each strain, whether to use steel that is rigid or that which is so
flexible that it can be tied in a knot. On the designer depends the
price asked for the work, and so it is his business to invent, for each
bridge is a separate problem in invention, a bridge that will carry the
required weight with the least expenditure of material and labour and at
the same time be strong enough to carry very much greater loads than it
is ever likely to be called upon to sustain. The designer is often the
constructor as well, and he is always a man of great practical
experience. He has in his time stepped out on a foot-wide girder over a
rushing stream, directing his men, and he has floundered in the mud of a
river bottom in a caisson far below the surface of the stream, while the
compressed air kept the ooze from flowing in and drowning him and his
workmen.

The second operation of making the pieces that go into the structure is
simply the following out of the clearly drawn plans furnished by the
designing engineers. Different grades of steel and iron are moulded or
forged into shape and riveted together, each part being made the exact
size and shape required, even the position of the holes through which
the bolts or rivets are to go that are to secure it to the neighbouring
section being marked on the plan.

The foundations for bridges are not always put down by the builders of
the bridge proper; that is a work by itself and requires special
experience. On the strength and permanency of the foundation depends the
life of the bridge. While the foundries and steel mills are making the
metal-work the foundations are being laid. If the bridge is to cross a
valley, or carry the roadway on the level across a depression, the
placing of the foundations is a simple matter of digging or blasting out
a big hole and laying courses of masonry; but if a pier is to be built