1896.J on the Tunnel under the Thames at BlacJcwall. 89 



a similar purpose, and act in a like manner, to the locks on a canal. 

 la one case we have to overcome a difference of water-level, and 

 in the other a difference of pressure between that of the ordinary 

 atmosphere outside and the working pressure produced by the 

 air-compressing engines inside the bulkhead, be it 20, 26, or 

 30 lbs. per square inch. The air-locks consist of wrought-iron 

 cylinders, 15 feet in length and 6 feet in diameter, securely built 

 into the brickwork of the bulkhead. There are two of these air- 

 locks at the level of the road near the bottom, each provided with two 

 doors 5 feet by 4 feet fixed at either end of the lock and opening 

 inwards towards the pressure inside. There is another but smaller 

 air-lock placed near the top of the tunnel to permit of escape in case 

 of accident. Supposing you wish to enter, the outer door is open, 

 but the inner one closed and pressed against by a force of say 30 tons. 

 It is clear that you cannot open this door until you have equalised 

 the pressure on both sides of it. To do this you enter the lock and 

 close the outer door to prevent the escape of air, after which a tap or 

 cock is opened which permits the compressed air from inside to rush 

 into the lock until the pressure within it is equal to that on the inner 

 side of the balkhead. As soon as this equality is established the 

 inner door can be opened and you step into the working pressure. 



Visit to tie Tunnel. — In attempting to describe the work of 

 construction, I do not think I can do better than in imagination to 

 conduct you over the work during a visit of inspection. It is first 

 necessary for ladies and gentlemen alike to put on waterproof boots, 

 woollen overalls and caps so as to keep dry and clean ; these are in 

 readiness for the purpose at the tunnel. Descending by the steps at 

 the end of the open approach on the south side of the river, we pass 

 for over 300 yards through the finished cut and cover portion of the 

 work and have an opportunity of noticing what will be the general 

 size and appearance of the interior of the tunnel, and that, although 

 it is all below high-water mark and its lower end beneath the level 

 of the bed of the river, yet it is quite dry and dusty under foot. On 

 reaching the bottom of shaft No. 4 the large steam pumps for lifting 

 out the water during construction will be noticed, for it must be 

 remembered that although the work when finished will be quite dry 

 and water-tight, yet during construction, even with the use of com- 

 pressed air, a large volume of water enters the work and has to be 

 got rid of. This mainly arises from the fact that the difference in 

 hydrostatic external pressure due to the 27 feet in height of the 

 shield amounts to about 12 lbs. per square inch. So that if the full 

 air pressure due to the external hydrostatic pressure at the bottom 

 of the shield and working face were always kept up, it would escape 

 in too large volumes from the top of the excavation and through 

 any porous soil. In fact, to prevent this too rapid escape of air 

 through the gravel, as well as to weight the material over the 

 shield, where the covering was least in thickness above it, clay 

 was deposited in the bed of the river for a width of 150 feet, 



