466 



THE POPULAR SCIENCE MONTHLY. 



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 in- 

 road 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 



It 1, I^^Bl ^ half-hour shifts. The use of com- 



pressed 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 tliick, and the flanges are 

 six inches wide. Each section weighs 

 about one thousand pounds. The 

 1 )ieces were lifted and placed in posi- 

 tion 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 sec- 

 tions were also heated and dipped in pitch. The tube being only 

 twenty-one feet in diameter, while the shield was twenty-one feet. 



