APPLICATIONS OF GEOTHERMICS TO GEOLOGY 17 
temperatures of the air for 41 Weather Bureau stations surrounding 
the tunnel. From the data included between the dotted lines in Figure 
I, a reciprocal gradient (1° F. in 75.8 feet) was computed for the apex 
of the mountain, and then having given the profile of the mountain 
and the diminution of air temperature along the mountain slopes, the 
reciprocal gradient (1° F. in 57.6 feet) beneath the adjacent plain was 
determined. The results of the calculations are shown in Figure 2. 
The computed surface of the mountain passes through the apex of 
the mountain and a point on the profile which is at an elevation above 
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ELEVATION REFERRED TO LEVEL OF PLAIN-A/LOMETERS 




: Die ee RS 7 OPP SURO meee cm 
6 we 70°C. ‘MPTOTE 
_t = 
= 
DISTANCE FROM CENTRAL PLANE OF MOUNTAIN —-HILOMETERS. 

Fic. 2.—Computed profile of mountain and corresponding isogeotherms. 
the plain equal to the half-height of the mountain. This approxima- 
tion to the original profile of the mountain is sufficiently accurate for 
our purpose. 
It is important to note that the profile of the mountain, the 
gradients at the apex of the mountain and beneath the adjacent 
plains, and the rate of diminution of air temperature along the moun- 
tain slope—four quantities in all—serve to fix the isogeotherms in 
space just as the two sides and the included angle serve to fix a plane 
triangle. The calculation is thus somewhat of the nature of a geo- 
thermal triangulation rather than an extrapolation to great depths. 
In Table II is given the rise of certain isogeotherms as they pass be- 
neath the apex of the mountain. Thus, a-a in Figure 2 represents a 
rise of 2,207 feet. Similarly, the rise, b-b, is about 1,950 feet, and so on. 
The isotherms do not approximate to planes until a depth of 100 
539 
