in different parts of the Globe. 127 



4. From the combined action of the variations of the aqueous 

 vapour and of the dry air, we now derive immediately the 

 periodical variations of the whole atmospheric pressure. As 

 the dry air and the aqueous vapour mixed with it press in com- 

 mon on the barometer, so that the upborne column of mercury 

 consists of two parts, one borne by the dry air. the other by 

 the aqueous vapour, we may well understand that as with 

 increasing temperature the air expands, and by reason of its 

 augmented volume rises higher, and at its upper portion 

 overflows laterally, — while, at the same time, the increased 

 temperature causes increasing evaporation, and thus augments 

 the quantity of aqueous vapour in the atmosphere, — so it 

 naturally follows that the composite result in the periodical 

 variations of the barometric pressure should not everywhere 

 bear a simple and immediately obvious relation to the 

 periodical changes of temperature. It is only when we know 

 the relative preportions of the two variations which take 

 place in opposite directions, that we can determine whether 

 their joint effect will be an increase or a decrease with in- 

 creasing temperature, — whether in part of the period the one 

 variation may preponderate, and in other parts the other 

 variation. The following are the results which we are 

 enabled to derive from observation. 



5. Throughout Asia the increase in the elasticity of the 

 aqueous vapour with increasing heat is never sufficient to 

 compensate the diminished pressure of the dry air ; and the 

 annual variation of barometric pressure is therefore every- 

 where represented, in accordance with the variation of the 

 pressure of the dry air, by a simple concave curve having its 

 lowest part or minimum in July. The observations in Taimyr 

 Land, at Iakoutsk, Udskoi, and Aiansk, shew that this is true 

 up to the Icy Sea on the north, and to the sea of Ochotsk on 

 the east. On the west a tendency towards these conditions 

 begins to be perceived in European Russia in the meridian 

 of St Petersburg, and becomes more marked as the range 

 of the Ural is approached. On the Caspian and in the Cau- 

 casus the phenomenon is already very distinctly marked ; its 

 limit runs south from the western shore of the Black Sea, so 

 that Syria, Egypt, and Abyssinia, fall within the region over 



