And these applications of principle not only tend to diminish deflection, but the required purpose is also more effectively attained, and that by lighter pieces of timber.

To ascertain the absolute strength of a cast iron beam of the preceding form, or that of strongest section, amounts to one-fifth of the cylinder's external diameter; the relative stregth a to that of a hollow cylinder being as the diameters of their section Talks.)

RULE.-Multiply the sectional area of the bottom flange in inches by the depth of the beam in inches, and divide the product by the distance between the tupports, also in inches; and 514 times the quotient equal the absolute strength of the beam in ewis.

The strongest form in which any given quantity of matter can be disposed that of a hollow cylinder; and it has been demonstrated that the maximum of strength is obtained in cast iron, when the thickness of the annulus, or ring.

A Table showing the Weight or Pressure a beam of Cast Iron, 1 inch in breadth. wul sustain, without destroying its elastic force when it is supported at each end, and loaded in the middle of its length, and also the defection in the middle which that weight will produce. By Mr. Hodgkinson, Manchester.

Note.-This Table shows the greatest weight that ever ought to be laid upon a beam for permanent load; and, if there be any liability to jerks, etc, ample allowance must be made; also, the weight of the beam itself must be included. [See Tables of Cast Iron.)

To find the weight of a cast iron beam of given dimensions.

Rule.-Multiply the sectional area in inches by the length in feet, and by 3.2, the product equal the weight in lbs.

Ex. Required the weight of a uniform rectangular beam of cast iron, 16 feet in length, 11 inches in breadth,and 1½ inch in thickness.

11 X 1.5 X 16 X 3.2 = 844.8 lbs.