In this first class of compression, experiment has shown that the resistance is in proportion to the number of fibres pressed, that is, in proportion to the area. For example, if 5000 pounds is required to crush a prism with a base 1 inch square, it will require 20,000 pounds to crush a prism having a base of 2 by 2 inches, equal to 4 inches area; because 4 times 5000 equals 20000.

Therefore, if any given surface pressed be multiplied by the pressure per inch which the kind of material pressed may be safely trusted with, the product will be the total pressure which may with safety be put upon the given surface. Now, the capability for this kind of resistance is given in column P, in Table I., for each kind of material named in the table. Therefore, to find the limit of weight, proceed as follows:

99 - The Limit of Weight. - To ascertain what weight a post may be loaded with, so as not to crush the surface against which it presses, we have -

Rule 1. - Multiply the area of the post in inches by the value of P, Table I., and the product will be the weight required in pounds; or -

W=AP. (1.)

Example. - A post, 8 by 10 inches, stands upon a white-pine girder; the area equals 8 x 10 = 80 inches. This being multiplied by 320, the value of P, Table I., set opposite white pine, the product, 25600, is the required weight in pounds.