296 M. Dove on the theory of the Variations 



was to be investigated, without seeking for physical causes, it 

 was natural that a phaenomenon, in which opposite effects result- 

 ing from two different causes counterbalance each other, should 

 altogether escape notice. It is, perhaps, more remarkable that 

 no surprise should have been excited when the atmospheric pres- 

 sui'e was not found to diminish from winter to summer, with 

 increasing heat 



When, by the labours of Prinsep more particularly, the phae- 

 nomena of the tropical atmosphere in Hindostan became more 

 known, there was seen to be a great difference between the baro- 

 metric variation there and in tropical America ; inasmuch as the 

 Indian observations showed a decidedly well-marked annual 

 variation. A new en-or was now fallen into, and it was supposed 

 that the phsenomenon did not extend beyond the torrid zone, 

 and that it was an immediate consequence of the periodical 

 change of wind, i. e. of the monsoons. This erroneous view was 

 completely refuted when the barometric relations at the Siberian 

 stations became known ; for it was then found, that north of the 

 Himalaya (which in the supposed hypothesis must have formed 

 the limit of the ph?enomenon), the annual barometric variation 

 was exhibited on a large scale, and over a region so extensive, 

 that the shores of the Icy Sea itself could hardly be assumed as 

 its boundary. A greatly diminished atmospheric pressure taking 

 place in summer over the whole continent of Asia must produce 

 an influx from all surrounding parts ; and thus we have west 

 winds in Europe, north winds in the Icy Sea, east winds on the 

 east coasts of Asia, and south winds in India. The monsoon 

 itself becomes, as we see, in this point of view only a secondary 

 or subordinate phsenomenon. 



I have endeavoured to establish the reality of the above phse- 

 nomenon and its climatological bearings in several memoirs ; and 

 I must refer for the numerical values to Poggendorff's Annalen, 

 vol. Iviii. p. 177; vol. Ixxvii. p. 309; and to the Berichte oi the 

 Berlin Academy, 1852, p. 285. I will here embody the results 

 in distinct pro])ositions, in order to show, in connexion there- 

 with, the importance of the bearing of the Hobarton observations. 



1. At all stations of observation in the torrid and temperate 

 zones the elasticity of the aqueous vapour contained in the atmo- 

 sphere increases with increasing temperature. In the region of 

 the monsoons this increase from the colder to the warmer months 

 is greatest near their northern limit. Hindostan and China pre- 

 sent in this respect the most excessive climate. No differences 

 of similar magnitude are found in the southern hemisphere. 

 The form of the curve of elasticity of the aqueous vapour shows, 

 however, a less decidedly convex summit in the region of the 

 monsoons than beyond it, having in that i*egion rather the cha- 



