_To illustrate:_ Take a rod, as a meter stick, drill a hole at _S_ and place through it a screw fastened at the top of the blackboard. Attach by cords two spring balances and draw to the right and left, _A_ and _B_ as in Fig. 66. Draw out the balance _B_ about half way, hold it steadily, or fasten the cord at the side of the blackboard, and read both balances. Note also the distance _AS_ and _BS_. Since the rod is at rest, the tendency to rotate to the right and left must be equal. That is, the moments of the forces at _A_ and _B_ about _S_ are equal. Since these are computed by the product of the _force times the force_ arm, multiply _B_ by _BS_ and _A_ by _AS_ and see if the computed moments are equal. _Hence a force that tends to turn or rotate a body to the right can be balanced by another of equal moment that acts toward the left._ [Illustration: FIG. 67.--Law of parallel forces illustrated.] =85. Parallel Forces.=--Objects are frequently supported by two or more upward forces acting at different points and forming in this way a system of parallel forces; as when two boys carry a string of fish on a rod between them or when a bridge is supported at its ends. The principle of moments just described aids in determining the magnitude of such forces and of their resultant. To illustrate this take a wooden board 4 in. wide and 4 ft. long of uniform dimensions. (See Fig. 67.) Place several screw hooks on one edge with one set at _O_ where the board will hang horizontally when the board is suspended there. Weigh the board by a spring balance hung at _O_. This will be the resultant in the following tests. Now hang the board from two spring balances at _M_ and _N_ and read both _balances_. Call readings _f_ and _f´_. To test the forces consider _M_ as a fixed point (see Fig. 67) and the weight of the board to act at _O_. Then the moment of the weight of the board should be equal the moment of the force at _N_ since the board does not move, or _w_ times _OM_ equals _f´_ times _NM_. If _N_ is considered the fixed point then the moment of the weight of the board and of _f with reference to the point N_ should be equal, or _w_ times _ON_ = _f_ times _NM_. Keeping this illustration in mind, the law of parallel forces may be stated at follows: 1. _The resultant of two parallel forces acting in the same direction at different points in a body is equal to their sum and has the same direction as the components._ _The moment of one of the components about the point of application of the other is equal and opposite to the moment of the supported weight about the other._ =Problem.=--If two boys carry a string of fish weighing 40 lbs. on a rod 8 ft. long between them, what force must each boy exert if the string is 5 ft. from the rear boy?