224 THE ATMOSPHEEE AND METEOEOLOGY. 



tlie one to the soutli-west, the other to the north-east ; that is to say, 

 each of them forms a right angle with the monsoon from the south-east. 

 It is the aerial current which has forced the long peninsulas of the 

 Mississippi to spread thus over the waters like the branches of a great 

 fallen tree.* 



The geological labours of the winds are, however, accomplished, for 

 the most part, in an indirect manner, either by the evaporation of 

 the moisture of the continents, or by causing considerable down- 

 falls of rain. During the course of ages the contours of the land 

 and sea have not ceased to change, and, in consequence of these 

 gradual modifications, the winds themselves have been subjected to 

 analogous variations. Some are saturated with the vapour of water, 

 and the clouds that they carry are deposited in rivers and lakes 

 in the midst of land. Other atmospheric currents have lost their 

 moisture in great part, and then in passing over inland seas they have 

 absorbed them ; pumped them, so to speak, leaving behind them 

 smiling plains transformed into deserts. Without any doubt it is 

 tlie winds which have now dried the lands of Cape Natal and 

 Transvaal ; it is they that have been the great agents in the work of 

 drying up central Asia, they have drunk the vast extent of water 

 that formerly stretched from the Euxine to the Caspian Sea, and from 

 the Lake of Aral to the Gulf of Obi, and left steppes of salt in the place 

 of this ancient Mediterranean. f 



It is by means of the atmosphere, too, that the exchange of particles 

 between the earth and bodies wandering in space is accomplished. 

 When an aerolite, shot like an enormous bullet through space, meets 

 the exterior strata of gas that surround the earth, it is instantly set 

 on fire, and bursts either entirely or on the surface ; and hurling with 

 violence some fragments to the ground, it leaves behind it a long train 

 of luminous matter resembling a fiery track. Owing to the resistance 

 opposed by the atmosphere to the passage of the strange star, the 

 globe is every year enriched in this manner with material brought 

 from the sky. Tlie strata of air, moreover, are the vehicle of all 

 sounds ; they also convey the vibrations of light and heat. Deprived 

 of this envelope the globe would immediately be wrapt in complete 

 darkness. But if the atmosphere allows the rays of luminous heat, 

 emitted by the sun, to pass, it intercepts in return a great part of the 

 daik rays, which escape from the earth into space. It is thus that 



* Humphreys and Abbot, Report on the Mississippi River, p. 450. 

 t Maury, Physical Geography of the Sea. 



