ON ITS EXTERIOK. THERMAL SPRINGS. 187 



Schouw, and Thurmann. ( 258 ) Wahlenberg maintained 

 that in very high latitudes the mean temperature of 

 variable springs is somewhat higher than the mean 

 temperature of the air ; he looked for the cause of this, 

 not in the dryness of very cold air and the consequent 

 less abundance of winter rain, but in the protecting 

 covering of snow diminishing the radiation of heat 

 from the ground. In those parts of the great plains of 

 Northern Asia in which a perpetual " stratum of ice," 

 or at least frozen soil with interspersed pieces of ice, is 

 found at a depth of a few feet, ( 259 ) the temperature of 

 springs cannot, without great caution, be used in making 

 out Kupffer's important theory of isogeothermal lines. 

 In such situations there takes place a twofold radiation 

 of heat from the upper stratum of the earth ; upwards 

 towards the atmosphere, and downwards towards the ice 

 stratum. An extensive series of valuable observations 

 collected by my friend and companion Grustav Eose, in 

 the Siberian Expedition, in the heat of summer (often 

 in wells still surrounded with ice), between the Irtysch, 

 the Obi, and the Caspian Sea, showed a great complica- 

 tion of local disturbances. Similarly perturbing in- 

 fluences, arising from entirely different causes, show 

 themselves in results obtained within the tropics, from 

 mountain springs issuing forth on vast table-lands eight 

 or ten thousand feet above the sea (Micuipampa, Quito, 

 Bogota), or on slender isolated summits many thousand 

 feet still higher; these comprehend a more extensive 

 part of the earth's surface, and are useful in leading 

 to the consideration of analogous thermic relations in 

 mountainous countries in the temperate zone. 



In this important subject it is first of all necessary to 



