CHAP, iv.] APPLICATIONS OF COMPRESSED AIR. 71 



The air-gun produces a noise, but much less than that of fire-arms 

 of the same size, and a light is visible from the gun, which may be 

 due to the ignition of the solid particles suddenly shot out by the 

 aerial current ; but, according to M. Daguin, this effect proceeds from 

 the electricity developed/by the friction of the wad and of the 

 particles in question against the inner walls of the barrel. 



IT. THE BoKtxG OF TUNNELS BY COMPRESSED AIR. 



In contemporary industrial works the power of compressed air has 

 been and is still utilized in various ways. We will mention the most 

 remarkable examples of this application. 



In the first rank we must mention the boring of the immense 

 tunnel which runs through the Alps, a little to the south of Mont 

 Cenis, and joins the stations of Bardonneche and Modane, the 

 extreme stations, the one French and the other Italian, belonging to 

 the Victor-Emmanuel line. In this there were 12,000 metres of 

 archway to open through the rock, at depths which prevented the use 

 of the ordinary process for boring tunnels, that is, by shafts sunk 

 from above along the line of the intended tunnel. 



The boring of this long tunnel could only be done from two opposite 

 points : it appeared almost impossible to use steam and powder for 

 excavating and for breaking down and crushing the rocks, because in 

 proportion as the miners advanced further into the mountain the diffi- 

 culties connected with the ventilation of the workings would increase, 

 the air being vitiated by the mixture of the gases of the powder and 

 steam, by the burning of fires and lamps, and by the carbonic acid 

 given oft' by the workmen. The engineers l determined to adopt an 

 idea which Colladon and, later on, Caligny had put forward that of 

 employing compressed air" as the motive power of the machines to be 

 used for boring the rock. The compression-pumps, or machines 

 employed to compress the air in the reservoirs or receivers, themselves 

 borrowing their power from a neighbouring fall of water (the stream 

 from Melezet to Bardonneche, and at Modane the little river of Arc). 

 At the commencement, the air-compression pumps, thus called from 



1 MM. Sommeiller, Grandis and Grattoni. 



