416 



Mr. J. K. Laughton on the Natural Forces that 



The expansion of the air over the desert of Gobi, and Central 

 Asia generally, is laid down by Captain Maury as entirely, or in 

 very great part, the cause of the monsoons of the Indian Ocean. 

 Mr. Blodgett, in his interesting work ' On the Climatology of the 

 United States/ and almost every other writer on the subject, have 

 expressed the same opinion. But in opposition to this we have 

 the positive assurance of Humboldt, after an almost exhaustive 

 inquiry into the physical geography of Central Asia, that " the 

 lofty chain of the Himalayas is a boundary to the climate of 

 India : the monsoons, and the changes of dry and wet seasons 

 which depend on them, do not extend beyond it." (L'Asie Cen- 

 trale, vol. iii. p. 291.) " Thibet," he continues, " has a climate of 

 its own, about which we as yet know only the extreme dryness, 

 and that enormous difference of temperatures, day and night, sum- 

 mer and winter, which is peculiar to all tablelands where the air 

 has but little density." 



This is exactly what we might expect : a range of lofty moun- 

 tains is a barrier that surface-wind is unable to cross ; air can- 

 not bound over peaks of great elevation ; nor can the air which 

 at great elevations has become extremely rarefied descend with 

 a steady rush into the denser air of low-lying plains. The mean 

 elevation of the Gobi desert, according to Humboldt, is, at the 

 most, 4000 feet ; whilst the central part of it is somewhat under 

 2500 feet. To whatever extent the air over this desert plain 

 may be rarefied by the heat of summer, the diminution of its 

 density can certainly never approach to an equality with that 

 due to a height of from 20,000 to 30,000 feet. 



In opposition to the views held by Captain Maury, Professor 

 Dove considers that the monsoons are caused by the difference of 

 the elastic force of the aqueous vapour in Siberia and India. 

 (Gesetz der Stiirme, p. 50, 3rd edit.) It is difficult to under- 

 stand the force of his argument ; for according to his own Tables 

 the barometric pressure (that is, the entire elastic force of the 

 atmosphere) is very nearly the same at the two places which he 

 names in illustration of his meaning ; these Tables, when re- 

 duced to English inches, give : — 





Mean for the 

 year. 



Variation for 

 July. 



Variation for 

 January. 



Calcutta 



Barnaul 



29-636 

 29-329 



•303— 

 •354- 



•387 + 

 •301 + 



Elastic force will produce the same pressure, and will affect 

 equilibrium in the same manner, whether it is the elasticity of dry 

 air or of aqueous vapour ; and, volume for volume, under equal 

 temperatures, dry air is heavier than moist. It is impossible to 



