IN PaODTJOINd ASCENDING ATMOSPHERIC OUBKENTS. 



201 



or near to the tropics ; some, however, extending considerably 

 beyond that limit. From the equator up to the latitude of 

 30°, or, in certain parts, to 40"^ north, much sun-heated land 

 is found in the summer ; and an inquiry into the state of the 

 air over these lands at that time, and its various movements 

 about them, will enable us to judge of the influence of direct 

 solar heating of the surface of the land, on the atmosphere 

 which rests upon it. The deserts of Bokhara and Tartary, 

 for instance, extend far north, and they are raised to a very 

 high temperature in the summer; we may therefore begin 

 by adverting to the influence of direct solar heat on their 

 atmospheres. They may be considered as extending from 

 the Sea of Aral, in 45° north, towards the south. Accounts 

 pretty generally agree in representing the summer tempera- 

 ture here as reaching to 100=^ of Fahrenheit. Burnes, in his 

 Travels, says — " After having passed the Oxus, the country 

 was entirely destitute of water, but the wind blew steadily 

 from the north, (the hot country in the summer,) from which 

 quarter it blows constantly, — nor do I believe that it would 

 be possible to traverse this tract in the summer, if it ceased 

 to blow." Yet nearly south of this part there exist extensive 

 ranges of mountains covered with snow during the whole ol 

 the summer, which of course have a very low temperature. 

 Here then, if there were truth in the theory under consi- 

 deration, we might reasonably expect to find air ascending 

 from the heated surface, and other air flowing in from colder 

 districts. But not a breath of air appears to pass from the 

 parts that are covered with snow to the hot land of the desert. 

 On the contrary, the wind blows steadily from the part which 

 is highly heated at the time ; tliat is, the wind blows from the 

 hot desert towards the cold mountains, giving no countenance 

 to the theory. 



It may be imagined, however, that the atmosphere, though 

 it does not ascend here, rises from surface-heated land some- 

 where further south ; but this we shall find on inquiry is not 



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