I9I0.] DAVIES— TUNNEL CONSTRUCTION. 171 



without the renewal and replenishment of the air. The greatest 

 possible capacity of bell could only, by any possibility, enable work- 

 men to continue work therein for a very short period, and to renew 

 the air the bell had to be brought to the surface. The admiral con- 

 ceived the idea that if air were pumped into a working chamber 

 it could be applied at a pressure equal to the full head and thereby 

 exclude water; and, going further still, that if such a chamber 

 were connected with the surface or outer air by a tube or other con- 

 nection and fitted with large valves, it would be feasible for the 

 workmen to freely pass into and out of the working chamber by 

 simple operation of the valves. This simple method he considered 

 as equally applicable to a caisson or tunnel and for the purpose of 

 excluding water. This is all the air lock is today — a large recep- 

 tacle of sufficient size for cars, buckets or men, fitted with flap 

 doors, one at each end, and opening the same way, that is inwards, 

 against the higher pressure, and connecting the working chamber 

 with the outer air at atmospheric pressure. It is immaterial whether 

 this air lock is fitted vertical, as is usual in caisson work, or horizon- 

 tal, as commonly applicable to tunnels, the essential condition being 

 that the lock be built into a bulk-head or diaphragm enabling a pres- 

 sure of air higher than normal atmosphere to be maintained inside 

 the working chamber, which, in tunnel construction, is usually a 

 length of completed tunnel, while in caisson work it is the lower 

 portion, between the air floor or diaphragm and the cutting edge. 

 The " modus operandi " is extremely simple. To enter air pressure, 

 the person enters through the open outer door or valve and closes 

 it behind him. He is then, so to speak, in a room with both doors 

 shut and at atmospheric pressure. He then admits air under pres- 

 sure until the pressure within the room is equal to that of the 

 working chamber inside, when, obviously, the inner door can be 

 opened and the passage effected. The use of double and triple locks 

 allowing varying stages of pressure are usual in tunnel work, when 

 high pressures of over twenty pounds are used, as increasing the 

 personal safety of men and expediting the operation in the decom- 

 pression. The honest observance of rigid rules of health makes the 

 the risks to workmen by air pressure almost nil. These are: (i) 

 Selection of men, limiting the age of men who have not been in the 



