Page:Popular Science Monthly Volume 83.djvu/346

342 over to Scotland would lead in a northwesterly direction, the present water route to England and London would he shorter and quicker, and it is doubtful if the causeway would be practicable.

Investigations show that the best way of establishing a channel railway to Ireland is either by means of a submarine tunnel or a floating tube, the cost in both cases being much less than a bridge or causeway. The deep water of Beaufort's Dyke could be avoided by selecting a location from Laggan Head to Maiden Island and thence over to Antrim, or by bending the tunnel to the north around the end of this depression, and thereby increasing the tunnel length under the water to twenty-five miles, with a total length of thirty-five miles, including the approaches. A two-track bore, 150 feet below sea bottom, under water 500 feet deep, would probably cost $50,000,000 to $60,000,000 and the possibility of building it would depend wholly upon finding rock strata absolutely water-tight and impervious. Tunneling with compressed air to exclude water is possible only at depths not exceeding about 150 feet below the surface of the water, for then the atmospheric pressure reaches 75 pounds per square inch, and pressures and depths not exceeding half these amounts are usually enough for effective work. It is evident, therefore, that water could not be excluded by means of compressed air at the depth which would be necessary beneath the Irish Channel, and, as previously stated, the possibility of carrying on such work would depend entirely on finding impervious strata, the presence of which could not be definitely determined until the bore was made. The overhead thickness of rock in the Mersey tunnel is 30 feet, and in the Severn tunnel it is 40 feet.

It appears, therefore, that a tunnel would cost from $35,000,000 to $50,000,000, according to its location and other conditions, and would require from ten to twelve years for its construction.

Another method of crossing the channel is by means of floating tubes, lying either on the surface of the water or anchored far enough beneath the surface to allow ships to pass over out. While no structure of this kind has been built, somewhat similar submerged floating piers have occasionally been used, such as those under two swing bridges at Dublin, and the later one at Norwich, England. Very interesting designs for floating piers also appeared in a recent competition for a new bridge over the Hooghly River at Calcutta. The type is probably the most practical of all methods for crossing a navigable channel of so great a depth.

If the tubes were to float upon the surface, openings must be left for water travel, and this can be done by depressing occasional sections, four to five miles apart, leaving openings of 1,000 feet, the presence of the openings being indicated by signals. The depressed sections would connect with those on the surface by means of suitable grades. As the