ATMOSPHERIC FEATURES OF THE DESERT. 135 



receives it ; it absorbs but insignificant quantities, which it loses in 

 a few minutes when the calorific source begins to fail. On the other 

 hand, in these immense plains where no inequality of surface can 

 oppose the atmospheric movements, the wind acquires an increasing 

 force and swiftness, vires acquirit eundo, and soon assumes all the 

 characteristics of a tempest. Hence arise those terrible typhoons, 

 those appalling hurricanes, of whose destructive effects history 

 records so many instances, and of which I shall presently be called 

 upon to speak. As for water, we have seen that its entire absence 

 is a characteristic feature of the Sandy Desert. 



To sum up, an overpowering degree of heat during the day, a 

 freshness, often even an excessive cold, during the night (in the 

 Sahara the thermometer frequently rises above 120 F. at noon, and 

 not infrequently sinks below 32 about two or three o'clock A.M.) ; 

 an ever transparent and azure sky, 



" Darkly, deeply, beautifully blue ; " 



the absence of rains and dews, of gales and thunder ; but a frequent 

 recurrence of terrible hurricanes : such is the meteorological constitu- 

 tion of the arid zone, which embraces all the northern districts of 

 Africa, except the Mediterranean region that is, from the snowy 

 heights of Atlas to the fertile pastures of Soudan and which extends 

 in Asia from the west to the north-east, for all but one narrow belt, 

 as far as the 1 1 9th meridian of longitude. 



Foremost among the phenomena peculiar to this zone we must 

 place those famous tempests which, in default of humid clouds, tra- 

 verse with startling swiftness the changing surface of the Desert, 

 driving before them whirlwinds of burning sand, and striking the 

 traveller's heart with a sense of unconquerable awe. The wind of the 

 Desert is called by the Arabs the choum. or khamsin ; but is more 

 generally known in European books as the Simoun, Simoom, 

 or Samoun. It is the Samiel of the Turks ; and, under a somewhat 

 milder form, the Scirocco of the Mediterranean. Wherever, or how- 



