162 
POPULAR SCIENCE REVIEW. 
each perforator, working at its ordinary rate, consumed about 
190 cubic inches of compressed air per second, so that the nine 
machines expended nearly one cubic foot per second, or about 
3,425 cubic feet, English, of air, compressed to a pressure of 
six atmospheres, or, in round numbers, with a working force 
of 90 lb. per square inch of surface. When the air, so com- 
pressed to six atmospheres, is liberated in the tunnel, and 
expands to its normal state of pressure, it is found to absorb 
nearly the same quantity of heat which it gave out in the act 
of compression. By this absorption of heat from the air in 
the tunnel, the temperature, which has been raised by the 
workmen, the lamps, and gas is considerably lowered, thus 
rendering it more cool, fresh, and suitable for respiration. 
. At Bardonneche, during the 325 useful working days, the 
number of holes bored to a depth from 30 to 32 inches 
amounted to the number of 45,751; the number of chisels 
employed was 72,538 ; the gunpowder consumed weighed 
40,735 1b.; and the length of match for firing the mines 
amounted to 251,000 feet. In 1861 the machines advanced 
the tunnel 557 feet, by boring 20,000 holes; in 1862 they 
bored 46,000, besides 6,000 made during the working experi- 
ments ; amounting altogether to 72,000 holes, divided amongst 
80 perforators ; from this it results as a mean, that 900 holes 
were drilled by each of the perforators. The eight machines 
worked ten perforators, and made 80 holes in 7 hours 
39 minutes, so that each machine took 7 hours 39 minutes to 
bore 10 holes, or three-quarters of an hour for each mine hole. 
In three-quarters of an hour each machine worked at the rate 
of three blows per second, making 8,100 evolutions complete, 
thus giving 8,100 blows of the chisel to execute one hole. 
The 900 holes bored on an average, as above stated, gives for 
each perforator 8,100 x 900 = 7,290,000 blows, worked by air. 
By way of comparison of cost, it may be observed that the 
Bletchingly tunnel on the South-Eastern Railway, in blue clay, 
shale, and sand, cost at the rate of £71. 18s. 7d. per yard 
forward; the Saltwood tunnel, on the same line, £118; the 
Kilsby tunnel, on the London and Birmingham, £125; and 
the Clay Cross, on the Midland Railway, £100 per yard 
forward. The estimated cost of the Mont Cenis tunnel is 
stated to be at about the rate £166 per lineal yard. Assuming 
this to be near the mark, the entire tunnel will cost, at this 
rate, upwards of two millions sterling. One item of cost in 
this tunnel, which must not be overlooked, is the * heavy 
amount of transport, the length from the middle of the tunnel 
to either mouth being upwards of three miles and a half. The 
tunnel is for a double line of rails. 
The wear and tear of the perforating machinery has been 
found greater than was expected ; for the hard, sharp- cutting 
