1882.] on the Makinij and Worlcing of a Channel Tunnel. 131 



while when compressed air is used as the motive power, the ventilation 

 is effected by the very spent, or exhaust, air from the machine itself. 



Next, as regards the lining of the tunnel. It must be remem- 

 bered that in excavating through ordinary soil needing lining, heavy 

 timbering has to be put up as the work goes forward, and then the 

 lining has to be executea n\ 1th the greatest care, following up the work 

 as closely as possible, to enable the timber to be removed. In the case 

 of the Channel tunnel, made of a truly cylindrical form through such 

 a material as the grey chalk, which upholds itself so well that it is 

 doubtful whether there need be any lining at all, no timbering will 

 be required, and there will be no urgency in putting the linino" in. 

 Moreover, the excavation being of absolutely uniform dimensions, 

 and of regular shape all round, it will be the simplest thing imagin- 

 able to make on the surface concrete blocks of just such size as to be 

 easily lifted by small hydraulic, or compressed air, cranes, and of the 

 form of the 17 feet cm-vature outside and the 14 feet curvature 

 inside ; and without any scaffolding or permanent centering of any 

 kind, with no more than the appropriate elevating ajDparatus, this 

 lining would be laid in the excavation, beginning at the bottom and 

 working upwards at the two sides, and then over the crown, where 

 it would be supported by travelling centering mounted upon wheels, 

 and moved forward as the work progresses. 



Let me ask you to compare in your own minds the condition of 

 things, as regards ease of excavation, economy, and ventilation which 

 would obtain in a timnel thus constructed with that which obtains in a 

 tunnel such as the St. Gothard, or the Mont Cenis, carried throuo-h hard 

 rock. In the tunnel through the chalk, in lieu of men crowded close 

 together at the working face tending upon machines to drill holes to 

 receive cartridges, we have a machine needing the j)resence of certainly 

 not more than three men at the outside ; in lieu of the delays occa- 

 sioned by the withdrawing of the machinery to admit of the holes 

 being charged with dynamite, and the need of the retreat of the men 

 to a safe place while the dynamite is exploded, and the interference 

 with the ventilation caused by this explosion (coupled with the 

 uncertainty when the explosion has taken place that the shots will 

 have been satisfactory), and the further delay caused by getting out 

 the blasted material to put it into waggons and carry it away ; we 

 have continuous action of the machine, we have no blasting and there- 

 fore no bad atmosphere produced by it, and no uncertainty as to the 

 effect of the blasts, neither have we any delay in the work while the 

 material is being removed and put into the waggons. Moreover, we 

 are free from all the dangers attendant upon the use of explosives. 



Then again, consider om* condition as compared with a tunnel 

 through an ordinary non-rocky material : we have not the exj)ense of 

 providing the timber supports, neither do we require a number of 

 men at work to put them up, thus little ventilation will be needed 

 because there will be but few men present to vitiate the aii', and as 

 we employ compressed air locomotives to move our trains of materials 



K 2 



