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(76.) A single force, which is thus mechanically equivalent to two or more other forces, is called their resultant, and relatively to it they are called its components. In any mechanical investigation, when the resultant is used for the components, which it always may be, the process is called “the composition of force.” It is, however, frequently expedient to substitute for a single force two or more forces, to which it is mechanically equivalent, or of which it is the resultant. This process is called “the resolution of force.”

(77.) To verify experimentally the theorem of the parallelogram of forces is not difficult. Let two small wheels, MN, fig.8., with grooves in their edges to receive a thread, be attached to an upright board, or to a wall. Let a thread be passed over them, having weights A and B, hooked upon loops at its extremities. From any part P of the thread between the wheels let a weight C be suspended: it will draw the thread downwards, so as to form an angle MPN, and the apparatus will settle itself at rest in some determinate position. In this state it is evident that since the weight C, acting in the direction PC, balances the weights A and B, acting in the directions PM and PN, these two forces must be mechanically equivalent to a force equal to the weight C, and acting directly upwards from P. The weight C is therefore the quantity of the resultant of the forces PM and PN; and the direction of the resultant is that of a line drawn directly upwards from P.