Let P, _fig. 7._, be the point on which the two forces act, and let their directions be P A and P B. From the point P, upon the line P A, take a length P _a_, consisting of as many inches as there are ounces in the force P A; and, in like manner, take P _b_, in the direction P B, consisting of as many inches as there are ounces in the force P B. Through _a_ draw a line parallel to P B, and through _b_ draw a line parallel to P A, and suppose that these lines meet at _c_. Then draw P C. A single force, acting in the direction P C, and consisting of as many ounces as the line P c consists of inches, will produce upon the point P the same effect as the two forces P A and P B produce acting together. (75.) The figure P _a c b_ is called in GEOMETRY a _parallelogram_; the lines P _a_, P _b_, are called its _sides_, and the line P _c_ is called its _diagonal_. Thus the method of finding an equivalent for two forces, which we have just explained, is generally called “the parallelogram of forces,” and is usually expressed thus: “If two forces be represented in quantity and direction by the sides of a parallelogram, an equivalent force will be represented in quantity and direction by its diagonal.” (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, M N, _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 M P N, 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 P C, balances the weights A and B, acting in the directions P M and P N, 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 P M and P N; and the direction of the resultant is that of a line drawn directly upwards from P.