4. It is through a knowledge of the composition and resolution of forces alone that the carpenter can expect to arrive at excellence in the art of designing frames of timber, as without this knowledge it would be impossible for him to understand clearly what is to be aimed at in such designs: or even to know whether a design of his own would answer its intended purpose or not.

The first step towards obtaining this knowledge, is to acquire just notions of the action of forces.

5. A heavy body exerts in a vertical direction a force equal to its own weight; and it would always descend in a vertical line, if not moved out of that direction by some other force.

6. But when a heavy body W (Fig. 1) is sustained by two beams A C and B C, its effects on these beams depend on their position; the farther the ends A and B are set apart, the greater will be the sum of the strains on the beams; and the contrary. Hero it is obvious the weight resolves itself into two forces, one in the direction of each beam, or we may consider that the abutments A and B exert reactionary forces through the beams in the opposite direction, which are sufficient to support the weight. These are evidently equal to the forces caused by the weight, for action and reaction are equal and opposite.

Fig 1.

We may now proceed to explain what is meant by the composition and resolution of forces.