TUNNEL. An artificial arch or passage under ground. They are employed as the means of conducting canals under elevated ground ; for the formation of roads under rivers and canals, and in the construction of sewers and drains, &c., &c. Tunnels are now almost as common as canals and bridges. Amongst the many important works of this kind, may be mentioned, the eanal tunnel under Standidge, between Manchester and Huddersfield, which extends under ground upwards of three miles, and is 220 yards below the surface. The railway tunnel under Liverpool. The road tunnel under the Thames, at Rotherhithe, which, although completed only half-way, is an undertaking of great national interest, and will, whenever finished, prove of great public utility. It is thirty-eight feet in width, and in the style of a double arcade, as shown in a sectional representa tion, which we shall have shortly to introduce. The work was commenced in 182:5, by the building on the surface of the ground a circular brick tower, fifty feet in diameter, and three feet thick ; this tower was gradually undermined all round, and sunk, until it rested on clay, forty feet below the surface • a wall was then built from beneath, to meet the kirb on which it stood, till from the depth of sixty-four feet, the shaft was completed, and a well formed seventeen feet deep, and twenty-five feet diameter, in the centre of the area, to serve as a receptacle for any water that might collect in the works, and which always brings it under the command of the steam-engine pumps. The shaft was then broken through, to commence the tunnel, in which, it is said, considerable diffi culty was experienced. To give security and confidence to the men in exca vating, Mr. Brunel invented a cast-iron shield or frame, of great solidity, so as to be capable of withstanding an immense pressure. Its extreme dimensions were thirty-seven feet in width, twenty-one feet six inches in height, and seven feet in depth, horizontally. This shield was divided into twelve perpendicular frames, and each frame subdivided into three stories, called cells or boxes. The
utility of the framing consisted in its supporting the superincumbent weight, and in protecting and shielding the workmen employed from accident. One miner worked in each of the stories or cells, consequently, thirty-six men were enabled to pursue their operations at the same time. Each division had a roof of cast iron plates, polished on the upper surface, so as to slip easily over the stratum of clay which rested upon it; and was supported by two strong cast-iron plates, called shoes, and which rest upon gravel at the hue. The motion of each divi sion was thus effected :—Each of the miners in the three cells excavated the ground in front of him, to the depth of nine inches, until the perpendicular height of the soil in front of the division, which was to be advanced, was exca vated. He then supported the face of the soil by means of small planks called poling*, and shut them with screws to the adjoining divisions, which were at rest. The next operation consisted in unscrewing and slackening one of the legs, while the other supported the weight of the machine. The slackened leg was then advanced at two separate times to the length of nine inches, and then screwed up tight. When properly secured, the other leg was advanced, together with the shoes, in the same manner; and the division was then moved forward nine inches, by means of two horizontal screws and levers, one at the top and the other at the lower part of the division. One end of these screws was fixed in the frame, and the other abutted on the brickwork. Each of the divisions was moved in a similar manner, until the whole twelve were advanced nine inches, when the bricklayers immediately followed up with the brickwork and Dement, building one brick in length in straight joints. This brickwork again formed an abutment for the horizontal screws; thus the work proceeded, miter nately moving the machinery forward nine incbee, and following it up with a course of brickwork in cement.