THE RAILWAY TUNNEL THROUGH THE ALPS. 161 
I, 515 feet, and if we add the 1,197 feet for the progress at 
Modane during the same period, the total progress forward at 
both ends for an entire year will be 2,712 feet; so that at this 
rate of working, it will require 14f years to complete the exca- 
vation : and taking all the circumstances of the case into 
consideration, it is doubtful if the tunnel can be bored through 
in a less period of time. 
The total estimated quantity of rock to be excavated is 
about 784,666 cubic yards. As the tunnel is 13,354 yards 
long, this will give for the lineal yard forward, and for the 
whole face of the tunnel to be removed, nearly 58 cubic yards. 
At the average advance of 4 feet 5 inches per day, as above 
stated, this will give a daily removal of 73^ yards cube; and 
if both faces are worked at the same progressive rate, it will 
yield 147 cubic yards of daily excavation for the joint work of 
all the boring machines in operation. This supposes con- 
tinuous working, which, however, is scarcely possible, for there 
is always time required to charge the bore holes, fire the blast, 
break down shattered rock, and remove the whole of the 
loosened material out of the tunnel. 
There will also be a sensible loss of time occasioned by pro- 
longing the pipes and tubes for the compressed air to work 
the machines, for the water and gas pipes ; for all these must 
keep pace with and follow up the advance of the perforating 
machines in the tunnel, as well as the timbers and rails for 
carrying the machines, and other indispensable work of the 
kind, all of which must advance simultaneously. 
The rock in which the excavation is at present being made 
is of a quality exceedingly difficult to work, having what the 
engineers have called an “ infelicitous stratification,” with 
great diversity of texture, which has proved very trying to 
the perforators. The rock is a crystallised calcareous schist, 
very much broken and contorted ; being interspersed in almost 
every direction with large masses of pure quartz, in veins of 
variable dimensions, straggling irregularly through the general 
body of the rock. 
It has proved difficult to work in a diversified material of 
this character from the unequal resistance presented to the 
perforators by the calcareous schist and quartz, it not unfre- 
quently happening that the chisels have to be turned from 
their proper direction, through meeting with a block of quartz, 
which, of course, leads to loss of time. 
The production of compressed air at Bardonneche, for the 
year 1862, amounted to 152,100 cubic yards per month, with 
an actual pressure of 90 lb. per square inch ; the entire year 
giving the immense amount of 1,825,000 cubic yards ; which 
is equivalent to a volume of ah* at its ordinary pressure of 
II, 024,000 cubic yards. It was ascertained by experiment that 
