126 Annual Variation of Atmospheric Pressure 



wind is either comparatively constant throughout the year, 

 or where it changes in the contrary sense to that above de- 

 scribed. At the last-named class of places the rate of de- 

 crease in the mean annual tension of the aqueous vapour 

 with increasing distance from the equator is more rapid than 

 in the first class. 



2. At all stations in Europe and Asia, the pressure of the 

 dry air decreases from the colder to the warmer months, 

 and everywhere in the temperate zone has its minimum in 

 the warmest month. 



3. If we compare the annual variation of the pressure of 

 the dry air in Northern Asia and Hindostan with the varia- 

 tion in Australia and the Indian Ocean, we shall be satisfied 

 that something more takes place than a simple periodical 

 exchange of the same mass of air in the direction of the 

 meridian, between the northern and southern hemispheres. 

 From the magnitude of the variation in the northern hemi- 

 sphere, and the extent of the region over which it prevails, 

 we must infer that at the time of diminished pressure a 

 lateral overflow probably takes place ; that it actually does 

 so may be considered as proved for the northern part of the 

 region, by the fact that at Sitka, on the north-west coast of 

 America, the pressure of the dry air increases from winter 

 to summer. It is not probable that the overflow takes place 

 exclusively to the east, it probably occurs also to the west ; 

 and on this supposition the small amount of the diminution 

 of the pressure of the dry air from winter to summer in 

 Europe would be caused, not solely by the moderate amount 

 of the difference of temperature in the hotter and colder 

 seasons, but also by the lateral afflux of air in the upper 

 regions of the atmosphere tending to compensate the pres- 

 sure lost by thermic expansion. As at the northern limit of 

 the monsoon, at Chusan and Pekin, the annual variation of 

 the pressure of the dry air is most considerable, while at the 

 northern limit of the trade-wind in the Atlantic Ocean, i. e. 

 at Madeira and the Azores, it is very small, it is probable 

 that there is in the torrid zone also a lateral overflow in the 

 upper strati < f the atmosphere, from the region of the mon- 

 soons to that of the trades. 



