Galilei, Galileo, Mechanics, 1665

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Theſe Principles being declared, we will paſſe to the Contem-
plation of Pullies, the compoſition and ſtructure of which, together
with their uſe, ſhall be deſcribed by us.
And firſt let us ſuppoſe the

^{*} Little Pulley A B C, made of Mettall or hard Wood, voluble a-
bout it's Axis which paſſeth thorow it's Center D, and about this

Figure: /permanent/archimedes/galil_mecha_070_en_1665/figures/070.01.016.1.jpg not scanned
[Figure 11]

Pulley let the Rope E A B C be put,
at one end of whichlet the Weight E
hang, and at the other let us ſuppoſe
the Force F.
I ſay, that the Weight
being ſuſtained by a Force equall to
it ſelf in the upper Nut or Pulley
A B C, bringeth ſome benefit, as the
moving or ſuſtaining of the ſaid
Weight with the Force placed in F:
For if we ſhall underſtand, that from
the Center D, which is the place of the Fulciment, two Lines be
drawn out as far as the Circumference of the Pulley in the points
A and C, in which the pendent Cords touch the Circumference, we
ſhall have a Ballance of equal Arms which determine the Diſtance
of the two Suſpenſions from the Center and Fulciment D: Where-
upon it is manifeſt, that the Weight hanging at A cannot be ſuſtain-
ed by a leſſer Weight hanging at G, but by one equal to it; ſuch
is the nature of equal Weights hanging at equal Diſtances.
And
although in moving downwards, the Force F cometh to turn about
the Pulley A B C, yet there followeth no alteration of the Alti-
tude or Reſpect, that the Weight and Force have unto the two
Diſtances A D and D C, nay, the Pulley encompaſſed becometh a
Ballance equal to A C, but perpetuall.
Whence we may learn,
how childiſhly Ariſtotle deceiveth himſelf, who holds, that by making
the ſmall Pulley A B C bigger, one might draw up the Weight with
a leſſer Force; he conſidering that upon the enlargement of the
ſaid Pulley, the Diſtance D C encreaſed, but not conſidering that
there was as great an encreaſe of the other Diſtance of the Weight,
that is, the other Semidiameter D A.
The benefit therefore that may
be drawn from the Inſtrument above ſaid, is nothing at all as to the
diminution of the labour: and if any one ſhould ask how it hap-
pens, that on many occaſions of raiſing Weights, this means is made
uſe of to help the Axis, as we ſee, for example, in drawing up the
Water of Wells; it is anſwered, that that is done, becauſe that
by this means the manner of employing the Force is found more
commodious: for being to pull downwards, the proper Gravity of
our Arms and other parts help us, whereas if we were to draw
the fame Weight upwards with a meer Rope, by the ſole ſtrength

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