SUBMARINE TUNNELING 225 



wood, with the heading in white limestone, there was every 

 reason to expect further soft seams where the rock should, 

 change to the granite gneiss of Blackwell's Island. These- 

 expectations were not disappointed ; for after passing throughi 

 350 ft. of the limestone, and when within 200 ft of Blackwell's- 

 Island, a soft seam was met, and air pressure had to be once- 

 more used in the heading. As this seam was but 14 ft. in 

 width, and presented no especial difficulties, the tunnel was- 

 carried through it without using the shield, the cast-iron seg- 

 ments being erected under a timber roof. Gneiss was encoun- 

 tered on the other side of this soft vein, which brought with it 

 the assurance that the last of the soft ground had been passed. 

 On May 16 serious loss and delay were caused by a fire which 

 destroyed the New York works. The fire started in an adjoin- 

 ing picnic ' ground, containing many light frame structures, 

 which caused so fierce a conflagration that it was impossible to 

 save our works. This caused a delay of three weeks in the 

 time of the tunnel's completion. On July 11, 1894, the re- 

 maining 1.") ft of rock l>etween the headings was blasted away, 

 thus opening the pioneer tunnel under the East River, two 

 years from the time when ground was first broken. Some 

 weeks were spent in clearing up and shutting out the water in 

 the wet places. A 3-ft gas main was now laid through to 

 New York, and nn Oct. 15 gas was delivered into the city, 

 accomplishing the purpose of the tunnel. 



VAN BUREN STREET TUNNEL, CHICAGO. 



The Van Buren Street tunnel in Chicago belongs to that 

 class of submarine tunnels which has been designated as 

 tunnels on the river Ix-d, by which it is meant that the top of 

 the tunnel is flush with, or extends slightly above, the bed of 

 the stream. Two methods an- available for constructing these 

 tunnels; vi/.., the cofferdam method and the caisson method. 

 The cofferdam iiictli<d has been actually employed in several 



