There is another point in regard to loading a highway bridge, which is to be considered. It often happens that a very heavy load is carried over such bridges upon a single truck, thus throwing a heavy and concentrated load upon each point as it passes. Mr. Stoney states that a wagon with a crank-shaft of the British ship "Hercules," weighing about forty-five tons, was refused a passage over Westminster iron bridge, for fear of damage to the structure, and had to be carried over Waterloo bridge, which is of stone; and he says that in many cases large boilers, heavy forgings, or castings reach as high as twelve tons upon a single wheel. The report of the American Society of Civil Engineers, above referred to, advises that the floor system be strong enough to carry the following loads upon four wheels: Class A, 24 tons; Class B, 16 tons; Class C, 8 tons; though it is stated that these do not include the extraordinary loads sometimes taken over highways. "This provision for local loads," says Mr. Boller, one of the committee, "may seem extreme; but the jar and jolt of heavy, spring-less loads come directly on all parts of the flooring at successive intervals, and admonish us that any errors should be on the safe side." To pass now to railroad bridges, we find here a very heavy load coming upon the structure in a sudden, and often very violent, manner. Experiment and observation both indicate that a rapidly moving load produces an effect equal to double the same load at rest. This effect is seen much more upon short bridges, where the moving load is large in proportion to the weight of the bridge, than upon long spans, where the weight of the bridge itself is considerable. The actual load upon a short bridge is also more per foot than upon a long one, because the locomotive, which is much heavier than an equal length of cars, may cover the whole of a short span, but only a part of a longer one. The largest engines in use upon our railroads weigh from 75,000 to 80,000 pounds on a wheel-base of not over twelve feet in length, or 2,800 pounds per foot for the whole length of the engine, and from 20,000 to 24,000 pounds on a single pair of wheels. The heaviest coal-trains will weigh nearly a ton to the foot, ordinary freight-trains from 1,600 to 1,800 pounds, and passenger-trains from 1,000 to 1,200 pounds per foot. Any bridge is liable to be traversed by two heavy freight-engines followed by a load of three-quarters of a ton to the foot; so that if we proportion a bridge to carry 3,000 pounds per foot for the total engine length, and one ton per foot for the rest of the bridge, bearing in mind that any one point may be called upon to sustain 24,000 pounds, and regarding the increase of strain upon short spans due to high speeds, we have the following loads for different spans exclusive of the weight of the bridge:--