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

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SALV. I believe that you very much deceive your ſelf, and am
certain, that experience will ſhew you the contrary, and that the ball
being once arrived at the ground, will run together with the horſe,
not ſtaying behind him, unleſs ſo far as the aſperity and
neſs of the Earth ſhall hinder it.
And the reaſon ſeems to me
very manifeſt: for if you, ſtanding ſtill, throw the ſaid ball
long the ground, do you think it would not continue its motion
and that for ſo
much a greater ſpace, by how much the ſuperficies were more
ſmooth, ſo that v. g. upon ice it would run a great way?
SIMP. There is no doubt of it, if I give it impetus with my
arm; but in the other caſe it is ſuppoſed, that he who is upon the
horſe, onely drops it out of his hand:
SALV. So I deſire that it ſhould be: but when you throw it
with your arm, what other remaineth to the ball being once gone
motion being conſerved in the boul, it doth continue to carry it
forward?
Now, what doth it import, that that impetus be
ferred on the ball rather from the arm than from the horſe?
Whilſt
you were on horſeback, did not your hand, and conſequently the
ball run as faſt as the horſe it ſelf?
Doubtleſs it did: therefore
in onely opening of the hand, the ball departs with the motion
but from the motion dependant on the ſaid horſe, which cometh to
be communicated to you, to your arm, to your hand, and laſtly to
the ball.
Nay, I will tell you farther, that if the rider upon his
ſpeed fling the ball with his arm to the part contrary to the courſe,
it ſhall, after it is fallen to the ground, ſometimes (albeit thrown to
the contrary part) follow the courſe of the horſe, and ſometimes lie
ſtill on the ground; and ſhall onely move contrary to the ſaid
courſe, when the motion received from the arm, ſhall exceed that
of the carrier in velocity.
And it is a vanity, that of ſome, who
ſay that a horſeman is able to caſt a javelin thorow the air, that
way which the horſe runs, and with the horſe to follow and
take the ſame; and laſtly, to catch it again.
It is, I ſay, a vanity,
for that to make the project return into the hand, it is requiſite to
caſt it upwards, in the ſame manner as if you ſtood ſtill.
For, let
the carrier be never ſo ſwift, provided it be uniform, and the
ject not over-light, it ſhall always fall back again into the hand of
the projicient, though never ſo high thrown.
SAGR. By this Doctrine I come to know ſome Problems very

curious upon this ſubject of projections; the firſt of which muſt
ſeem very ſtrange to Simplicius. And the Problem is this; I
firm it to be poſſible, that the ball being barely dropt or let fall,
by one that any way runneth very ſwiftly, being arrived at the