4,000 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:--

+---------+-----------------+
| 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