with Geographic Botany. — Humidity. 325 



the ocean^ and left by nature destiture of moisture; over 

 them the atmosphere is dry to an extreme, and they are free 

 from changes of temperature, cold winds, or currents of air ; 

 as instances may be mentioned the large deserts, as those of 

 Africa, Egypt and Arabia, and the great central desert of 

 Asia. The sandy plains or karroos, which stretch to the 

 north from the Cape of Good Hope, rarely witness rains, 

 whilst so greedy are they of moisture, that rivers are absorbed 

 by their sands. 



Exposure to a wind constantly blowing from one direction 

 is another source of freedom from rain ; in the trade- winds 

 rain is said to be seldom seen, they being usually regarded as 

 having a tolerable exemption. Though rain is certainly not 

 so frequent here as elsewhere, yet, in crossing the trades at 

 several different periods, I have generally witnessed occasional 

 showers, and these sometimes heavy, whilst they also occur 

 more commonly in the night. They are more frequent in the 

 Pacific than in the Atlantic Ocean, and when prevailing, the 

 force of the breeze will be observed to fluctuate a good deal. 

 To the same cause is attributable that remarkable absence of 

 rain from a surface of territory in Peru, of which Lima forms 

 a portion : the garuas which supply the soil and vegetation 

 with moisture resemble dense mists ; during the night and 

 early part of the day they fall heavily, and a person exposed 

 to them is soon thoroughly wetted, whilst they convey much 

 chilliness to the feelings. This fine precipitation of moisture 

 sometimes borders very closely on fine rain ; it commences 

 about 2 A.M., and is often very heavy early in the morning; 

 perhaps during the day the sun succeeds in penetrating it, 

 but this is by no means always the case. This kind of 

 weather continues from May to August, and the thermometer 

 ranges from 60° to 70°, which is cold and chilly compared 

 with the remainder of the year. Instead of the garuas, were 

 the city of Lima to be visited for any length of time by the 

 rains usual in its latitude, many of the buildings would be 

 completely destroyed ; for aware that they are not subject to 

 rains, the inhabitants occupy houses built of a material very 

 like hardened mud, and all the houses being flat-topped 

 would retain much of the rain that fell. So great indeed is 

 the usual torrent of rain in the tropics, that in those towns 

 exposed to them the houses are suppUed with a number of 

 shoots to carry off the water with all possible speed ; and as 

 it often happens that these are decorated with fantastic co- 

 lours, the perspective of the streets is unique to a foreigner. 

 Ulloa has laboured to prove that the rarity of rain in the re- 

 gion of the garuas is attributable to the constancy of the south 



