St I5.M A KIN E TUN N KLI N(J 



shield. Although Barlow made vigorous efforts to* get hi* 

 shield used, it was not until 1868 that an opportunity pre- 

 sented itself. In the meantime the inventor had been studying, 

 how to improve his original device, and in 1868 he secured addi- 

 tional patents covering these improvements. Briefly described 1 ,, 

 they consisted in partly closing the shield with a diaphragm,, 

 as shown by Fig. 124. The iminclosed portion of the shield is= 

 here shown at the center, but the patent specified that it might 

 also be located below the center in the bottom part of the 

 shield. The idea of the construction was that in case of an 

 irruption of water the upper portion of the shield could be 

 kept open by air pressure, and work prosecuted in this open 

 space until the shield A 



had l>een driven ahead 

 sufficiently to close 

 the aperture, when 

 the normal condition 

 of affairs would be 

 rer.inned. This was 

 obviously an improve- 

 ment of real merit 

 The partial diaphragm 

 also served to stiffen the shield somewhat against collapse, but 

 tin- thin plate cutting-edges and most of the other structural 

 weaknesses were left unaltered. To summarize briefly the* 

 improvements due to Balow's work, we have : the construction 

 of the shield in a single piece ; the use of compressed air and 

 a partial diaphragm for keeping the upper part of the shield 

 open in case of irruptions of water; and the injection of liquid" 

 cement to fill the voids In-hind the lining. 



Turning now to the London Tower tunnel work, it may 

 first lx> noted that Ilarlow found some dilh'culty in finding a 

 contractor who \\as willing t<> undertake the job, so little 

 confidence had engineers generally in his shield system. One 

 man, however, Mr. J. II. (ircathead. jvreeived that Barlow's 



Longitudinal Section. Cross Section . 



FIG. 13*. Second Shield Invented by Barlow. 



