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

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1roof, if we remove it far from perpendicularity, and then let it go,
have you not obſerved that, it declining, will paſs freely, and well
near as far to the other ſide of the perpendicular?
SAGR. I have obſerved it very well, and find (eſpecially if the
plummet be of any conſiderable weight) that it riſeth ſo little leſs
than it deſcended, ſo that I have ſometimes thought, that the
ſcending arch is equal to that deſcending, and thereupon made it
a queſtion whether the vibrations might not perpetuate themſelves;
and I believe that they might, if that it were poſſible to remove

the impediment of the Air, which reſiſting penetration, doth ſome
ſmall matter retard and impede the motion of the pendulum,
though indeed that impediment is but ſmall: in favour of which
opinion the great number of vibrations that are made before the
moveable wholly ceaſeth to move, ſeems to plead.
The motion of
grave penduli
might be
ted, impediments
being removed.
SALV. The motion would not be perpetual, Sagredus,
though the impediment of the Air were totally removed, becauſe
there is another much more abſtruſe.
SAGR. And what is that? as for my part I can think of no
other?
SALV. You will be pleaſed when you hear it, but I ſhall not
tell it you till anon: in the mean time, let us proceed.
I have
propoſed the obſervation of this Pendulum, to the intent, that you
ſhould underſtand, that the impetus acquired in the deſcending
arch, where the motion is natural, is of it ſelf able to drive the
ſaid ball with a violent motion, as far on the other ſide in the like
aſcending arch; if ſo, I ſay, of it ſelf, all external impediments
being removed: I believe alſo that every one takes it for granted,
that as in the deſcending arch the velocity all the way increaſeth,
till it come to the loweſt point, or its perpendicularity; ſo from
this point, by the other aſcending arch, it all the wav diminiſheth,
untill it come to its extreme and higheſt point: and diminiſhing
with the ſame proportions, where with it did before increaſe, ſo that
the dgrees of the velocities in the points equidiſtant from the point
of perpendicularity, are equal to each other.
Hence it ſeemeth
to me (arguing with all due modeſty) that I might eaſily be induced
to believe, that if the Terreſtrial Globe were bored thorow the

centre, a Canon bullet deſcending through that Well, would
quire by that time it came to the centre, ſuch an impulſe of
city, that, it having paſſed beyond the centre, would ſpring it
wards the other way, as great a ſpace, as that was wherewith it had
deſcended, all the way beyond the centre diminiſhing the velocity
with decreaſements like to the increaſements acquired in the
ſcent: and the time ſpent in this ſecond motion of aſcent, I
lieve, would be equal to the time of deſcent.
Now if the
able by diminiſhing that its greateſt degree of velocity which it