85. When a weight is laid upon the middle of a piece of timber that is supported only at the ends, it always causes some amount of deflection. When the piece bends in a very small degree, it is said to be stiff, and when the bending is considerable, it is called flexible.

the specimen, and in the Table these are distinguished by +. On the other bearing sometimes the specimen broke during the motion of the weight, and therefore would have separated under a less force, with more time; these are marked - .

The stiffness of beams of the same length is proportional to the space through which they are bent by a given weight. That two pieces of different lengths may be equally stiff, the deflection, or bending, should be proportional to their lengths. For a deflection of one-fourth of an inch in a joist 20 feet long would not be attended with any bad effect; but if a joist 4 feet long were to bend one-fourth of an inch, it would be totally unfit for the purpose.