OF the Stiffness of Bodies when strained in the Direction of their Length.

79. When wood is strained in the direction of its length and within the so-called elastic limit of the material, the elongation produced by the strain is nearly in direct proportion to the straining force and original length, and inversely as the area of the cross section.

Let L represent the original length in inches, l the amount of elongation also in inches, W the weight in lbs., S the sectional area in square inches, and E the Modulus of Elasticity, or weight in lbs. per square inch required to extend the piece to double its original length, on the assumption that the rate of extension throughout is uniformly as the weight. Then, LxW/ExS = l [1]

From which the following rule is obtained.

Rule I. - Multiply the weight in lbs. to be suspended by the length of the piece in inches, and the product divided by the sectional area in square inches, multiplied by the value of E, Tables I. and II., will give the elongation in inches; or the same product divided by the value of E multiplied by the amount of elongation that would leave the elasticity of the piece uninjured, and the result will give the area of the section in square inches.