But it may be asked, Can the precise load which is coming upon any structure be exactly fixed? are not the circumstances under which bridges are loaded very different? Bridges in different localities are certainly subjected to very different loads, and under very different conditions; but the proper loads to be provided for have been fixed by the best authority for all cases within narrow enough limits for all practical purposes. Few persons are aware of the weight of a closely packed crowd of people. Mr. Stoney of Dublin, one of the best authorities, packed 30 persons upon an area of a little less than 30 square feet; and at another time he placed 58 persons upon an area of 57 square feet, the resulting load in the two cases being very nearly 150 pounds to the square foot. "Such cramming," says Mr. Stoney, "could scarcely occur in practice, except in portions of a strongly excited crowd; but I have no doubt that it does occasionally so occur." "In my own practice," he continues, "I adopt 100 pounds per square foot as the standard working-load distributed uniformly over the whole surface of a public bridge, and 140 pounds per square foot for certain portions of the structure, such, for example, as the foot-paths of a bridge crossing a navigable river in a city, which are liable to be severely tried by an excited crowd during a boat-race, or some similar occasion." Tredgold and Rankine estimate the weight of a dense crowd at 120 pounds per square foot. Mr. Brunel used 100 pounds in his calculations for the Hungerford Suspension Bridge. Mr. Drewry, an old but excellent authority, observes that any body of men marching in step at from 3 to 3-1/2 miles an hour will strain a bridge at least as much as double the same weight at rest; and he adds, "In prudence, not more than one-sixth the number of infantry that would fill a bridge should be permitted to march over it in step." Mr. Roebling says, in speaking of the Niagara Falls Suspension Bridge, "In my opinion, a heavy train, running at a speed of 20 miles an hour, does less injury to the structure than is caused by 20 heavy cattle under full trot. Public processions marching to the sound of music, or bodies of soldiers keeping regular step, will produce a still more injurious effect." Evidently a difference should be made in determining the load for London Bridge and the load for a highway bridge upon a New-England country road in a thinly settled district. A bridge that is strong enough is just as good and just as safe as one that is ten times stronger, and even better; for in a large bridge, if we make it too strong, we make it at the same time too heavy. The weight of the structure itself has to be sustained, and this part of the load is a perpetual drag on the material.