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

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1offer upon ſome other day: but I would not have Sagredus
fended at this digreſſion.
SAGR. I am rather very much pleaſed with it, for that I
member that when I ſtudied Logick, I could never comprehend that
ſo much cry'd up and moſt potent demonſtration of Ariſtotle.
SALV. Let us go on therefore; and let Simplicius, tell me
what that motion is which the ſtone maketh that is held faſt in the
ſlit of the ſling, when the boy ſwings it about to throw it a great
way?
SIMP. The motion of the ſtone, ſo long as it is in the ſlit, is
circular, that is, moveth by the arch of a circle, whoſe ſtedfaſt
centre is the knitting of the ſhoulder, and its ſemi-diameter the arm
and ſtick.
SALV. And when the ſtone leaveth the ſling, what is its
tion?
Doth it continue to follow its former circle, or doth it go
by another line?
SIMP. It will continue no longer to ſwing round, for then it
would not go farther from the arm of the projicient, whereas
we ſee it go a great way off.
SALV. With what motion doth it move then?
SIMP. Give me a little time to think thereof; For I have
ver conſidered it before.
SALV. Hark hither, Sagredus; this is the Quoddam reminiſci
in a ſubject well underſtood.
You have pauſed a great while,
Simplicius.
SIMP. As far as I can ſee, the motion received in going out of
the ſling, can be no other than by a right line; nay, it muſt
ceſſarily be ſo, if we ſpeak of the pure adventitious impetus. I
was a little puzled to ſee it make an arch, but becauſe that arch
bended all the way upwards, and no other way, I conceive that

that incurvation cometh from the gravity of the ſtone, which
turally draweth it downwards.
The impreſſed impetus, I ſay,
without reſpecting the natural, is by a right line.
The motion
preſſed by the
jicient is onely by a
right line.
SALV. But by what right line? Becauſe infinite, and towards
every ſide may be produced from the ſlit of the ſling, and from the
point of the ſtones ſeparation from the ſling.
SIMP. It moveth by that line which goeth directly from the
motion which the ſtone made in the ſling.
SALV. The motion of the ſtone whilſt it was in the ſlit, you
have affirmed already to be circular; now circularity oppoſeth
directneſs, there not being in the circular line any part that is
rect or ſtreight.
SIMP I mean not that the projected motion is direct in
ſpect of the whole circle, but in reference to that ultimate point,
where the circular motion determineth.
I know what I would