballance is ſuppor
ted, 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 which

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