The Viaduct Works' Handbook

49 handles, hy which they may he worked from above the surface of the water. The centre part of the scoop is formed into a leather bag, into which the earth is scooped and brought to the surface. The cylinder is weighted with a load, so that it may sink as the earth is excavated. If the bed of the river happens to con- sist of clay, when the cylinder reaches this stratum, the water is pumped out, leaving the bottom dry, as the clay will effectually exclude the water, and the operation is carried on by men going inside, as in well-sinking. If a water-tight stratum is not found, and the depth is too great for the use of scoops, the operation is carried on by divers, or by the pneumatic process, which is known as Hughes method, and may be briefly described as follows. A temporary air-tight cover is put upon the cylinder, in which is formed a box of sufficient dimensions to contain a man and a bucket of earth, with a door opening in- wards from the box to the cylinder, and another outwards from the box to the atmosphere; the cylinder is thus formed into a diving-bell, with entrance from the air through the cover or box. The air is forced into the cylinder with such force as to drive all the water out under the edges of the cylinder below, and the work- men carry on the sinking inside in the dry, and can go in or out at any time. A man going in passes into the box described above, where he shuts himself in from the atmosphere, and opens an air- cock which communicates with the cylinder, and immediately pro- duces equilibrium of pressure in the cylinder and box ; he is then able to open the inner door and enter the cylinder. If he wishes to come out, the operation is simply reversed—he shuts himself in the box, and opens a cock which communicates with the atmosphere, when equilibrium of pressure in the box and atmosphere is imme- diately restored; he can then open the door and come out, bringing with him the earth excavated. Many other processes of sinking have been tried from time to time, but none have been so successful as that described above, the just value of which may readily be appreciated when it is considered that, should a cylinder strike partially upon a large boulder stone or the trunk of a tree, as is sometimes the case, the process of sinking can be carried on when it might otherwise have become impossible. D