350 
POPULAR SCIENCE REVIEW. 
quantity entering might fairly have been expected rather to 
increase than diminish. Such has not been the case. The 
water supply that has been reached was very soon exhausted, 
and the water itself that issued from the fissures intersected 
was neither very hot, nor was it loaded with mineral matter, 
nor has it increased in quantity. It can hardly be said to have 
been in circulation. Thus, so far as metamorphic rocks are 
concerned, the metamorphosis seems to have been carried on 
away from water action of any kind. 
The temperature of rock in the earth’s interior is another 
enquiiy of very great interest and importance. It is to be 
regretted that, owing to want of proper arrangement from the 
first, the method of taking the temperature of the rock has not 
been fully and properly carried out on both sides. On the 
Italian side the observations are better than on the French 
side ; but no complete record of either has yet been prepared. 
The system adopted was to bore holes to about ten feet in the 
solid rock, at intervals of about 500 metres, and put in ther- 
mometers. But the thermometers themselves were not, in all 
cases, maximum and minimum instruments, and occasionally 
accidents have happened. It is understood, however, that 
further observations may at any time be made. 
The general result, as represented by Signor Borelli, the 
resident engineer on the Italian side, shows a remarkable 
uniformity in the temperature of the rock throughout. At the 
distance of 6,200 metres (6,506 yards), or nearly midway, and 
at a depth of about 5,000 feet, the temperature of the rock was 
found to be only 80°. The mean annual temperature of the 
surface at the mouth of the tunnel and above is not very 
closely determined ; but, under any circumstances, the incre- 
ment, compared with that recorded in observations made in 
mines, is exceedingly small. The greatest depth reached in 
mines has been 2,150 feet, and the average increment is gene- 
rally taken at 1° Fah. for every 60 feet. Allowing 80 feet to 
reach the stratum of invariable temperature, the permanent 
temperature at 5,000 feet should be at least 80° above the 
mean annual temperature. It is certainly not more than 50°, 
and probably less. There have, however, been other cases 
observed, at which the increment amounted to 1° for upwards 
of 100 feet; and as there is a general absence of mineral veins 
and metalliferous deposits of every kind in the rocks through 
which the tunnel is bored, it may be possible that this smaller 
increment belongs to the earth generally, the higher being due 
to chemical action, induced by the presence of certain metals, 
metalliferous minerals, and water. 
At the same time it must be remembered that the very large 
proportion of all the observations of subterranean temperature 
