186 COSMOS. 



shown by TVahlenberg and Erman the elder, in the averages 

 of the summer and winter months. But in accordance with 

 the criterion here indicated, a spring in one zone must be 

 denominated warm, which hardly attains the seventh cr 

 eighth part of temperature of one which in another zone, 

 near the equator, will be called cold. I may mention the 

 differences between the average temperature of St. Peters- 

 burg (38.12 F.) and of the shores of the Orinoco. The 

 purest spring water which I drank in the vicinity of the 

 cataracts of Atures 31 and Maypures (81. 14 F.) or in the 

 forest of Atabapo, had a temperature of more than 79 F. ; 

 even the temperature of the great rivers in tropical South 

 America, corresponds with the high degrees of heat of such 

 cold 32 springs. 



31 Humboldt, Voyage aux Regions Equinoxiales, t. ii, p. 376. 



32 Foi the sake of comparing the temperature of springs where they 

 break forti directly from the earth, with that of large rivers flowing 

 through open channels, I here bring together the following average 

 numbers from my journals : 



Rio Apure, lat. 7f ; temperature, 81. 



Orinoco, between 4 and 8 of latitude ; 81.5 85.3. 



Springs in the forest, near the cataract of Maypures, breaking forth 

 from the granite, 82. 



Cassiquiare, the branch of the Upper Orinoco, which forms the union 

 with the Amazon; only 75.7. 



Rio Negro, above San Carlos (scarcely 1 53' to the north of the 

 equator); only 74. 8. 



Rio Atabapo, 79.2 (lat. 3 500- 



Orinoco, near the entrance of the Atabapo, 82. 



Rio Grande de la Magdalena (lat. 5 12' to 9 56'), 79 9'. 



Amazon, 5 31' south latitude, opposite to the Pongo of Rentema 

 (Provincia Jaen de Bracamoros), scarcely 1300 f 'et above ths 

 South Sea, only 72. 5. 



The great mass of water of the Orinoco consequently pproaches the 

 average temperature of the air of the vicinity. During great inunda- 

 tions of the Savannahs, the yellowish brown waters, which smell of 

 sulphuretted hydrogen, acquire a temperature of 92. 8 ; this I found to 

 be the temperature in the Lagartero,to the east of Guayaqtiil, which 

 swarmed with crocodiles. The soil there becomes heated, as in shallow 

 rivers, by the warmth produced in it by the sun's rays falling upon it. 

 With regard to the multifarious causes of the low temperature of the 

 water of the Rio Negro, which is of a coffee-brown colour by reflected 

 light, and of the white waters of the Cassiquiare (a constantly clouded 

 eky, the quantity of rain, the evaporation from the dense forests, and 



