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

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1of the Members and Muſcles, and as we uſe to ſay, by Force of
Armes
, beſides the extern Weight, we are to lift up the Weight of
our
own Armes, in which greater pains is required.
Conclude we,
therefore
, that this upper Pulley doth not bring any Facility to the
Force
ſimply conſidered, but onely to the manner of applying it:
but
if we ſhall make uſe of the like Machine
191[Figure 191]
in
another manner, as we are now about to
declare
; we may raiſe the Weight with di­
minution
of Forces: For let the Pulley
B
D C be voluble about the Center E placed
in
it's Frame B L C, at which hang the
Grave
G; and let the Rope A B D C F
paſſe
about the Pulley; of which let the end
A
be faſtned to ſome fixed ſtay, and in the
other
F let the Force be placed; which
moving
to wards H ſhall raiſe the Machine
B
L C, and conſequently the Weight G:
and
in this operation I ſay, that the Force in
F
is the half of the Weight ſuſtained by it.
For the ſaid Weight being kept to Rights by the two ^{*} Ropes A B

and
F C, it is manifeſt, that the Labour is equally ſhared betwixt
the
Force F and the Fulciment A: and more ſubtilly examining the
nature
of this Inſtrument, if we but continue forth the Diameter
B
E C, we ſhall ſee a Leaver to be made, at the midſt of which, that
is
at the point E, the Grave doth hang, and the Fulciment cometh
to
be at the end B, and the Force in the Term C: whereupon, by
what
hath been above demonſtrated, the Force ſhall have the ſame
proportion
to the Weight, that the Diſtance E B hath to the Di­
ſtance
; Therefore it ſhall be the half of the ſaid Weight: And
becauſe
the Force riſing towards A, the Pulley turneth round,
therefore
that Reſpect or Conſtitution which the Fulciment B and
Center
E, on which the Weight and Term C, in which the Force
is
employed do depend, ſhall not change all the while; but yet in
the
Circuinduction the Terms B and C happen to vary in number,
but
not in vertue, others and others continually ſucceeding in their
place
, whereby the Leaver B C cometh to be perpetuated.
And
here
(as hath been done in the other Inſtruments, and ſhall be in
thoſe
that follow) we will not paſſe without conſidering how that
the
journey that the Force maketh, is double to the Moment of the
Weight
.
For in caſe the Weight ſhall be moved ſo far, till that
the
Line B C come to arrive with it's points B and C, at the points
A
and F, it is neceſſary that the two equal Ropes be diſtended in
one
ſole Line F H, and conſequently, when the Weight ſhall have
aſcended
along the Intervall B A, the Force ſhall have been moved
twice
as far, that is, from F unto H. Then conſidering that the

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