88 Mr. Alexander B. Binnie [March 6, 



the least depth was somewhat less. It was due to these considera- 

 tions coniDled with the widths of the busy streets above spoken of 

 and the size of the largest vehicles, such as furniture vans, &c., 

 that the outside diameter was fixed at 27 feet. 



Having now described the work and some of the main conditions 

 under which it had to be constructed, you will have noticed that to 

 keep out the water, compressed air is employed, and that to drive the 

 shield forward, hydraulic pressure is used, the macliinery for which 

 requires a few words of description. For the purpose of air com- 

 pression, six steam engines and air pumps are provided, and these are 

 situate on the south bank of the river near shafts Nos. 3 and 4. They 

 have a united capacity of 1500 horse-power, but only about 1000 to 

 1200 horse-power are used continuously as one engine has to be kept 

 idle in case of accident or breakdown. When working at 1000 to 

 1200 horse-power, these engines and pumps force into the tunnel 

 about 8000 cubic feet of air per minute, or 17 tons weight per hour. 

 The air from these various engines is first conducted into a wrought- 

 iron reservoir 28 feet in length and 7 feet in diameter, formed like a 

 steam boiler. The first effect of compression, it is needless to say, is 

 to raise the temperature of the air very much, in fact to about 90° or 

 100° F., consequently before it can be conducted into the tunnel it has 

 to be cooled by passing it through a series of smaller tubes surrounded 

 with cold water like the surface condenser of a steam engine. From 

 the coolers it is led in pipes down shaft No. 4 and along the tunnel 

 through the air-tight bulkhead, presently to be dealt with, to the 

 working face. 



In describing the shield, I mentioned that it weighs about 250 

 tons, and that it has to be thrust forward as the excavation is com- 

 pleted by the twenty-eight hydraulic rams which abut or press upon 

 the last completed ring of the tunnel. To produce the necessary 

 total pressure of about 2800 to 3000 tons an hydraulic pressure up to 

 2| tons per square inch has to be maintained. This is developed by 

 two hydraulic engines of 70 horse-power, and transmitted in pipes 

 down shaft No. 4, along the tunnel and through the air-tight bulk- 

 head to the working face. 



I have previously spoken of a certain structure which I have called 

 the air-tight bulkhead. This I must now describe. It is clear that if 

 we are to use the compressed air in the tunnel to press against the 

 working face and keep out the water, it must in some way be con- 

 fined, or it would rush out backwards and escape up the shaft. To 

 confine the air in the tunnel, temporary air-tight walls or partitions 

 called bulkheads are built across it. As these have to bear an 

 outward thrust or bursting pressure of about 1000 tons, they are 

 formed of massive walls 12 feet in thickness, built of brickwork in 

 Portland cement. It is, however, obvious that they must not be 

 solid but must have means of access formed to allow of entrance 

 and exit both for men and materials. To permit of this access 

 air-locks have to be formed through the bulkhead. These are for 



