I9I0.] DAVIES— TUXXEL COXSTRUCTIOX. 169 



some material to work in — portions in river silt, where, theoretically, 

 there should have existed twelve feet of silt cover and where at 

 times there was actually none at all until clay was dumped to make 

 an artificial cover, and that finally the work was carried on in every 

 combination of these materials. Below the Hudson River, where 

 the deep channel of 65 feet of water flowed, the tubes on both the 

 up-town and down-town systems passed from silt to full rock and 

 from full rock to full sand ; consequently the sections at every point 

 continually changed. 



The essential factors in all tunnel construction may be summed 

 up as follows: (i) The removal and disposition of the material; 

 (2) the support of roof and sides during construction; (3) the 

 elimination or disposition of water entering the work; (4) the 

 provision of a safe place for workmen engaged on construction ; 

 (5) the construction of a permanent lining and waterproofing it. 



Carlyle, after disserting on the weakness of man, writes: " Nev- 

 ertheless he can use tools, can devise tools. . . . Nowhere do we 

 find him without tools, without tools he is nothing, with tools he is 

 all." 



In no line of engineering work are the tools more essential than 

 in the modern art of tunnelling. In every type of structure and 

 in every material the proper and efficient tools are the prime factor 

 in the advanced new art of this work. 



In rock tunnels the combination of the modern compressed air 

 operated rock drill with fumeless high explosives; in soft ground the 

 use of compressed air, mechanical haulage, electric light and power, 

 steel temporary false works and reinforced concrete or iron plate 

 lining; and for subaqueous work the use of a shield and other me- 

 chanical appliances, has to-day converted into an essentially me- 

 chanical process what at one time was replenished by brute force. 

 The power plant needed for carrying out such a piece of construc- 

 tion is of tremendous extent. For the Hudson and Manhattan 

 tunnels, for example, and exclusive of the Sixth Avenue subway, 

 the combined power plants aggregate some 13,500 horse power 

 boiler capacity, operating high and low pressure air compressors, 

 electric generators, and hydraulic pumps delivering water to the 

 shields at a pressure of 5,000 pounds per square inch, as well as 



