216 CLIMATOLOGY. 



radiation, and the other of the conduction of heat. The 

 limit here spoken of, or the depth to which the periodical 

 changes of summer and winter are felt, is influenced by a 

 variety of circumstances, and differs in different localities. 

 The temperature of that limit would be, it is supposed, 

 that of the mean of a number of years (fourteen or fifteen 

 perhaps) forming a complete cycle of the annual variations. 



The limit or commencement of permanently frozen soil, 

 or ground ice, is coincident, according to Baer, with the 

 isothermal line of 32° F. ; and its thickness increases in 

 proportion as the mean temperature of the locality falls 

 below that degree, its unlimited descent being checked 

 by the interior heat of the earth. 



Observations of the temperatures of mines and of arte- 

 sian wells have established the fact that the temperature 

 rises as Ave penetrate into the crust of the earth ; but the 

 rate of increment has not yet been satisfactorily deter- 

 mined. The temperatures of mines in the same district, 

 and of different parts of the same mine at equal depths, 

 vary 'greatly. Some authors fix the increment at one de- 

 gree of Fahrenheit for every forty-five feet of descent, 

 after the superficial stratum which is directly influenced 

 by the solar heat has been passed. 



At Yakutzk, in Siberia, on the sixty-second parallel, 

 with a mean annual temperature of 14° F., a well dug 

 to the depth of 382 feet just penetrated the frozen earth, 

 and the resulting increment of heat there is one degree for 

 twenty-eight feet of descent. At Fort Simpson, on the 

 Mackenzie, very nearly in the parallel of Yakutzk, but 

 having a mean annual temperature of 25° F., the frozen 

 substratum was found to terminate at the depth of seven- 

 teen feet from the surface, the underlying bed being loose 

 and sandy. The surface soil there was thawed at the close 

 of summer (19th October, 1837), to the depth of nearly 



