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

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1it will paſſe any further, or elſe that there it would immediately
ſtand ſtill, and move no further?
SIMP. I believe that it would continue to move a great way
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
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
You ſee it demonſtrated then, that a moveable may be
moved by one and the ſame internal principle, with contrary
The natural
tion changeth it
ſelfe into that
which is called
ternatural and
SIMP. I believe there are ſolutions to all theſe objections,
though for the preſent I do not remember them; but however it
be, the Author continueth to demand, on what principle this
cular motion of grave and light bodies dependeth; that is,
ther on a principle internal, or external; and proceeding
wards, ſheweth, that it can be neither on the one, nor on the other,
ſaying; Si ab externo; Deuſne illum excitat per continuum
an verò Angelus, an aër? Et hunc quidem multi
Sed contra----[In Engliſh thus] If from an externe
ciple; Whether God doth not excite it by a continued Miracle?
or an Angel, or the Air? And indeed many do aſſign this. But
on the contrary-----.
SALV. Trouble not your ſelf to read his argument; for I am

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