120 RAILWAY CONSTRUCTION. 



top or upper working length of the cylinder, and no access can 

 be obtained to the interior of the cylinder without passing 

 through this air-lock, which has one lower door or valve opening 

 into the cylinder, and an upper door opening out into the open 

 air. Temporary inside staging is formed by putting planks across 

 from flange to flange, and placing short ladders on these landings 

 for the use of workmen descending or ascending. The excavated 

 material is hoisted by a winch, generally placed on the landing 

 just under the air-lock. The air-pump is placed in some con- 

 venient position outside, near at hand, the pressure-pipe passing 

 through the air-lock into the interior of the cylinder. Air is 

 forced into the cylinder to a pressure sufficient to drive out and 

 keep out the water from the interior, and allow the workmen 

 free access for excavating the material round the cutting edge 

 and base of cylinder. The amount of pressure required will 

 depend upon the depth of the working below the level of the 

 water alongside. Men accustomed to the process can work with- 

 out much inconvenience under a pressure of 20 to 22 pounds per 

 square inch, equal to a depth of 45 to 50 feet; but when the 

 pressure exceeds 25 pounds, the duty becomes very trying, and 

 is attended with considerable risk. Instances are recorded of 

 men working at depths of 105 and 110 feet, necessitating a 

 pressure of over 45 pounds per square inch; but it is very 

 questionable whether the men exposed to such a severe ordeal 

 were not permanently affected, if some of them did not actually 

 succumb. 



It will sometimes occur that, after sinking through soft porous 

 strata to a considerable depth, a layer of clayey material is 

 penetrated sufficiently retentive to keep out the water and per- 

 mit of the removal of the air-lock and the completion of the 

 sinking as an open-top cylinder. 



When working on the plenum system everything must pass 

 through the air-lock, both materials and men. The excavated 

 material is hoisted up to the level of the air-lock, the upper and 

 lower doors of which must be closed, and the pressure inside the 

 air-lock brought to the same as that inside the cylinder by means 

 of a regulating valve. The lower door is then opened to admit 

 the excavated material, and then closed again to cut off all com- 

 munication with the interior of the cylinder. The upper door is 

 then opened, and the material hoisted out into the open air. 

 The same process has to be adopted for the egress of the work- 



