TUNNEL (OF. tonnel, tunnel. tun. pipe. ton nellr. arbor• arched vault, tunnel, diminutive of tonne, pipe, tun, AIL. tunna, OHO. muss, Ger. Tonne, AS. t unne, Eng. Ion, both the and Germanic words apparently borrowed from Ir., Gael. tunna, tun). An artificial gallery, passage, or roadway beneath the ground, under the bed of a stream, or through a hill or moun tain. The art of tunneling has been known to man since very early times. A king of Eupt upon ascending the throne began at once to drive the long narrow passage or tunnel leading to the inner chamber of the rock-cut tomb at Thebes which was to form his final resting place. Simi lar rock-cut tunneling work was performed by the Nubians and 'Indians in building their temples, by the Aztecs in America, and, in fact, by most of the ancient civilized peoples. The first built up tun nels of which there are any existing records were those constructed by the Assyrians. The vaulted drain or passage under the southeast palace of Nimrud, built by Shalmaneser 11. (B.c. 860-824), is in all essentials a true soft-ground tunnel. A much better example is the tunnel un der the Euphrates River, which may quite ac curately be claimed to be the first subaqueous tunnel of which there exists any record. It was, however, built under the dry bed of the river, the waters of which were temporarily diverted and then turned back into their normal channel after the tunnel work was completed, thus mak ing it a true subfluvial tunnel only when finished. The Euphrates River tunnel was built through soft ground, and was lined with brick masonry, having interior dimensions of 12 feet in width and 15 feet in height. Only hand work was em ployed by these ancient peoples in their tunnel construction. In soft ground the tools used were the pick and shovels or scoops. For rock work they possessed a greater range of appliances.
Research has shown that among, the Egyptians, by whom the art of quarrying was highly de veloped, use was made of tube drills and saws provided with mating edges of corundum o• other hard, gritty material. The usual tools for rock work were, however, the hammer, the chisel. and wedges; and the excellence and magnitude of the worlds accomplished with these limited appliances attest the unlimited time and labor which must have been available for their accomplishment.
The 'Romans should doubtless rank as the greatest tunnel-builders of antiquity in the num ber, magnitude, and useful character of their works and in the improvements which they de vised in the methods of tunnel-building. They
introduced fire as an agent for hastening the breaking down of the rock, and also developed the familiar principle of prosecuting the work at several points at once by means of shafts. Their method of operation was simply to build large fires in front of the rock to be broken down, and when it had reached a high temperature to cool it smblenly by throwing water upon the hot sur face, thus producing cracks and fissures.
The Roman tunnels were designed for 'public utility, especially for aqueducts and roads. One of the most notable of the tunnels of larger sec tion is that which gives the road between Naples and Pozzuoli passage through the Posi lipo bills. It is exe•iyated through volcanic tufa, and is about 3000 feet long and 25 feet wide, with a section of the form of a pointed arch. In order to facilitate the illumination of this tunnel, its floor and roof were made gradu ally converging from the ends toward the mid dle; at the entrances the section was 75 feet high. This double funnel-like construction caused the rays of light to concentrate as they approach the centre, and thus to improve the natural illumination. This tunnel was probably excavated during the time of Augustus, although some authorities place its construction at an earlier date.
During the Middle Ages the art of tumul was practiced for military purposes, hut seldom for the public need and comfort. Every great castle had its private underground passage from the central tower or keep to some distant concealed place to permit the escape of the family and its retainers in case of victory by the enemy, and during the defense to allow of sorties and the entrance of supplies. The tunneldmild ers of the Middle Ages added little to the knowl edge of the art. Indeed, until the seventeenth century and the introduction of gunpowder no particular improvement was made in the tunnel ing methods of the Romans. Although gun powder had been previously employed in mining, the first important use of it in tunnel work was at Malpas, France. in 1679-81, in the tunnel for the Languedoc Canal. This tunnel was 510 feet long. 22 feet wide. and 29 feet high, and was excavated through tufa.