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

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1principle, ſeeing that if it be not hindered, it moveth
ouſly: and on the contrary, I know that the principle which
veth it upwards, is external, although that I do not know, what
thing that vertue is, impreſſed on it by the projicient.
SALV. Into how many queſtions muſt we excurre, if we would
decide all the difficulties, which ſucceſſively have dependance one
upon another! You call that an external (and you alſo call it a
preternatural and violent) principle, which moveth the grave
ject upwards; but its poſſible that it may be no leſſe interne and
natural, than that which moveth it downwards; it may

ture be called external and violent, ſo long as the moveable is
ned to the projicient; but being ſeparated, what external thing
remaineth for a mover of the arrow, or ball?
In ſumme, it muſt
neceſſarliy be granted, that that vertue which carrieth ſuch a
able upwards, is no leſſe interne, than that which moveth it
wards; and I think the motion of grave bodies aſcending by the
impetus conceived, to be altogether as natural, as the motion of
deſcent depending on gravity.
The vertue which
carrieth grave
jects upwards, is
no leſſe natural to
them, than the
gravity which
veth them
wards.
SIMP. I will never grant this; for the motion of deſcent hath
its principle internal, natural, and perpetual, and the motion of
aſcent hath its principle externe, violent, and finite.
SALV. If you refuſe to grant me, that the principles of the
motions of grave bodies downwards and upwards, are equally

ternal and natural; what would you do, if I ſhould ſay, that they
may alſo be the ſame in number?
Contrary
ciples cannot
rally reſide in the
ſame ſubject.
SIMP. I leave it to you to judge.
SALV. But I deſire you your ſelf to be the Judge: Therefore
tell me, Do you believe that in the ſame natural body, there may
reſide interne principles, that are contrary to one another?
SIMP. I do verily believe there cannot.
SALV. What do you think to be the natural inclination of
Earth, of Lead, of Gold, and in ſum, of the moſt ponderous
ters; that is, to what motion do you believe that their interne
principle draweth them?
SIMP. To that towards the centre of things grave, that is, to
the centre of the Univerſe, and of the Earth, whither, if they be
not hindered, it will carry them.
SALV. So that, if the Terreſtrial Globe were bored thorow,
and a Well made that ſhould paſſe through the centre of it, a
Cannon bullet being let fall into the ſame, as being moved by a
natural and intrinſick principle, would paſſe to the centre; and it
would make all this motion ſpontaneouſly, and by intrinſick
ciple, is it not ſo?
SIMP. So I verily believe.
SALV. But when it is arrived at the centre, do you think that