CAUSES OF ARIDITY AND MOISTURE. 515 



rain on the Kalahari 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 moun- 

 tains 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 hu- 

 midity. 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 southeasters and north- 

 easters probably cause a confluence of the two great atmospheric 

 currents, he will perceive an accumulation of humidity on the 

 flanks and crown of the partition, instead of, as elsewhere, 

 opposite the Kalahari and Darfur, a deposition of the atmos- 

 pheric moisture on the eastern slopes of the subtending ridges. 

 This explanation is offered with all deference to those who have 

 made meteorology their special study, and as a hint to travelers 

 who may have opportunity to examine the subject more fully. 



selves 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 185G. 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 tempera- 

 tures 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 toward 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 185G. 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 introduction 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." 



