Salusbury, Thomas
,
Mathematical collections and translations (Tome I)
,
1667
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principle, ſeeing that if it be not hindered, it moveth
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ouſly: and on the contrary, I know that the principle which
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veth it upwards, is external, although that I do not know, what
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thing that vertue is, impreſſed on it by the projicient.</
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>SALV. </
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>Into how many queſtions muſt we excurre, if we would
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decide all the difficulties, which ſucceſſively have dependance one
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upon another! You call that an external (and you alſo call it a
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preternatural and violent) principle, which moveth the grave
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ject upwards; but its poſſible that it may be no leſſe interne and
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natural, than that which moveth it downwards; it may
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ture be called external and violent, ſo long as the moveable is
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ned to the projicient; but being ſeparated, what external thing
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remaineth for a mover of the arrow, or ball? </
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<
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>In ſumme, it muſt
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neceſſarliy be granted, that that vertue which carrieth ſuch a
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able upwards, is no leſſe interne, than that which moveth it
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wards; and I think the motion of grave bodies aſcending by the
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impetus
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conceived, to be altogether as natural, as the motion of
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deſcent depending on gravity.</
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The vertue which
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carrieth grave
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jects upwards, is
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no leſſe natural to
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them, than the
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gravity which
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veth them
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wards.
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<
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>SIMP. </
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>I will never grant this; for the motion of deſcent hath
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its principle internal, natural, and perpetual, and the motion of
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aſcent hath its principle externe, violent, and finite.</
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<
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>SALV. </
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<
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>If you refuſe to grant me, that the principles of the
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motions of grave bodies downwards and upwards, are equally
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ternal and natural; what would you do, if I ſhould ſay, that they
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may alſo be the ſame in number?</
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Contrary
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ciples cannot
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rally reſide in the
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ſame ſubject.
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<
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>SIMP. </
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>I leave it to you to judge.</
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<
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>SALV. </
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<
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>But I deſire you your ſelf to be the Judge: Therefore
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tell me, Do you believe that in the ſame natural body, there may
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reſide interne principles, that are contrary to one another?</
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<
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>SIMP. </
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<
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>I do verily believe there cannot.</
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<
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>SALV. </
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<
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>What do you think to be the natural inclination of
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Earth, of Lead, of Gold, and in ſum, of the moſt ponderous
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ters; that is, to what motion do you believe that their interne
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principle draweth them?</
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<
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>SIMP. </
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>To that towards the centre of things grave, that is, to
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the centre of the Univerſe, and of the Earth, whither, if they be
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not hindered, it will carry them.</
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<
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>SALV. </
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<
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>So that, if the Terreſtrial Globe were bored thorow,
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and a Well made that ſhould paſſe through the centre of it, a
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Cannon bullet being let fall into the ſame, as being moved by a
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natural and intrinſick principle, would paſſe to the centre; and it
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would make all this motion ſpontaneouſly, and by intrinſick
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ciple, is it not ſo?</
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<
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>SIMP. </
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<
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>So I verily believe.</
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<
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>SALV. </
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<
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>But when it is arrived at the centre, do you think that </
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