558. The ratio between the compressive and tensile strengths of steel is nearly unity. The same thing is approximately true of wood and some other materials of construction. In cement and concrete, however, the conditions are quite different, the strength in compression being from five to ten times the strength in tension. Concrete cannot, therefore, be economically used to resist tension, and in structures requiring transverse strength concrete is at a great disadvantage.

559. The idea of supplementing the tensile strength of concrete by the use of iron in combination with it, seems to have been suggested independently by a number of men. It is known that combination beams were tested by Mr. R. G. Hatfield as early as 1855. In 1875 Mr. W. E. Ward,1 M. Am. Soc. Mech. Engrs., constructed a dwelling entirely of "beton," the floors, roofs, etc., being reinforced with light iron beams and rods. These early uses of the combination have some bearing upon the ability of patentees to cover in their blanket patents more than the peculiar form of the steel member which they advocate in their particular system.