ow. In view of the above, and remembering that a
moving load produces a much greater strain upon a bridge than one
which is at rest, we may be sure, that, as the committee above
referred to recommend, the loads should not be less than those given
in the table. We can easily see that in special cases they should be
more.

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 2