i 879 «] The Heat of the Comstock Mines . 219 
sensibly to any depth in the course of twenty-four hours. 
The shallow holes which were made use of always lay in 
new ground, and exhibited results which may be accepted 
with as much confidence as if they were 20 feet or more 
deep. Very often they were in ground which had been ex- 
posed only one or two hours, having been sunk immediately 
after a blast which threw off 4 or 5 feet of the rock. The 
surface which was thus thrown down had not been exposed 
more than twenty-four hours. The high temperature and 
small flow of air in the heading forbid the supposition that 
any sensible diminution of heat could have taken place at 
the bottom of the drill-hole under such circumstances. The 
surface of the rock exposed to the air of the drift was found 
to be about 123 0 F., the experiment being made near the 
“ header ” or end of the drift. The air itself was found to 
show considerable uniformity when its temperature was 
taken under circumstances that were at all similar. In 
freshly opened ground it varied from 108 0 to 116 0 F., and 
higher temperatures are reported at various points, reaching 
in fa<ft as high as 123 0 F. in the 1900 level of the Gould and 
Curry. 
The temperature of the air is subjedt to more fluctuations 
than that of the rock, for the simple reason that it is artifi- 
cially supplied to the mine, and varies according to the dis- 
tance to which it is carried, the quantity, velocity in the 
pipe, and its initial temperature. All of these elements of 
the problem vary within wide limits. The initial tempera- 
ture of the air which supplies a particular drift will, for 
instance, depend upon whether it is drawn from the surface, 
the bottom of a shaft, where it is often cooler than above 
ground, or from some old air-way, where it has had time 
and opportunity to take up heat. 
In the Comstock mines it is the custom, without excep- 
tion, to blow the air through galvanised iron pipes, the 
diameter of which is usually from 8 to 20 inches. The size 
most used is 11 inches in diameter, and the usual amount 
of air blown is about 700 cubic feet per minute, this being 
the supply for two to six or more men, working in one or 
two “ headers.” 
In most cases the air is not sent down from the surface, 
but taken from some point in the incline or at the bottom of 
the shaft. Its temperature may be assumed at about 8o° or 
90° F. in summer, though it is sometimes higher than this. 
Its velocity in the air-pipe is not very far from 1000 feet per 
minute. From these data it will be seen that about 15 or 
20 degrees of heat are added to the air, in a period of time 
