THE STORY OF A GREAT WORK. 



465 



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The tunnel could not have been built without this shield. The 

 credit of its invention appears to be due to Mr. Alfred E. Beach, 

 of New York, who designed it in 1868 for use in the construction 

 of the tunnel under Broadway. It was subsequently used in 

 Buffalo, Chicago, at the Hudson 

 River Tunnel, and other places. 

 The use of the shield in tunneling 

 was first introduced by Sir Mark 

 I. Brunei in 1825, and it was after- 

 ward employed by Mr. Greathead, 

 in the Thames Tunnel and other 

 works ; but the St. Clair shield dif- 

 fers in some important respects 

 from any before employed. It is a 

 cylinder of iron, twenty-one feet 

 and six inches in diameter and six- 

 teen feet long, built of steel one 

 inch thick, and with a sharp cut- 

 ting edge in front. It is divided 

 into twelve compartments by two 

 horizontal and three vertical stays. 

 It weighs fifty tons, and was built 

 on the spot, the material having 

 been prepared in the workshops at 

 Hamilton. Against the rear end 

 of the shield were ranged twenty- 

 four hydraulic rams, eight inches 

 in diameter and having a stroke of 

 twenty-four inches. These forced 

 the cutting edge forward into the 

 clay, which was then excavated 

 within the shield. By means of a 

 Worthington pump, a pressure of 

 five thousand pounds per square 

 inch, or three thousand tons in all, 

 could be exerted. The greatest 

 pressure used was seventeen hun- 

 dred pounds per square inch, or a 

 thousand and sixty tons in all. The 

 pressure could be exerted on any 

 or all of the rams so as to preserve 

 the true direction of the shield. 

 The keeping of this direction was 

 one of the interesting engineering feats of the work. It was done 

 by means of a specially made London transit, set on masonry, a 

 series of disks and cross-wires indicating the slightest deviation. 



VOL. XLV. ."(5 



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