86 SKETCH OF THE LIFE AND ACHIEVEMENTS 



New Spain. They left the ship at Cumana, and 

 thus the accidental disease became the more 

 immediate cause of the great discoveries of 

 Humboldt in these territories on the Orinoco 

 to the confines of the Portuguese possessions 

 on the Rio Negro. 



The forty- one days which occupied their 

 passage from Corunna to Cumana enabled both 

 Humboldt and Bonpland to make important 

 observations relative to the temperature of the 

 air, based on the material which science had 

 already afforded them. Another very interest- 

 ing phenomenon engaged Humboldt during this 

 voyage, viz., the colour of the sky. He not 

 only enjoyed the splendid changes of colours, 

 from soft green to a beautiful yellow and red, 

 as reflected in the sea, but he yielded involun- 

 tarily to the impressions which the azure above 

 produced. He examined the cause and effect 

 of this phenomenon, and became the first na- 

 turalist who instituted scientific observations 

 upon the sea near the equator, with reference 

 to the colour of the sky. Already, in 1765, 

 Deluc called the attention of scientific men to 

 the azure of the sky, and inquired into the cause 

 and the condition of this phenomenon ; and, in 

 1791, Saussure invented an instrument, which 

 he called " Cyanometer," This instrument was 



