126 STEPPES AND DE3E2.TS. 



Centrale, T. iii. p. 176.) The Savannahs, which between 

 the Missouri and the Mississipi are called Prairies, and 

 which appear in South America as the Llanos of Venezuela 

 and the Pampas of Buenos Ayres, are covered with small 

 monocotyledonous plants of the family of Cyperaceae, and 

 with grasses of which the thin pointed stalks or ears, and 

 the delicate lanceolate leaves or blades, radiate towards the 

 unclouded sky, and possess an extraordinary power of " emis- 

 sion." Wells and Daniell (Meteor. Essays, 1827, p. 230 

 and 278) have even seen in our latitude, where the atmos- 

 phere has so much less transparency, the thermometer sink 

 6.5 or 8 of Reaumur (14.5 or 18 Fahrenheit), on 

 being placed on the grass. Melloni, in a memoir, " Sull 

 abassamento di temperatura durante le notti placide e 

 serene," 1847, p. 47 and 53, has shewn how in a calm 

 state of the atmosphere, which is a necessary condition of 

 strong radiation and of the formation of dew, the cooling of 

 the grassy surface is also promoted by the particles of air 

 which are already cooled sinking to the ground as being the 

 heaviest. In the vicinity of the equator, under the clouded 

 sky of the Upper Orinoco, the Rio Negro, and the Amazons 

 River, the plains are clothed with dense primeval forests ; but 

 to the north and south of this wooded region there extend 

 from the zone of palms and lofty dicotyledonous trees, in 

 the northern hemisphere, the Llanos of the Lower Orinoco 

 the Meta and the Guaviare, and in the southern hemisphere 

 the Pampas of the Rio de la Plata and of Patagonia. The 

 space thus occupied by Savannahs or grassy plains in South 

 America is at least nine times as great as the area of France, 

 The wooded region acts in a threefold manner in diminish- 



