to a number of vehicles. The time of crossing is
3 or 4 minutes. The total cost of the structure was L133,000.

14. In the United States few railway companies design or build their own
bridges. General specifications as to span, loading, &c., are furnished to
bridge-building companies, which make the design under the direction of
engineers who are experts in this kind of work. The design, with strain
sheets and detail drawings, is submitted to the railway engineer with
estimates. The result is that American bridges are generally of
well-settled types and their members of uniform design, carefully
considered with reference to convenient and accurate manufacture. Standard
patterns of details are largely adopted, and more system is introduced in
the workshop than is possible where the designs are more varied. Riveted
plate girders are used up to 50 ft. span, riveted braced girders for spans
of 50 ft. to 75 ft., and pin-connected girders for longer spans. Since the
erection of the Forth bridge, cantilever bridges have been extensively
used, and some remarkable steel arch and suspension bridges have also been
constructed. Overhead railways are virtually continuous bridge
constructions, and much attention has been given to a study of the special
conditions appertaining to that case.

_Substructure._

15. The substructure of a bridge comprises the piers, abutments and
foundations. These portions usually consist of masonry in some form,
including under that general head stone masonry, brickwork and concrete.
Occasionally metal work or woodwork is used for intermediate piers.

When girders form the superstructure, the resultant pressure on the piers
or abutments is vertical, and the dimensions of these are simply regulated
by the sufficiency to bear this vertical load.

When arches form the superstructure, the abutment must be so designed as to
transmit the resultant thrust to the foundation in a safe direction, and so
distributed that no part may be unduly compressed. The intermediate piers
should also have considerable stability, so as to counterbalance the thrust
arising when one arch is loaded while the other is free from load.

For suspension bridges the abutment forming the anchorage must be so
designed as to be thoroughly stable under the greatest pull which the
chains can exert. The piers require to be carried above the platform, and
their design must be modified according to the type of suspension bridge
adopted. W