1882.] on the Making and Working of a Channel Tunnel. 127 



the tunnel to the face where the engine is employed to make the 

 excavation. The compressed air presses on the pistons of these 

 engines in the same manner as that in which steam presses on the 

 piston of a steam engine, but, for the purpose we are now consider- 

 ing, there is this important difference, that whereas the steam engine 

 delivers its exhaust steam into the air, thereby filling it with uu- 

 breathable vapour, the compressed air engines deliver the exhaust 

 air into the workings, and in that manner the very machine which 

 is doing the work is, as an accident of this work, effecting the venti- 

 lation. It must not be supposed that compressed air engines are 

 any novelty, or that their efficient working is a matter of doubt. 

 They have been employed in various situations, and for underground 

 purposes for many years. They were used, as you probably well 

 know, in the Mont Cenis tunnel, and in the St. Gothard tunnel, for 

 working the drills which bored the holes in which the cartridges 

 for the blasting of the rock were placed. In fact the mode of 

 construction of these engines, and the manner of their using, are as 

 well understood as are the mode of construction, and the manner of 

 use, of the steam engine. 



There is known exactly, what would be the loss by friction of the 

 compressed air in passing through a given-sized pipe, a loss varying 

 with the velocity and with the pressure; and upon this point I may 

 say that compressed air has been worked at from as much as 1000 lbs. 

 on the square inch down to as little as 30 or 40 lbs. on the square 

 inch, and I may tell you that a pipe little more than 11 inches dia- 

 meter would deliver at 10 miles distance air which had originally 

 been at 50 lbs. pressure in quantities sufficient to develop 150 horse- 

 power at the working face, with a loss of pressure of about 25 per 

 cent., while if the air had been brought up to 500 lbs. pressure a pipe 

 a little over 4 inches diameter would enable, at the same percentage 

 of loss of pressure, a similar horse-power to be obtained at the end of 

 10 miles of such pipe. 



Having determined how the machine is to be driven, let us next 

 consider in what manner it shall attack a stratification capable of 

 being easily cut. An obvious way would be to scoop out channels in 

 the face of the work, and to make these of such depth as to enable 

 the block contained between the channels to be broken off and taken 

 away as a whole ; but you will see that if this mode were adopted, the 

 work of cutting must be suspended while the block was being removed, 

 and the block must be got out past the machine, and that this would 

 be a matter of difficulty, especially in a small trial heading, and must 

 cause delay. 



On the whole, therefore, it becomes much more simple and 

 economical, and much more rapid — and in a case like this rapidity is 

 the true economy — to use such a number of cutters as will cut the 

 whole face of the heading to pieces, making it, as it were, into small 

 fragments, or shavings. A case containing these shavings is before 

 you. The advantage of thus dealing with the material you will perceive 



