1896.] on the Tunnel under the Thames at Blackwall. 91 



inch, probably caused by the larger amount of oxygen absorbed by 

 the lungs ; every one appears to speak with a nasal intonation, you 

 cannot whistle, and the skin acts more freely than at the same tem- 

 perature under normal conditions. I should here note that no one 

 becomes ill from the effects of compressed air while under its pressure, 

 the baneful effects, if experienced at all, usually show themselves on 

 coming out of it. But I have arrived at the conclusion that among 

 otherwise healthy persons some can and some cannot withstand air 

 pressure, and 1 have had the pleasure of conducting many persons 

 over the works, from little girls of thirteen up to gentlemen of over 

 seventy years of age, who have not felt the least ill effects from com- 

 pressed air. 



Passing on to the shield and the working face we see the two main 

 operations in progress: (1) excavating; and (2) erecting the cast- 

 iron rings of the tunnel. 



Excavation. — As to the excavation, the mode of conducting it 

 depends on the kind of ground being pushed through. If it be hard 

 or stiff enough to stand with a vertical face when pressed against by 

 the various partitions of the shield, the men simj^ly dig or pick it 

 away in front for a few inches or a foot or two, passing the excavated 

 material out to the stage behind the shield, from which it is tipped 

 into wagons and removed. After a sufficient amount has been cleared 

 and loosened in front of the shield, the latter is, by the hydraulic jacks, 

 pressed forward, it may be a few inches or perhaps 2 feet 6 inches, 

 the distance depending on the nature of the ground. Each ring dis- 

 places 54 cubic yards, and progress has varied from 1 foot up to 

 10 feet a day. If, however, the material be gravel the progress is 

 very slow as this material will not remain vertical when dug into, 

 but runs down as fast as it is excavated. Besides which, so rapid is 

 the escape of air that if precautions were not taken it would pass out 

 in dangerous quantities. To obviate this and to support the face, 

 the front of each pocket or working face is closed with three wrought- 

 iron shutters pressed forward by powerful screws, and all the joints 

 luted with clay. In these circumstances the excavation is made either 

 by raking out the gravel through holes in the shutters, or by drawing 

 them back one at a time, digging out a small portion and then screw- 

 ing forward the shutter again. When all the shutters have been 

 screwed forward the shield is advanced, and as the screws are so 

 arranged as to allow of their slipping through the nuts attached to 

 the shield, the result is that it moves forward past the shutters which 

 remain in the positions into which they have been screwed. It need 

 not be said that this is slow and tedious work requiring great skill 

 and patience. 



Erecting the Mings. — After the shield has been pressed forward so 

 as to leave a clear space in the tail of 2 feet 6 inches, the erectors are 

 brought into work, and, as before described, the various segments and 

 the key-piece erected. It will be noticed that as the tail of the 

 shield overlaps the last finished ring of plates it leaves an annular 



