"Fairbairn has deduced the following formula from experiments made mostly on very thin cylindrical tubes of various lengths and diameters—viz., for wrought-iron cylindrical tubes let l = the length, d = the diameter, and t the thickness of the shell, all expressed in the same unit of measure, and let p = the collapsing pressure in pounds per unit of area; thenp = 9,672,000 \frac{t^{2.19}}{l d}. { IV.}In case a tube is stiffened by T-iron rings or by flanges, l represents the distance between two such adjacent rings or flanges."