161. Relining Timber-Lined Tunnel, Great Northern Ry 317 162. Relining Timber-Lined Tunnel, Great Northern Ry 318 163. Relining Timber-Lined Tunnel, Great Northern Ry 319 164. Construction of Centering Mullan Tunnel 320 165. Centering Mullan Tunnel 321 166. Relining Timber-Lined Tunnel, Norfolk & Western Ry 322 167. Relining Timber-Lined Tunnel, Norfolk & Western Ry 323 INTRODUCTION THE HISTORICAL DEVELOPMENT OF TUNNEL BUILDING. A tunnel, defined as an engineering structure, is an artificial gallery, passage, or roadway beneath the ground, under the bed of a stream, or through a hill or mountain. The art of tunneling has been known to man since very ancient times. A Theban king on ascending the throne began at once to drive the long, narrow passage or tunnel leading to the inner chamber or sepulcher of the rock-cut tomb which was to form his final resting-place. Some of these rock-cut galleries of the ancient Egyptian kings were over 750 ft. long. Similar 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 tunnels 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 II. (860-824 B.C.), is in all essentials a true soft-ground tunnel, with a masonry lining. A much better example, however, is the tunnel under the Euphrates River, which may quite accurately be claimed as the first submarine 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 making it a true submarine tunnel only when finished. The Euphrates River tunnel was built through soft ground, and was lined with brick masonry, having interior dimensions of 12 ft. in width and 15 ft. in height. Only hand labor was employed by these ancient peoples in their tunnel work. 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 developed, use was made of tube drills and saws provided with cutting edges of corundum or 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 works accomplished by these limited appliances attest the unlimited time and labor which must have been available for their accomplishment.