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

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Of Natation,
Lib. 1. Prop.
7.
Of Natation,
Lib.
1. Prop. 4.
Plato denyeth
Poſitive Levi­
ty.
No cauſe of
the motion of
A cent, ſave the
Impulſe of the
Medium, exceed­
ing the Move­
able in Gravi­
tie.
To that which for a finall concluſion, Signor Buonamico produceth

of going about to reduce the deſcending or not deſcending, to the
eaſie and uneaſie Diviſion of the Medium, and to the predominancy
of the Elements: I anſwer, as to the firſt part, that that cannot in any
manner be admitted as a Cauſe, being that in none of the Fluid
Mediums, as the Air, the Water, and other Liquids, there is any

Reſiſtance againſt Diviſion, but all by every the leaſt Force, are di­
vided and penetrated, as I will anon demonſtrate: ſo, that of ſuch
Reſiſtance of Diviſion there can be no Act, ſince it ſelf is not in be­
ing.
As to the other part, I ſay, that the predominancy of the Ele­

ments in Moveables, is to be conſidered, as far as to the exceſſe or
defectus of Gravity, in relation to the Medium: for in that Action,
the Elements operate not, but only, ſo far as they are grave or light:
therefore, to ſay that the Wood of the Firre ſinks not, becauſe Air
predominateth in it, is no more than to ſay, becauſe it is leſs grave
than the Water.
Yea, even the immediate Cauſe, is its being leſs
grave than the Water: and it being under the predominancy of the

Air, is the Cauſe of its leſs Gravity: Therefore, he that alledgeth the
predominancy of the Element for a Cauſe, brings the Cauſe of the
Cauſe, and not the neereſt and immediate Cauſe.
Now, who knows
not that the true Cauſe is the immediate, and not the mediate?

Moreover, he that alledgeth Gravity, brings a Cauſe moſt perſpicuous
to Sence: The cauſe we may very eaſily aſſertain our ſelves;
whether Ebony, for example, and Firre, be more or leſs grave than
water: but whether Earth or Air predominates in them, who ſhall

make that manifeſt?
Certainly, no Experiment can better do it
than to obſerve whether they ſwim or ſink.
So, that he who knows,
not whether ſuch a Solid ſwims, unleſs when he knows that Air pre­
dominates in it, knows not whether it ſwim, unleſs he ſees it ſwim,
for then he knows that it ſwims, when he knows that it is Air that
predominates, but knows not that Air hath the predominance, unleſs
he ſees it ſwim: therefore, he knows not if it ſwims, till ſuch time
as he hath ſeen it ſwim.

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