IN PRODUCING ASCENDlNa ATMOSPHERIC CURRENTS. 



203 



round the bases of the snowy mountains of Hindoo-Koosh 

 and Afghanistan, passing over the comparatively cool waters 

 of the Persian Gulf and the Arabian Sea, and finally rushes 

 over the northern plains of Hindoostan to the Himalaya 

 Mountains, where it really does ascend. These plains, 

 however, are not then highly heated by the direct action 

 of the sun upon them, as the surfaces of the deserts were 

 over which the wind had passed, the plains of Hindoostan 

 being at this season screened from the solar rays by a thick 

 canopy of clouds. 



There appears to be a partial exception to this general 

 state of Hindoostan, over the mouth of the Indus and Cutch, 

 where the Monsoon generally sets in at the end of June, 

 but during the months of July, August, and September the 

 winds are observed to cause drought.* Now these dry winds 

 evidently come from Beloochistan and Arabia, and not having 

 crossed a sufiicient extent of sea to become charged with 

 vapour, they pass towards the great area of condensation 

 near the mountains as dry winds. The state of this localily 

 therefore proves the real nature of the causes that are in 

 operation to produce the effects that are experienced. 



The whole of the facts show that the cause of the continued 

 flow of air over the extensive surfaces of the dry, heated 

 deserts that have been named, is to be found in the large 

 amount of condensation of vapour that is taking place against 

 the lofty Himalaya Mountains, which vapour is known to 

 have been brought from the Indian Ocean ; and its con- 

 densation is shown to be able to produce a comparative 

 vacuum, and to create an ascending current powerful enough 

 to draw air from the remote distances that have been named. 

 The moving mass constitutes a general north wind in the 

 deserts of Bokhara and Persia, and a north-west wind over 

 Arabia, and it is converted into a west wind in its passage 

 across the Arabian Sea ; these various directions being 



* See Mrs, Postans' book On CtUch, p. 259. 



