Galilei, Galileo - Discourse concerning the natation of bodies

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The Authors
anſwer to the
fourth Object
ion.

Of Natation,
Lib. 1. Prop. 7.

Of Natation,
Lib. 1. Prop. 4.

Plato denyeth
Poſitive Levi
ty.

The Authors
defence of the
doctrine of Plato
and the Ancients,
who abſolutely
deny Levity:

According to
Plato there is no
Principle of the
Motion of de
ſcent in Naturall
Bodies, ſave that
to the Centre.

No cauſe of
the motion of
A cent, ſave the
Impulſe of the
Medium, exceed
ing the Move
able in Gravi
tie.

Bodies aſcend
much ſwifter in
the Water, than
in the Air.

All Bodies aſ
cending through
Water, loſe
their Motion,
comming to the
confines of the
Air.

And, howbeit, Experience ſhewes, that the Bodies, ſucceſſively

leſs grave, do moſt expeditiouſly aſcend in water, it cannot be doubt
ed, but that the Ignean Exhalations do aſcend more ſwiftly

through the water, than doth the Air: which Air is ſeen by Experi
ence to aſcend more ſwiftly through the Water, than the Fiery Exha
lations through the Air: Therefore, we muſt of neceſſity conclude,
that the ſaid Exhalations do much more expeditiouſly aſcend through
the Water, than through the Air; and that, conſequently, they are
moved by the Impulſe of the Ambient Medium, and not by an intrin
ſick Principle that is in them, of avoiding the Centre of the Earth;
to which other grave Bodies tend.

The lighter
Bodies alſend
more ſwiftly
through Water.

Fiery Exhalati
ons ascend tho
row the Water
more ſwiftly
than doth the
Air; & the Air
aſcends more
ſwiftly thorow
the Water, than
F ire thorow the
Air.

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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