Engineering Wonders of the World
Volume I

THE TUBE RAILWAYS OF LONDON. 303 Fig. 9.—price’s rotary digger, back view. m, electric motor; gc, gear case; p, pinion driving b, rack. feet from the working pit at Golder’s Green, and this stoppage naturally delayed the work considerably. Under favourable conditions the rotary pro- gresses much faster than the Greathead shield. The superiority in this respect was very Greathead and Rotary Shields compared. noticeable in extremely hard clay, which offered excessive resistance to the cutting edge of the Greathead, but was pared away with comparative ease by the rotary, the “ feed ” of which could be adjusted to suit the material exactly. On the other hand, in some kinds of ground the Greathead worked better ; and on the score of accurate alignment and freedom from subsidence of tunnel lining set with its aid, the same type proved more satisfactory. TUNNELLING IN WATER- BEARING STRATA WITH COMPRESSED AIR. Shield - tunnelling through dry strata such as the London clay, in which by far the greater part of the Tube rail- ways has been driven, does not in itself require the em- ployment of compressed air to help the operations of mining at the face, although in some cases where tunnels have been driven under or close to im- portant buildings compressed air has been used, so that the ground exposed at the work- ing face may have the addi- tional support due to the elastic reaction of the air. In water - bearing strata compressed air is used prim- arily to exclude water which otherwise would enter the workings and have to be re- moved by continuous pumping ; and it should also be noticed that this method prevents the entrance of sand and dangerous settlements of land and buildings above. The employment of compressed air as an aid to underground mining operations and shaft- An Admiral’s Invention. sinking was first suggested by the famous British admiral, Sir Thomas Cochrane, afterwards Lord Dundonald. In 1830 he took out a patent for an apparatus for maintaining high pressure at the working face of a tunnel. This apparatus included air- locks, through which men and materials could pass from the ordinary air into the com- pressed air, and vice versd. An air-lock, as now applied to tunnels, con-