[Illustration:  THE SPIDER-WEB-LIKE VIADUCT ACROSS CANON DIABLO The slender steel structure supporting a loaded train that stretches along its entire length.]

The pneumatic tube, which is practically a steel caisson on a small scale operated in the same way, is often used for small towers, and many of the steel sky-scrapers of the cities are built on foundations of this sort when the ground is unstable.

Foundations of wooden and iron piles, driven deep in the ground below the river bottom, are perhaps the most common in use.  The piles are sawed off below the surface of the water and a platform built upon them, which in turn serves as the foundation for the masonry.

The great Eads Bridge, which was built across the Mississippi at St. Louis, is supported by towers the foundations of which are sunk 107 feet below the ordinary level of the water; at this depth the men working in the caissons were subjected to a pressure of nearly fifty pounds to the square inch, almost equal to that used to run some steam-engines.

The bridge across the Hudson at Poughkeepsie was built on a crib or caisson open at the top and sunk by means of a dredge operated from above taking out the material from the inside.  The wonder of this is hard to realise unless it is remembered that the steel hands of the dredge were worked entirely from above, and the steel rope sinews reached down below the surface more than one hundred feet sometimes; yet so cleverly was the work managed that the excavation was perfect all around, and the crib sank absolutely straight and square.

It is the fourth department of bridge-building that requires the greatest amount not only of knowledge but of resourcefulness.  In the final process of erection conditions are likely to arise that were not considered when the plans were drawn.

The chief engineer in charge of the erection of a bridge far from civilisation is a little king, for it is necessary for him to have the power of an absolute monarch over his army of workmen, which is often composed of many different races.

With so many thousand tons of steel and stone dumped on the ground at the bridge site, with a small force of expert workmen and a greater number of unskilled labourers, in spite of bad weather, floods, or fearful heat, the constructing engineer is expected to finish the work within the specified time, and yet it must withstand the most exacting tests.

In the heart of Africa, five hundred miles from the coast and the source of supplies, an American engineer, aided by twenty-one American bridgemen, built twenty-seven viaducts from 128 to 888 feet long within a year.

The work was done in half the time and at half the cost demanded by the English bidders.  Mr. Lueder, the chief engineer, tells, in his account of the work, of shooting lions from the car windows of the temporary railroad, and of seeing ostriches try to keep pace with the locomotive, but he said little of his difficulties with unskilled workmen, foreign customs, and almost unspeakable languages.  The bridge engineer the world over is a man who accomplishes things, and who, furthermore, talks little of his achievements.