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, etc, etc. Tunnels are now almost as common as canals and bridges. Amongst the many important works of this kind, may be mentioned, the canal 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, and the tunnel under the Thames, at Rotherhithe, were undertakings of great national interest. The latter tunnel is now completed; but unfortunately, for want of means to complete the approaches, it is of less practical utility than it would otherwise be. It is thirty-eight feet in width, and in the style of a double arcade. The work was commenced in 1825, 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 difficulty was experienced. To give security and confidence to the men in excavating, Mr. Brunei 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 base.

The motion of each division 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 excavated. He then supported the face of the soil by means of small planks called poling8, and «hut 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 oement, building one brick in length in straight joints.

This brickwork again formed an abutment for the horizontal screws; thus the work proceeded, alternately moving the machinery forward nine inches, and following it up with a course of brickwork in cement Notwithstanding these ingenious contrivances for ensuring the progress of the work (which reflect great credit upon the talents of the engineer), an irruption of water took place on the 18th of May, 1827; and as some account of the circumstances attending it may prove of importance to persons engaged in, or about to undertake, similar works, we shall here give it from the pages of a periodical journal published at the time.

For several weeks previous to the irruption of the water, it was discovered, by the frequent descent of pieces of bone, brickbats, coals, etc, from the bed of the river to the works, that the earth, or rather the mud between the water and the tunnel, was exceedingly loose, and even at times in motion. Although much water had occasionally penetrated the works, the engine was found sufficient to remove it, and the work proceeded with very little interruption, till that time when the irruption of water between the shield and the brickwork was so great, as to oblige the men to make a hasty retreat, which they all did in safety. This irruption, which soon filled the tunnel, was much augmented by the action of the water on the last row of brickwork, before it was completed, and the cement had had time to set. On examining the bed of the river, after the accident, with the diving-bell, a spacious cavity was discovered over the spot, which terminated in a small hole, descending into the tunnel between the shield and the brickwork, as represented in the annexed sectional sketch.

This hole, as well as a second, which subsequently broke out in another part of the cavity, was afterwards filled up with bags of clay, and large quantities of loose clay and gravel, thus making an artificial bed to the river; and this new-made part was protected from the effects of the tide, by a raft thirty-five feet square, skirted with a tarpaulin, covering, in all, about 8,800 square feet. After a while, this artificial covering having sufficiently settled, the water was drawn off by the engine, and the workings recommenced: after clearing away all obstructions before the shield, that piece of mechanism was found to be quite uninjured.