ancient kings. Similar rock cut tunnels were made by the early inhabitants of India in building their temples and by many other ancient races. The tunnels constructed by the Assyrians are probably the earliest examples of built up tunnels. The vaulted drain under the palace of Nimrud, built about 860 B. C., may be considered a genuine soft ground tunnel. A similar example and what might quite properly be considered as the first subaqueous tunnel on record was under the Euphrates River. This was built of brick masonry and was 12 feet wide and 15 feet high. When this was being constructed, however, it was not strictly speaking, a subaqueous tunnel for it was built under the dry bed of the river, the waters of which were turned aside until the tunnel was completed.
Of course among the ancient peoples such work was all done by hand, the tools being pick and shovel for the soft ground; and hammer, and chisel and wedges for rock tunnelling.
The Romans were the greatest tunnel builders of ancient times and devised many ingenious methods for the work. Their tunnels were constructed principally for acqueducts and roads, and some of them are wonderful examples of engineering when the absence of modern facilities for such work is considered.
During the middle ages tunnels were only built for military purposes and little progress was made in the methods of work until the introduction of gunpowder. Up to the beginning of the 19th century most all the tunnels were built through rock or hard ground, the soft ground tunnel being scarcely ever attempted.
In 1803 a tunnel 24 feet wide was cut through soft soil for the St. Augustine Canal in France. Timbering was used to support the roof and sides while the earth was being removed, and was followed by a lining of masonry.
One of the greatest factors in bringing the art of tunnelling to its present importance was the development of the steam railway. Almost immediately tunnel building increased. About 1820 two tunnels were constructed on the Liverpool and Manchester Railway in England. In the United States the first railway tunnel was built on the Allegheny Portage Railroad in Pennsylvania, in 1831-33. This increase in tunnel building was accompanied by corresponding progress in methods of construction and the introduction of improved machinery and special devices.
One of the greatest of these improvements was the invention and development of the shield system without which the construction of the present great submarine tunnels would have been impossible. In the construction of mountain tunnels such as those under the Alps and other rock tunnels, the principal difficulties encountered were the hardness of the rocks, the great length of tunnel, the lack of ventilation, etc. Tunnelling under water, however, is radically different. The mere excavation presents little trouble; but flooding has constantly to be guarded against, and as the work is usually through
