Page:Popular Science Monthly Volume 83.djvu/347

 heaviest ships now afloat have a draft of about 36 feet, a depth of 40 to 45 feet in anticipation of increased draft, should be provided, as is wisely made in the Panama Canal. The roadway level would then lie about 60 feet below the surface, instead of down beneath Beaufort's Dyke, and the approaches would be proportionately shorter. In a submerged position they would not be subject to pressure from wind and wave, but would lie in comparatively quiet water. The chief objection to these tubes is their deterioration from rust, and the difficulty or impossibility of repairing them under water. When the metal is corroded through, the concrete or other lining would be the only remaining material for resisting external pressures. Since the tubes would be supported along their whole length, they would need a comparatively small section for strength, and if lying on the surface, the tubes would be proportioned like ships, to receive varying support, and to bridge the waves from one crest to another. The weight of the submerged tubes should be such as to nearly equal the weight of water displaced. If slightly less than the weight of water, there would be an upward pull on the anchors when empty, and a corresponding downward thrust under moving load, to be resisted by surface floats. If so arranged that the upward and downward pressures are equal, the forces under normal conditions would be a minimum, and the cross section of the tubes might be nearly or quite uniform throughout. The tubes would be made in convenient lengths of 200 to 400 feet, with their ends temporarily closed, and floated out into position and sunk to their desired depth, where they would be connected. The temporary dams should be 3 to 4 feet back from the ends, leaving space for the divers while bolting the sections together. A submerged viaduct of this kind across the Irish Channel could probably be built for $25,000,000 to $30,000,000, and the type is the most promising of all to put into execution.

Whether or not a channel railway would be profitable as a financial investment is uncertain, but it would accomplish a far greater purpose than merely earning dividends, for it would increase commercial activity in Ireland, and with government security for interest on the investment, it might in the end be one of the easiest means of bettering conditions there.

A high-level bridge over water of such great depth would necessarily be on floating piers, and its cost would be from $150,000,000 to $200,000,000—so great indeed as to be prohibitive.

The construction of a channel railway to Ireland, and a ship canal from Galway Bay to Dublin, the cost of which would be about $40,000,000, are of the utmost importance to the prosperity of the island and its people, for ocean ports would then be established on the west coast of Ireland, and this long neglected part of Great Britain would participate to a greater extent in the general welfare.