128 Annual Variation of Atmospheric Pressure 



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 inflection or secondary minimum ; 

 it is only beyond the Ural that the curves become uniformly 

 concave, with a single summer minimum and winter maxi- 

 mum, which character they retain throughout the rest of the 

 Asiatic continent, even to its eastern coast. In winter, the 

 absolute height of the barometer at the northern limit of i^he 

 monsoon is very great. The still considerable amount of the 

 annual variation at Nangasaki, and the little difference 

 between the curve of Manilla and that of Madras, shew that 

 the region in question extends beyond the eastern coast of 

 Asia into the Pacific Ocean ; in higher latitudes, hewever, 

 its limits appear to be reached in Kamtschatka. As the an- 

 nual variation which is greater at Madras than at Manilla is 

 found greater at Aden than at Madras, the western limit of 

 the region would appear to extend far on the African side. 



6. In middle and western Europe the barometric pressure 

 appears to decrease everywhere from the month of January 

 to the spring, usually attaining a minimum in April ; it then 

 rises slowly but steadily to September, and sinks rapidly 

 to November, when it usually reaches a second minimum. 

 In summer, therefore, the whole atmospheric pressure gains 

 more by increased evaporation than it loses by expansion. 

 This over- compensation is probably, as we have seen above, 

 to be explained by the lateral overflow received in the upper 

 regions from Asia. In Sitka the whole annual curve is con- 

 vex, a result only found in Europe at considerable mountain 

 elevations, where it is a consequence of the expansion, and 

 extension upwards, of the whole mass of the atmosphere in 

 summer. 



7. The region of great annual barometric variation, on the 

 Asiatic side of the globe, where monsoons prevail, extends 

 much farther to the north in the northern hemisphere, than 

 it does to the south in the southern hemisphere ; for the 

 variation reaches its maximum at Pekin, while at Hobarton, 

 in nearly a corresponding latitude, it has already become in- 



