754 REPORT — 1894. 



the shield. A hanging iron screen in each compartment ahout 6 ft. back from 

 the cutting-edge forms a safety chamber at its back, where men could stand with 

 their heads abore water in case of a rush of water in the face due to air blowing 

 out suddenly, or from other causes. Provision is made for using iron poling boards 

 at the face, shoved forward by jacks, when in ballast, if necessary. The shield, which 

 weighs about 250 tons, is shoved forward by twenty-eight hydraulic jacks fixed at 

 the back and butting against the cast-iron lining, and able to exert a total pressure 

 of over 3,000 tons. 



At the present time the portion of the tunnel between shafts No. 4 and No. .3 

 has been constructed. The shield was started from No. 4 shaft in June 1893, 

 the first permanent ring being erected on June 0. When first starting the face 

 was nearly all clay, there being a little ballast at top and a little tine sand at the 

 bottom. It was therefore decided to go on without compressed air for some time, 

 and in three months seventy-seven rings, being equivalent to a length of 192 lineal 

 feet, were erected. This length was done with practically no timbering, except in 

 the top ballast for a short length, in which a good deal of water was met. A little 

 previous to this time the cutting-edge at the bottom of the shield got bent by 

 shoving the shield against a layer of rock which was met in the clay, and it was 

 found now to have got much worse. It was therefore considered better to drive 

 a timbered bottom heading in front of the shield, and put in a concrete foundation 

 on which the shield could be shoved forward to the next shaft. This was done 

 for the bottom 6 ft. of the shield, all the excavation above this level being done 

 without timbering. In this way by the middle of December, six months after 

 starting, 191 rings, equivalent to 477 lineal feet, had been erected. 



As the tunnel was going down a gradient of 1 in 3G, and the top of the sand 

 remained nearly level, the latter was now up to the middle of the shield, and was 

 carrying a great deal of water. On December 16 there was a rush of water and 

 sand, which filled the heading with sand, and the water rose 15 ft. in the tunnel 

 at the shield. 



As the shield was now close to No. 3 shaft, which was not yet sunk to its full 

 depth, it was considered safer not to continue the tunnel until the shaft was sunk, 

 and in the meantime to mal<e preparations for using compressed air when work 

 was again started. A concrete bulkhead was therefore built across the tunnel, with 

 air-locks in it, a little back from the shield, and when tlie shaft had been sunk to 

 its full depth, work was recommenced in the tunnel on March 23, under air pressure 

 of about 15 lb. to the square inch. 



No further trouble from water was then experienced, and by going on in the 

 same way as above described with a bottom heading, the shield was got into No. 3 

 sh alt in May. It is now being strengthened and repaired there, preparatory to 

 starting under the river from No. 3 to No. 2 shaft. 



In the deepest portion of the river, where the top of the tunnel comes within 

 7 ft. of the river bed, which is all gravel at this point, it is proposed to lay a 

 bed of clay 10 ft. thick in the bottom of the river, to prevent the compressed air 

 blowing out ; and with this precaution it is hoped that no difficulty will be met 

 with in this portion of the work which cannot be overcome by the experience 

 already gained in the portion between shafts No. 3 and No. 4. 



SATURDAY, AUGUST 11. 



The following Papers were read : — • 



On Methods that have been adopted /or measuring Pressures in the Bores 

 of Guns. By Sir A. Noble, K.C.B., F.B.S.See Reports, p. 523. 



