2 88 THE POPULAR SCIENCE MONTHLY 



The place at which air gains access to these passages need not be a 

 single opening, but consists, in all probability, of numerous small aper- 

 tures, covered possibly by a thin coating of moss, loose shale or other 

 porous substance. 



In the summer time the warm outside air entering these apertures 

 comes in contact with the rocks which have been chilled by the reverse 

 currents of the preceding winter and in doing so gives up its heat to 

 them, becoming specifically heavier. It then forces its way on down, 

 displacing the warmer and lighter column of air above the pit. 



It is evident that the rapidity with which this circulation takes 

 place depends upon the difference in temperature of the two air col- 

 umns. That is, the cold outward current is much more noticeable on 

 hot days than on cool days in summer, and in winter the strongest in- 

 ward current is noticed on the coldest days. 



This fact accounts for the common belief that the freezing takes 

 place more rapidly and that the mine is colder on hot than on cool days. 



The temperature of the mine, or, in other words, of the air as it 

 issues from the crevices, remains practically constant throughout the 

 summer, which is proved by thermometer readings. However, the dif- 

 ference between this constant temperature and the temperature pre- 

 vailing outside the mine is obviously greatest on the hottest days and 

 therefore, as one enters the mine, the contrast is more noticeable. This 

 causes one to believe that the mine is colder when it really is not. It 

 is true, however, that the ice is formed most rapidly during the hottest 

 weather. This is not because the temperature of the mine is lower, as 

 is generally supposed, but is due to the fact that the circulation of air 

 is more rapid; that is, a greater quantity of cold air issues from the 

 numerous apertures and consequently a greater amount of "cold" is 

 available for the formation of ice. 



As soon as the supply of '" cold " in the rocks is exhausted the inter- 

 nal and external air columns become gradually equal in temperature 

 and weight, the circulation ceases and the ice begins to melt. This gen- 

 erally occurs about September of each year. 



If this is the true explanation of this phenomenon, we may say, 

 with truth, that in this particular instance it is the heat of summer 

 which causes the ice to form, but, at the same time, we can not disre- 

 gard the fact that it is the severity of the preceding winter and the nat- 

 ural arrangement of the rock strata which make it possible for the heat 

 of summer to produce this peculiar phenomenon. 



