Page:Popular Science Monthly Volume 45.djvu/485

Rh and a half, the space under the tube when the shield moved forward was filled with cement. The clay was allowed to settle down on the upper part. When the shields met, the tube was built up within them to the junction and the shells of the shields allowed to remain. The inside of the tube is finished with a preparation to keep it from rusting.

On Sunday, August 24, 1890, the two excavations had approached so nearly that an opening was made with an earth auger, and the workmen talked and passed articles to each other. The earth was soon removed, and Mr. Hobson, the chief engineer, and others connected with the tunnel company, stepped through. Six days later the shields came together and the success of the great undertaking was assured. In its construction about seven hundred men were employed, of more than average intelligence, who took great interest in the work.

The actual length of the tunnel, from portal to portal, is six thousand and twenty-six feet. Of this, two thousand three hundred and ten feet is under the river, one thousand nine hundred and eighty-two feet under dry ground on the Canada side, and seventeen hundred and thirty-four feet under dry ground on the United States side. The open excavation to reach the ground level.on the Canadian side is three thousand and sixty-one feet, and on the United States side two thousand four hundred and sixty-six feet. The grade is one in fifty, except under the river, where it is practically level, only sufficient incline—one tenth per cent—being given toward the Canadian side to provide for drainage. The depth of the lowest part under the mean level of the river is 77·83 feet. The minimum depth between the top of the tube and the bottom of the river is fifteen feet, the average being twenty-five feet. It was necessary to place it as far down as possible in the clay, consistent with the grade, so as to overcome the tendency of a tube filled with air to rise to the surface in water or mud. The bottom is about nine feet above the rock which underlies the clay. On the Canada side the bottom is sixty feet below the surface of the ground at the portal, on the United States side it is eight feet less. The bottom of the tunnel at its lowest point is one hundred feet below the railway track on the level, which indicates the total ascent and descent which trains have to make in passing through. Ventilation is secured by the motion of the trains, which is found to be ample for the purpose.

The trains are drawn through the tunnel by powerful locomotives belonging to the tunnel company, specially built for the purpose. They take eighteen loaded cars at a trip.

The track in the tube is supported on solid brickwork, as shown in the accompanying cross-section. It was at first proposed to build the tunnel wide enough for two tracks, but it was found