THERMAL SPRINGS. 181 



ions comparative observations by De Gasparin, Schouw, and 

 Thurmann have thrown considerable light, in a geographical 

 and hypsometrical point of view, in accordance with latitude 

 and elevation, upon this influence. Wahlenbcrg asserted 

 that in very high latitudes the average temperature of vari- 

 able springs is rather higher than that of the atmosphere ; 

 he sought the cause of this, not in the dryness of a very cold 

 atmosphere and in the less abundant winter rain caused 

 thereby, but in the snowy covering diminishing the radiation 

 of heat from the soil. In those parts of the plain of North- 

 ern Asia in which a perpetual icy stratum, or at least a 

 frozen alluvial soil, mixed with fragments of ice, is found at 

 a depth of a» few feet,* the temperature of springs can only 

 be employed with great caution for the investigation of 

 Kupffer's important theory of the isogeothermal lines. A 

 two-fold radiation of heat is then produced in the upper 

 stratum of the earth : one upward toward the atmosphere, 

 and another downward toward the icy stratum. A long se- 

 ries of valuable observations made by my friend and com- 

 panion, Gustav Rose, during our Siberian expedition in the 

 heat of summer (often in springs -still surrounded by ice), be- 

 tween the Irtysch, the Obi, and the Caspian Sea, revealed a 

 great complication of local disturbances. Those which pre- 

 sent themselves from perfectly different causes in the tropic- 

 al zone, in places where mountain springs burst forth upon 

 vast elevated plateaux, eight or ten thousand feet above the 

 sea (Micuipampa, Quito, Bogota), or in narrow, isolated 

 mountain peaks many thousand feet higher, not only include 

 a far greater part of the surface of the earth, but also lead 

 to the consideration of analogous thermic conditions in the 

 mountainous countries of the temperate zones. 



In this important subject it is above all things necessary 

 to separate the cycle of actual observations from the theoret- 

 ical conclusions which are founded upon them. What we 

 seek, expressed in the most general way, is of a triple nature 

 — the distribution of heat in the crust of the earth which is 

 accessible to us, in the aqueous covering (the ocean) and in 

 the atmosphere. In the two envelopes of the body of the 

 earth, the liquid and gaseous, an opposite alteration of tem- 



summer maximum in Germany ; where, therefore, a temporary want 

 of rain ceases altogether." See the section " Geothermik," in the 

 excellent Lehrbuck der Geognosie, by Naumann, bd. i. (1850), s. 

 41-73. 



* See above, p. 47. 



