188 COSMOS. 



comparative observations by De Gasparin, Schouw and Thur- 

 mann have thrown considerable light, in a geographical and 

 hypsometrical point of view, in accordance with latitude and 

 elevation upon this influence. Wahlenberg asserted that in 

 very high latitudes the average temperature of variable 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 Northern 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, 85 the 

 temperature of springs can only be employed with great cau- 

 tion 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 up- 

 wards towards the atmosphere, and another downwards 

 towards the icy stratum. A long series of valuable observa- 

 tions made by my friend and companion, Gustav Rose, during 

 our Siberian expedition in the heat of summer (often in 

 springs still surrounded by ice) between the Irtysch, the 

 Obi, and the Caspian Sea, revealed a great complication of 

 local disturbances. Those which present themselves from 

 perfectly different causes in the tropical zone, in places where 



1826, p. 178; Schouw, Tableau du Ciimat et de la Vegetation de TItalie 

 vol. i, 1839, pp. 133 195 ; Thurmann, Sur la temperature des sources de 

 l(t chatne du Jura, comparee a ceUe des sources de la plaine Suisse, des 

 Alpes et des Vosges, in the Annuaire Meteor ologique de la France, 1850, 

 Plj. 258 268. As regards the frequency of the summer and autumn 

 rains, De Gasparin divides Europe into two strongly contrasted regions. 

 Valuable materials are contained in Kamtz, Lehrbuch der Meteorologie, 

 Bd. i, s. 448 506. According to Dove (Poggend. Annalen, Bd. 

 xxxv, s. 376) in Italy. " at places to the north of which a chain of 

 mountains is situated, the maxima of the curves of monthly quantities 

 of rain fall in March and September ; and where the mountains lie to 

 the south, in April and October." The totality of the proportions of 

 rain in the temperate zones may be comprehended under the following 

 general point of view : " The period of winter rain in the borders of 

 the tropics constantly divides, the further we depart fi'om these, into 

 two maxima united by slighter falls, and these again unite into a 

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

 rain ceases altogether." See the section "Geothermik" in the excellent 

 Lehrbuch der Geoynosie, by Naumaun, Bd. i, (1850), s. 4173. 

 35 See above, p. 45. 



