444 CAUSES OF ARIDITY AND MOISTURE. 



Desert is caused by the mass of air losing its humidity 

 as it passes up, and glides over the subtending ridge, and 

 will turn to the map, he may perceive that the same cause 

 is in operation in an intense degree by the mountains 

 of Abyssinia, to render the region about Darfur still more 

 arid ; and that the flanking ranges mentioned, lie much 

 nearer the equator than those which rob the Kalahari 

 of humidity. The Nile, even while running through 

 a part of that region, receives remarkably few branches. 

 Observing also that there is no known abrupt lateral 

 mountain-range between 6° and 12 S., but that there is 

 an elevated partition there, and that the southing and 

 northing of the south-easters and north-easters probably 



Archipelago, Astrakan and the Caspian Sea, in order to merge again 

 into the great circuit of the general winds, and be thus carried again 

 into the equatorial current. Whenever these masses of air, impreg- 

 nated with humidity during their passage over the ocean, meet with 

 an obstacle, such as a chain of mountains, for example, they slide up 

 the acclivity, and, when they reach the crest, find themselves relieved 

 from a portion of the column of air which pressed upon them. Thus, 

 dilating by reason of their elasticity, they cause a considerable degree 

 of cold, and a precipitation of humidity in the form of fogs, clouds, 

 rain, or snow. A similar effect occurs whatever be the obstacle they find 

 in their way. Now, this is what had gradually taken place before 1856. 

 By some cause or other connected with the currents of the atmosphere, 

 the warm current from the west had annually ascended northward, so 

 that, instead of passing through France, it came from the Baltic and 

 the north of Germany, thus momentarily disturbing the ordinary law 

 of the temperatures of Europe. But in 1856 a sudden change occurred. 

 The western current again passed, as before, through the centre of 

 France. It met with an obstacle in the air which had not yet found 

 its usual outlet towards the west and south. Hence a stoppage, a 

 rising, a consequent dilation and fall of temperature, extraordinary 

 rains and inundations. But now that the natural state of things is 

 restored, nothing appears to prognosticate the return of similar 

 disasters. Were the western current found annually to move further 

 north, we might again experience meteorological effects similar to 

 those of 1856. Hence the regular seasons may be considered re- 

 established in France for several years to come. The important 

 meteorological communications which the imperial Observatory is 

 daily establishing with the other countries of Europe, and the intro- 

 duction of apparatus for measuring the velocity of the aerial currents 

 and prevailing winds, will soon afford prognostics sufficiently certain 

 to enable an enlightened Government to provide in time against 

 future evils." 



