The hollow cylinder is the strongest form of section under compressive force. The experiments by which this was proved were conducted upon hollow tapering columns of cast-iron, upon cross-sections, as used in the connecting-rods of steam-engines, and upon forms in which the metal was cast in the shape of the letter H. All these forms proved considerably weaker than the hollow cylinder of equal weight of metal. As the relative merits of these forms of casting metal are of constant use, we append their proportionate strengths: Hollow cylindrical pillar, 100; H-shaped pillar, 75; +-shaped pillar, 44. The examples were all of the same weight and length, with rounded ends. General Morin's rule for the thickness of cast-iron pillars may be relied upon, as it is based upon the founder's experience of the minimum thickness. Height, feet, 7 to 10, 10 to 13, 13 to 20, 20 to 27.; minimum thickness, inch, 0.5 0.6 0.8 1.0

Another rule is to make the thickness in no case less than 1/12 of the diameter. Cellular or tubular girders exemplify to a still greater degree the value of hollow construction.