1896.] on the Tunnel under the Thames at BlackwalL 87 



cells or pockets are open in front, and the shield is so used in hard or 

 stiff ground, but in the gravel beds the working face has, except when 

 the excavation is in progress, to be very carefully closed with the 

 wrought-iron shutters secured with screws as shown on the section, the 

 mode of working which will be presently described. 



Compressed Air. — We all know that air at the sea-level presses 

 with a force of from 14:| to 15 lbs. per square inch, and can support 

 a column of mercury of from 30 to 31 inches in height. We know 

 that it has bulk, for if we invert a tumbler in a basin of water there 

 will still be a space filled with air into which the water cannot enter. 

 If we try the experiment we shall find that this air space will be 

 larger or smaller depending on the depth to which we immerse the 

 tumbler, consequently we see that air is an elastic body. By proj)erly 

 constructed air-compressing pumps, we can force air down into a 

 diving bell until all the water is expelled from it and the surplus 

 escapes through the open bottom of the bell. If we then measure the 

 amount of compression of the confined air, we shall find it equivalent 

 to the weight of a column of water equal to the area of the open 

 bottom of the bell and as high as the depth of the water. 



It having been decided to use compressed air to keep out the 

 water from the tunnel during its construction, the question arose, what, 

 having regard to the health of workers, was the highest pressure 

 which could be adopted with safety, as on this clearly depended the 

 greatest depth to which the bottom of the tunnel and shafts could be 

 carried. In going into the matter, it was evident that it would not 

 be a case of one or two men occasionally going down to perform 

 some temporary work, but that gangs of from sixty to eighty men 

 would have to be kept at work night and day, for many months, con- 

 sequently a safe maximum had to be arrived at. In places in America, 

 men had worked under a pressure of 48 lbs. per square inch above 

 the atmospheric pressure, that is, 68 lbs. absolute ; at Stockwell on 

 the City and South London Kail way it was about 15 lbs. ; and 

 after many inquiries 35 lbs. per square inch or 50 lbs. absolute was 

 determined upon. 



I have stated it in this way because in addition to whatever 

 artificial pressure we may apply, it must be borne in mind that we 

 always have the initial pressure of the atmosphere to work under, 

 which is about 15 lbs. per square inch. In what follows, however, I 

 shall speak only of the artificial pressure, leaving it to be understood 

 that we always have the natural pressure in addition. If the extreme 

 safe pressure be fixed at 35 lbs. per square inch, it follows that the 

 bottom of the tunnel must not go lower than 80 feet below high 

 water mark. This being settled, the next point to be decided was how 

 large could we make a circular tunnel so that it did not project 

 upwards through the gravel into the river. In other words, what was 

 the safe minimum amount of cover that could be allowed over the top 

 of the tunnel and between it and the river bed. This, after much 

 consideration, was provisionally fixed at 6 feet, but in construction, 



