Salusbury, Thomas, Mathematical collections and translations (Tome I), 1667

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SALV. But this motion beyond the centre, would it not be
wards
, and according to your aſſertion preternatural, and violent?
And yet on what other principle do you make it to depend, but
only
upon the ſelf ſame, which did carry the ball to the centre,
and
which you called intrinſecal, and natural?
Finde, if you can,
another
external projicient, that overtaketh it again to drive it
upwards
.
And this that hath been ſaid of the motion thorow
the
centre, is alſo ſeen by us here above; for the interne impetus

of
a grave body falling along a declining ſuperficies, if the ſaid
ſuperficies
be reflected the other way, it ſhall carry it, without a
jot
interrupting the motion, alſo upwards.
A ball of lead that
hangeth
by a thread, being removed from its perpendicularity,
ſcendeth
ſpontaneouſly, as being drawn by its internal inclination,
and
without any interpoſure of reſt, paſſeth beyond the loweſt
point
of perpendicularity: and without any additional mover,
moveth
upwards.
I know that you will not deny, but that the
principle
of grave bodies that moveth them downwards, is no leſs
natural
, and intrinſecal, than that principle of light bodies, which
moveth
them upwards: ſo that I propoſe to your conſideration a
ball
of lead, which deſcending through the Air from a great
titude
, and ſo moving by an intern principle, and comming to a
depth
of water, continueth its deſcent, and without any other
terne
mover, ſubmergeth a great way; and yet the motion of
deſcent
in the water is preternatural unto it; but yet nevertheleſs
dependeth
on a principle that is internal, and not external to the
ball
.
You ſee it demonſtrated then, that a moveable may be
moved
by one and the ſame internal principle, with contrary
tions
.
The natural
tion
changeth it
ſelfe
into that
which
is called
ternatural
and
olent
.

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