An Explication of the Third PLATE.
Figure 1. Is a Sort of Compound Lever of the second Kind, where the Weight
H 6 is unequally born by the Weights F 4 and G 2, which are reciprocally
proportional to the Distances C B and C A; and are accordingly _in
Æquilibrio_. Whence we see how two Men may bear unequal Parts of the same
Weight, in Proportion to their Nearness thereto.
_Fig. 2._ Is another Engine of the same Nature with the former; where the
Lines D C, A E, B F, and the Lever A B, are parallel to the Horizon; but
the Lines on which the Weights hang D w 7, E w 5, F w 2, are perpendicular
thereto; and here a Force or Weight pulling at the Point C sustains the
unequal Weights w 5 and w 2 _in Æquilibrio_: Provided the Distances C B
and C A be reciprocally proportional to those Weights. Whence we learn,
how Horses of unequal Strength may be duly fitted to preserve equally in
their Labour; _viz._ by taking care that the Beam by which they both draw
a Weight or Waggon, may be divided at the Point of Traction as C, in
reciprocal Proportion to such their Strength.
_Fig. 3._ A B is an upper Pulley, of no direct Advantage, but for
Readiness of the Motion, as increasing not the Power at all; equal Weights
being ever required to raise others.
_Fig. 4._ Is an upper and an under Pulley connected together; where the
upper being of no Efficacy, the lower does however double the Force, as is
ever the Case in such Pulleys.
_Fig. 5._ Is a Compound Pulley of three upper and three under Pulleys, all
communicating together; where therefore the whole Weight is divided among
6 Strings; and so 1 Pound balances 6 Pound. The last String B M 1, as
passing beyond the last upper Pulley, not being here to be reckon'd of any
Consequence.
_Fig. 6._ and 7. These are Boxes of the same Number of upper and under
Pulleys with the former; only in other Positions, and depend on the same
Principle entirely.
[[Plate IV. ― Sutton Nicholls sculp:]]