2G2 THE ATMOSPHEEE AND METEOEOLOaY. 



ences of temperature produced by the progress of the sun. In the 

 morning, as soon as the sun rises, the temperature, which had fallen to 

 its lowest because of the nocturnal evaporation, increases rapidly, the 

 air expands and spreads towards the colder spaces which extend on 

 the side of the west ; a little wind from the east results from this, 

 which becomes gradually a wind from the south-east, in proportion as 

 the sun mounts above the horizon. At noon the expanded air spreads 

 in the direction of the north ; and finally, towards the evening, it is 

 on the eastern side, where the aerial strata are chilled, that the surplus 

 air, still heated by the solar rays, directs itself. Thus, when the at- 

 mosphere is not agitated by a general wind, a breeze turning regu- 

 larly round the horizon in the same direction as the sun must be pro- 

 duced. In the northern hemisphere this movement of gyration is 

 accomplished from east to west by the south ; in the opposite hemi- 

 sphere it is by the north that this diurnal breeze efiects its gradual 

 revolution from east to west. In mountains the phenomenon is 

 more complex, on account of the ascending and descending breezes, 

 which are intermixed with the gyrating ones. It is remarked, how- 

 ever, that the greater part of the local winds, determined by the dif- 

 ference of temperature, tend towards the west in the morning, then 

 turn gradually in an opposite direction, and blow towards the 

 east when the sun is sinking. These are the solaures {soils aura) or 

 solar winds of the Department of the Drome.* 



As to the local winds, which characterize certain regions, they 

 originate in the unequal distribution of heat. Such are the chamsin 

 of Egypt, the pampero of the Argentine Republic ; such, above all, 

 is that aerial current to which the name of Simoon, or "poison- 

 ous," is given, in the Sahara. As soon as this wind commences 

 to blow the panting traveller can scarcely breathe ; the air is burn- 

 ing and dried up, as if emerging from the mouth of an oven ; the 

 heat, increased by the radiation of innumerable grains of sand which 

 float in the atmosphere, rises rapidly to 113, 122, and even 133 

 degrees Fahr. ; the sun is veiled, and every object assumes a violet or 

 dark-red hue, while space is filled with dust. In order not to be 

 smothered by this irrespirable air, travellers must envelop their 

 faces in their garments, and the camels bury their necks in the 

 sand. But the simoon is not always accompanied by clouds of 

 dust. Palgrave, who endured a violent simoon in the desert of 

 Arabia, saw not a single cloud of sand or vapour in the sky, 

 * Fournet, Eydrologie du Rhone. 



