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To ascertain how far this is consistent with the theorem of “the parallelogram of forces,” let a line PO be drawn upon the upright board to which the wheels are attached, from the point P upward, in the direction of the thread CP. Also, let lines be drawn upon the board immediately under the threads PM and PN. From the point P, on the line PO, take as many inches as there are ounces in the weight C. Let the part of PO thus measured be P c, and from c draw ca parallel to PN, and cb parallel to PM. If the sides P a and P b of the parallelogram thus formed be measured, it will be found that P a will consist of as many inches as there are ounces in the weight A, and P b of as many inches as there are ounces in the weight B.

In this illustration, ounces and inches have been used as the subdivisions of weight and length. It is scarcely necessary to state, that any other measures of these quantities would serve as well, only observing that the same denominations must be preserved in all parts of the same investigation.