+---------+-----------------+ | SPAN. | LBS. PER FOOT. | +---------+-----------------+ | 12 | 7,000 | +---------+-----------------+ | 15 | 6,000 | +---------+-----------------+ | 20 | 4,800 | +---------+-----------------+ | 25 | 4,000 | +---------+-----------------+ | 30 | 3,600 | +---------+-----------------+ | 40 | 3,200 | +---------+-----------------+ | 50 | 3,000 | +---------+-----------------+ | 100 | 2,800 | +---------+-----------------+ | 200 | 2,600 | +---------+-----------------+ | 300 | 2,500 | +---------+-----------------+ | 400 | 2,450 | +---------+-----------------+ | 500 | 2,400 | +---------+-----------------+ The above does not vary essentially from the English practice, and is substantially the same as given by the committee of the American Society of Civil Engineers. The load which any bridge will be required to carry being determined, and the general plan and dimensions fixed, the several strains upon the different members follow by a simple process of arithmetic, leaving to be determined the actual dimensions of the various parts, a matter which depends upon the power of different kinds of material to resist different strains. This brings us to the exceedingly important subject of the nature and strength of materials. It has been said that we know what one square inch of iron will hold. Like the question of loads above examined, this is a matter which has been settled, at any rate within very narrow limits, by the experience of many years of both European and American engineers. A bar of the best wrought-iron an inch square will not break under a tensile strain of less than sixty thousand pounds. Only a small part of this, however, is to be used in practice. A bar or beam may be loaded with a greater weight applied as a permanent or dead-load than would be safe as a rolling or moving weight. A load may be brought upon any material in an easy and gradual manner, so as not to damage it; while the same load could not be suddenly and violently applied without injury. The margin for safety should be greater with a material liable to contain hidden defects, than with one which is not so; and it should be greater with any member of a bridge which is subjected to several different kinds of strain, than for one which has to resist only a single form of strain. Respect, also, should be had to the frequency with which any part is subjected to strain from a moving load, as this will influence its power of endurance. The rule in structures having so important an office to perform as railroad or highway bridges, should be, in all cases, absolute safety under all conditions.