With respect to the alternative of comparing the bridge with masonry or with carpentry, we may say, that the principles on which the equilibrium of bridges is calculated, are altogether elementary, and independent of any figurative expressions of strains and mechanical purchase, which are employed in considering many of the arrangements of carpentry, •and which may indeed, when they are accurately analysed, be resolved into forces opposed and com bined in the same manner as the thrusts of a bridge. It is, therefore, wholly unnecessary, when we inquire into the strength of such a fabric, to distinguish the thrusts of masonry from the strains of carpentry, the laws which govern them being not only similar but identical ; except that a strain is commonly under stood as implying an exertion of cohesive force, and we have seen that a cohesive force ought never to be called into action in a bridge, since it implies a great and unnecessary sacrifice of the strength of the ma terials employed. lf, indeed, we wanted to cross a mere ditch, without depending on the firmness of the bank, we might easily find a beam of wood or a bar of iron strong enough to afford a passage over it, unsupported by any abutment, because, in a sub stance of .inconsiderable length, we are sure of hav
ing more strength than we require. But to assert that an iron bridge of 600 feet span " is a lever ex erting a vertical pressure only on the abutments," is to pronounce a sentence from the lofty tribunal refined science, which the simplest workman must feel to be erronequs. But, in this instance, the er ror is not so much in the comparison with the lever, as in the inattention to the mode of fixing it for a. lever or beam of the dimensions of the proposed bridge, lying loosely on its abutments, would proba bly be at least a hundred times weaker than if it were firmly connected with the abutments as a bridge is, so as to be fixed in a determinate direction. And the true reason of the utility of cast iron for building bridges, consists not, as has often been sup posed, in its capability of being united so as to act like a frame of carpentry, but in the great resist ance which it seems to afford to any force tending to crush it.