FOREIGN CORRESPONDENCE. 
489 
air-condenser (compresseur hydrauUque), 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 afi'ords 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 efl'ected 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 
