588 



mometer in the air kept up at 30*3°; it was 

 only 26° centesimal, consequently 3° lower than 

 at the Great Cataracts, and 2° higher than the 

 waters of the Rio Negro. In the temperate 

 zone in Europe, the Danube and the Elbe* 

 attain in the middle of summer only from 17° 

 to 19°. I could never find a difference at the 

 Oroonoko between the diurnal and nocturnal 

 heat of the waters, except when I plunged the 

 thermometer into shallow parts of the river, 

 where it flows with extreme slowness over a 

 wide, sandy beach, as at Uruana, and toward 

 the mouth of the Apure. Although in the 

 forests of Guyana, the radiation from the soil is 

 much slackened beneath a sky constantly cloudy, 

 the temperature of the air diminishes sensibly 

 during the night. The upper stratum of water 

 is then hotter than the surrounding soil ; and if 

 the mingling of two portions of air almost satu- 



* The following are the differences founded on direct ex- 

 periments made before my departure from Eiftope, during a 

 ions- residence at Vienna and Dresden. 



Latitude 48 — 49°. 



Temperature of the rivers 

 in summer, 17 — 19° cent. 



Temperature of the air in the 

 hottest month, 18— 19 5°. 



Mean temperature of the 

 year, 10—12°. 



Latitude 5 — 8o. 



26— 29°. (The Oroonoko.) 

 28—29°. 



27— 28°. 



