An Explication of the Third Plate.

Figure 1. Is a Sort of Compound Lever of the second Kind, where the Weight H6 is unequally born by the Weights F4 and G2, which are reciprocally proportional to the Distances CB and CA; 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 DC, AE, BF, and the Lever AB, are parallel to the Horizon; but the Lines on which the Weights hang Dw7, Ew5, Fw2, are perpendicular thereto; and here a Force or Weight pulling at the Point C sustains the unequal Weights w5 and w2 in Æquilibrio: Provided the Distances CB and CA 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. AB 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 BM1, 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.