TUNNEL. 537 TUNNEL. ionne, pipe, tun, ML. tunna, OHG. tunna, Ger. Tonne, AS. tunne, Eng. tun, botli tlic Romance and Germanic words apparently lioriowed from Ir., Gael, tunna, tun). An artitieial 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 Egypt upon ascending the throne began at once to drive the long narrow passage or tunnel leading to the inner chamlier of the rock-cut tomb at Thehes 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 anv existing records were those constructed b_v the ."Assyrians. The vaulted drain or passage under the southeast palace of Nimrud, built by Shalmaneser IL (B.C. 860-824), is in all essentials a true soft-ground tunnel. A nuich better example is the tunnel un- der the Euphrates Eiver, 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, tlius mak- ing it a true subfluvial tunnel only wlicn finished. The Euphrates River tunnel was luiilt through soft ground, and was lined with brick masonry, having interior dimensions of 12 feet in width and 1.5 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 cutting edges of corimdum or other hard, gritty material. The usual tools for rock work were, however, the hanuiier, the chisel, and wedges: and the excellence and magnitude of the works 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. The.v 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 suddenly 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 hills. It is excivatcd through volcanic tufa, and is. about .SOOn feet long and 2.5 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 7o feet high. This double funnel-like construction caused the ra.ys of light to concentrate as they approach the centre, and thus to imjirove 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 tunnel- building was practiced for military purposes, but 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 bv the enemy, and during the defense to allow of sorties and the entrance of supplies. The tumiel-build- ers of the Middle Ages added little to the knowl- edge of the art. Indeed, imtil the seventeenth century and the introduction of giuipowder 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 20 feet high, and was excavated through tufa. With the advent of gunpowder and canal con- struction the first strong impetus was given to tunnel-liuilding in its modern sense as a commer- cial and public utilitarian construction. Canal tunnels of notable size were excavated in France and England during the last half of the seven- teenth century. These were all rock or hard- ground tunnels. Indeed, previous to 1800 the soft-ground tunnel was beyond the courage of the engineer except in sections of such small size as hardly to deserve the name of tunnels. In 1803, however, a tunne! of 24 feet wide was excavated through soft soil for the Saint- -ugustine Canal in France. Timbering was employed to support the walls and roof as fast as the earth was re- moved and the masonry lining was built closely following it. From the experience gained in this tunnel were developed the various s.vstems of soft-ground subterranean tunneling since em- ployed. It was through the development of the steam railway, however, that the art of tunneling was to be brought into its present prominence. In 1820-26 two tunnels were built on tlic Liverpool and JIanchester Railway in England. This was the beginning of the rapid development which has made the tunnel one of the most familiar of engineering structures. The first railway tunnel in the I'niteil States was l>uilt on the .MIegheny Portage Railroad in Pennsylvania, in IS.'?l-.'?3; and the first canal tunnel had been completed thirteen years previously (1818-21) by the Schuvlkill Navigation Company, near Altoona, Pa. The art of tunneling entered its last and great- est jiliase with the construction of the Mont Cenis Tunnel in Europe and the Hoosae Tunnel in .-merica. The Mont Cenis Tunnel was undertaken to facilitate railway comnuinication between Italy and France, or, more properly, between Piedmont and Savov. the two parts of the kingdom of Vic- tor Emmanuel II. separated by the Alps. It is 7.6 miles long and passes under the Col de Frf jus near Mont Cenis. Work was begun in 1857 and
