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A DISCOVRSE
Presented to the Most Serene DON COSIMO II.
GREATDUKE of TUSC ANY:
CONCERNING
The Natation of BODIES Upon, or Submersion
In, the WATER.

Considering (Most Serene Prince) that the
publishing this present Treatise, of so
different an Argument from that which

many expect, and which according to the
intentions I proposed in my ^{*} Astronomi­
call Adviso, I should before this time
have put forth, might peradventure make
some thinke, either that I had wholly
relinquished my farther imployment
about the new Celestiall Observations,
or that, at least, I handled them very
remissely; I have judged fit to render an account, aswell of my
deferring that, as of my writing, and publishing this treatise.

His Nuncio Sl­
derio.

As to the first, the last discoveries of Saturn to be tricorporeall, and
of the mutations of Figure in Venus, like to those that are seen in the
Moon, together with the Consequents depending thereupon, have
not so much occasioned the demur, as the investigation of the times
of the Conversions of each of the Four Medicean Planets about Ju­
piter, which I lighted upon in April the year past, 1611, at my being in
Rome; where, in the end, I assertained my selfe, that the first and neerest
to Jupiter, moved about 8 gr. & 29 m. of its Sphere in an houre, make­
ing its whole revolution in one naturall day, and 18 hours, and almost
an halfe. The second moves in its Orbe 14 gr. 13 min. or very neer,
in an hour, and its compleat conversion is consummate in 3 dayes, 13
hours, and one third, or thereabouts. The third passeth in an hour,
2 gr. 6 min. little more or less of its Circle, and measures it all in 7
dayes, 4 hours, or very neer. The fourth, and more remote than the
rest, goes in one houre, o gr 54 min. and almost an halfe of its Sphere,
and finisheth it all in 16 dayes, and very neer 18 hours. But be­
cause the excessive velocity of their returns or restitutions, requires a
most scrupulous precisenesse to calculate their places, in times past