uty,
and the American bridgemen had seen to it that every rivet was properly
headed and every bolt screwed tight--and no fault could be found.

The bridge engineer's work is very diversified, since no two bridges are
alike. At one time he might be ordered to span a stream in the midst of
a populous country where every aid is at hand, and his next commission
might be the building of a difficult bridge in a foreign wilderness far
beyond the edge of civilisation.

Bridge-building is really divided into four parts, and each part
requires a different kind of knowledge and experience.

First, the designer has to have the imagination to see the bridge as it
will be when it is completed, and then he must be able to lay it out on
paper section by section, estimating the size of the parts necessary for
the stress they will have to bear, the weight of the load they will have
to carry, the effect of the wind, the contraction and expansion of cold
and heat, and vibration; all these things must be thought of and
considered in planning every part and determining the size of each. Also
he must know what kind of material to use that is best fitted to stand
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 marke