f 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.

In 1875 the American Society of Civil Engineers, in view of the
repeated bridge disasters in this country, appointed a committee to
report upon