Six years after its commencement the tunnel was drowned, so to speak, for a long time by a large spring of water which burst out from limestone, and arrangements had to be made to provide for this flood. It is now conducted by a subsidiary tunnel or channel to a huge shaft, where it is raised by pumps of sufficient strength. Then there was the perilous Salmon Pool to be dealt with. The river burst through here, and the rent had to be stopped with clay. The tunnel is twenty-six feet wide by twenty feet high, and is cut through Pennant stone, shale, and marl. It is lined with Staffordshire vitrified bricks throughout—seventy-five million bricks it is estimated being used. The works are ventilated by a huge fan, and pumping continually proceeds, something like twenty-six million gallons of water, it is said, being raised in the twenty-four hours. The tunnel, of which the engineers were Messrs. Hawkshaw, Son, Hayter & Richardson, and Mr. T. A. Walker, Contractor, is for the use of the Great Western Railway, and saves that Company’s Welsh and Irish trains to Milford a long way round by Gloucester. [Illustration: THE BORING MACHINE USED IN THE PRELIMINARY CONSTRUCTION OF THE ENGLISH CHANNEL TUNNEL.] In cutting the Mersey Tunnel, which was completed in 1886, machinery was used for some of the work. The machine bored partly to a diameter of seven feet four inches, but hand labour had to be largely depended upon. The plan pursued was to sink a shaft on either side of the river and drive a heading, sloping upward through the sandstone to the centre; this heading acting as a drain for any water which might appear. The thickness between the arch of the tunnel and the river bed is thirty feet at its least, and the tunnel, which occupied about six years in construction, and of which the engineers were Messrs. Brunlees & Fox, is provided with pumps raising some thirteen million gallons of water daily. As in the case of the Severn Tunnel, ventilation is provided for by huge fans. A boring machine was also used in the preliminary efforts for the construction of a tunnel under the English Channel. Holes, seven feet across and to the length of 2000 yards, have been bored by a compressed air machine, working with two arms furnished with teeth of steel. The construction of the tunnel is held to be quite feasible from an engineering point of view, and it is believed that it would pass through strata impervious to water, such as chalk marl and grey chalk.