FOREIGN CORRESPONDENCE. 



489 



air-condenser {compresseur hydraulique), which is nothing more than a 

 syphon turned with its orifices upwards, and communicating by one of 

 them with a stream of water, by the other with a reservoir of air. 

 The water descending by the first branch enters the second, and, by the 

 pressure it occasions, condenses the air, which is then forced into the 

 reservoir. This done, a valve is opened by which the water contained 

 in the syphon is let out, and the operation recommences. The emission 

 and introduction valves are regulated by a small machine operating by 

 means of a column of water ; and the air in the reservoir is maintained 

 at a constant degree of pressure by a column of water communicating 

 with another reservoir above. 



Thus with a waterfall twenty metres in height, the air is condensed 

 to six atmospheres, equivalent to the pressure of sixty-two metres of 

 water. This condensed air is used for two purposes — first, as a motive 

 power ; second, for ventilation. Two kinds of perforators worked by 

 condensed air instead of steam, are employed ; and the manner in which 

 these machines perform their duty affords the first practical demonstra- 

 tion of the possibility of employing compressed air as a motive power 

 with advantage. By means of the perforators holes for blasting may 

 be bored through the hardest syenite in one-twelfth of the time which 

 would be required if ordinary means were employed. It may be stated 

 here that in blasting rock three- fourths of the time are employed in 

 boring holes, and the remainder in charging and blasting ; hence, 

 accelerating the former operation is a great advantage. Moreover, it is 

 said, that by making use of the apparatus mentioned above, eighteen 

 perforators may be set to work in a place where three couples of 

 miners would hardly find room. So that by these ingenious con- 

 trivances, as well as by others for clearing away, &c., the perforation of 

 the tunnel may be effected in six years instead of thirty-six. 



The air that has been employed as a motive power is used to ventilate 

 the gallery; but when the latter shall have reached a considerable 

 depth, it will require 85,924 cubic metres of air every twenty-four 

 hours to replace that which has been vitiated by respiration, torches, 

 and gunpowder; and this quantity, in the form of 14,320 cubic metres 

 condensed to six atmospheres, the reservoir can furnish. A curious 

 phenomenon has been observed during these works : — When the air, 

 condensed to the degree above-mentioned, is shot from the machine into 

 the gallery, any water happening to be near the apparatus suddenly 

 congeals, although the ambient temperature be about 18° (centigrade) ; 

 an effect owing to the rapid dilatation of the air the moment it is 

 expelled from the machine. Hence, when a large mass of compressed 

 air is driven into a gallery situated at 1,600 metres below the outer 

 surface of the earth — for such is the level of the Alpine tunnel — 

 and where, consequently, the temperature must be about 50° (centi- 

 grade), the dilatation of the compressed air will produce a diminution 

 of temperature sufficient to counterbalance the excess alluded to. 



The progress now making per day in boring this tunnel is three 

 metres on each side of the mountain, or six metres per day in all. The 

 thickness of the Alps between Modane and Bardoneche is estimated at 



* One invented by Mr. Bartlett ; the other by M. Sommeiller. 



2 N 



