ANNOTATIONS AND ADDITIONS. 185 



of 10,000 (10,660 Eng.) feet. (Huinboldt, Bonpland, and 

 Kunth, Synopsis Plantarum sequinoct. Orbis novi, T. iii. 

 p. 370). 



( 38 ) p. 19. " The scene in the Steppe is 

 suddenly changed." 



1 have endeavoured to depict the coming in of the rainy 

 season, and the signs by which it is announced. The usual 

 deep dark azure of the sky in the tropics arises from 

 the more complete solution of the vapour contained in the 

 atmosphere. The cyanometer indicates a paler blue as soon 

 as the vapours begin to be precipitated. The dark spot or 

 patch in the constellation of the Southern Cross gradually 

 comes indistinct as the transparency of the atmosphere 

 diminishes, and this alteration announces the near approach 

 of rain. The brightness of the Magellanic clouds, (Nubecula 

 major and minor), gradually vanishes in a similar manner. 

 The fixed stars, which before shone like planets with a 

 steady, tranquil, and not trembling light, now scintillate 

 even in the zenith, where the vapours are least. (See 

 Arago, in my Relation hist. T. i. p. 623). All these appear- 

 ances are the results of the increased quantity of vapour 

 diffused in the atmosphere. 



( 39 ) p. 20. " Awakened from a torpid state ly the 

 first fall of rain" 



Extreme dryness produces in plants and animals the same 

 phenomena as does the withdrawal of the stimulus of heat. 

 Many tropical trees and plants shed their leaves during the dry 



