Page:Popular Science Monthly Volume 45.djvu/484

466 Observations were made every day and the results marked on a diagram. The deviation was rarely found to exceed a quarter of an inch, and any error was corrected by adjustment of the hydraulic jacks. When the shields came together they were found to be exactly in line.



At one time it was feared the work would have to be abandoned. When the tunnel from the Canadian end reached the bed of the river, quicksand and water caused much trouble, but by the use of compressed air the difficulty was surmounted. At the line of the river on each side a bulkhead of brick and cement was built across the tunnel, with two air chambers, provided with airtight doors. The greatest atmospheric pressure necessary to prevent an inroad of sand and water was thirty-seven pounds per square inch, and under this pressure, after a short experience, the workmen found no difficulty in pursuing their task, in half-hour shifts. The use of compressed air had to be resorted to at two points.

The completed tunnel, as already stated, is an iron tube. This tube is built up of rings, eighteen inches in width, one of which was put together within the shield each time it was moved forward. Each ring consists of thirteen sections and a key piece, flanged to enable them to be bolted together. The body of the section is two inches thick, and the flanges are six inches wide. Each section weighs about one thousand pounds. The pieces were lifted and placed in position by a revolving crane, a complete ring being put up in about one hour. To ease the pressure and make the joints watertight, the edges were planed and strips of oak and tar canvas inserted. The sections were also heated and dipped in pitch. The tube being only twenty-one feet in diameter, while the shield was twenty-one feet.