39% M. Dove on tJie theory of the Variations 



upper strata of the atmosphere from the region of the monsoons 

 to that of the trades. 



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

 vapour and of the dry air we now derive immediately the perio- 

 dical variations of the whole atmospheric pressure. As the dry 

 air and the aqueous vapour mixed with it press in common 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 tempe- 

 rature 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 proportions 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 

 increasing 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 com- 

 pensate the diminished pressure of the dry air, and the annual 

 variation of barometric pressure is therefore everywhere repre- 

 sented, 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 lakousk, 

 Udskoi and Aiansk, show 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 Caucasus the phsenomenon is already vei-y 

 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 which it prevails. Towards the confines of 

 Europe there is almost everywhere a maximum in September or 

 October, the barometric pressure increasing rapidly from July to 

 the autumn. This maximum is followed towards the latter part 

 of the autumn by a slighter inflexion or secondary minimum ; it 

 is only beyond the Ural that the curves become uniformly con- 

 cave, with a single summer minimum and winter maximum, 



