RAILWAY CONSTRUCTION. 119 



with water-tight joints, the cylinder is placed in its proper 

 position on the ground or lowered into the water prepara- 

 tory to sinking. The lower length is made with a sharp 

 cutting edge to facilitate penetration. By excavating and 

 removing the material round the cutting edge and base inside 

 the lower length, the cylinder descends gradually either from 

 its own weight or by assisted weights, and length after length 

 is added until it is sunk to the depth required. The excavated 

 material is filled into buckets and hoisted to the surface by a 

 winch fixed on the top length. When sinking in water the 

 working top of the cylinder is always kept at a suitable height 

 above the water for convenience in removal of the earth or clay 

 from the interior to barges or gangways alongside. 



Some strata are more favourable for cylinder sinking than 

 others. Material of a strong clayey nature admits but a small 

 amount of. water into the excavation, and a moderate-sized 

 pump will keep the working fairly dry until considerable depth 

 has been reached. Some other materials are so open that the 

 water cannot be kept down with ordinary pumps, and the 

 cylinders can then only be lowered by the pneumatic process. 

 This process has been carried out in two methods, one of them 

 on the vacuum principle, and the other by air pressure, or, as 

 it is termed, the plenum system. With the former method 

 the cylinder is placed in position, and an air-tight cap, through 

 which a pipe passes, is secured on the top. Powerful air-pumps 

 are then set to work, and the partial vacuum thus created in the 

 interior causes the material round the cutting edge and base to 

 be loosened and drawn into the cylinder, the cylinder at the 

 same time going down or sinking by its own weight, or assisted, 

 if necessary, by added weights. The cap is then taken off, and 

 the material removed from the interior, the operation of exhaust- 

 ing and emptying the interior being repeated until the cylinder 

 is sunk to its proper depth. This method has been found to 

 work well in strata which contained a large proportion of clay 

 to assist in excluding the air and water, but was not nearly so 

 successful when applied to material containing stones and large 

 boulders. 



The plenum process is based on the principle of the diving- 

 bell, the water being prevented from entering at the bottom by 

 keeping the cylinder full of compressed air. An air-chamber, or 

 air-lock, with perfectly air-tight joints, is securely fixed to the 



