On the Underground Temperature Gradient. 



359 



gradient just before that close must have been one degree in 39 feet; 

 if the interval be 15,000 years, one degree in 41 feet; and if 

 27,000 years, one degree in 43 feet. Or, again, at the bottom of 

 a mine half-a-mile below the '' surface of invariable temperature," 

 i.e. about 2700 feet deep, the temperature would be 53° above that 

 at the surface with a gradient of one degree in 50 feet, 68° above 

 with a gradient of one degree in 39 feet, 65° with one of one degree 

 in 41 feet, and 61° with one of one degree in 43 feet. In any case, 

 it follows that the gradient now differs very sensibly from the value 

 it had at the close of the Glacial period. 



TABLE I. 



Showing Number of Feet of Vertical Descent corresponding to a rise of 1° F. in 

 underground temperature at the close of the Glacial period. 



The question whether the present gradient retains any traces of 

 the Ice Age depends upon several conditions, such as the mean 

 annual temperature existing before the Glacial period, and the 

 relative lengths of the Glacial period and the subsequent interval. 

 Obviously, this is a question to which it is difficult, perhaps 

 impossible at present, to furnish any decisive answer. If the 

 'pre-Glacial mean temperature were the same as it is now, if the 

 glacial conditions commenced and ended abruptly, and if, moreover, 

 the Glacial and post-Glacial periods were of equal duration, then 

 the gradient now would be the same as it was before the Glacial 

 period, that is assuming the normal gradient^ to have undergone no 

 sensible change during the interval. Supposing the first two of the 

 above conditions satisfied, however, it may be worth while giving 

 a few numerical estimates of the change of gradient depending on 

 different assumptions as to the lengths of Glacial and post-Glacial 

 times. These will be found in Table II, which supposes that the 

 present gradient is one degree in 50 feet, and that the mean annual 

 temperatures were constant during both periods and differed by 19°. 



As the general opinion seems to be that the duration of the Glacial 

 period was much greater that that of the succeeding interval, it is 

 therefore possible that some relics of the Ice Age are still to be 

 found in the present temperature gradient. 



1 The gradient due to the temperature of solidification, the surface temperature 

 having remained constant ever since. 



