of Atmospherical Phenomena. '^'( 297 



racter of a flattened summit or table-land, the elasticity con- 

 tinuing nearly the same throughout the period of the rainy mon- 

 soon. Near the equator the convex curve of the northern hemi- 

 sphere becomes, first flattened, and then gradually transformed 

 into the concave curve of the southern hemisphere. In the Atlantic 

 this transition takes place in a rather more northerly parallel. 

 In regard to the magnitude of the annual variation, the following 

 rule appears generally applicable in the torrid zone : the annual 

 variation is considerable at all places where equatorial currents 

 prevail when the sun^s altitude is greatest, and polar currents 

 when the sun's altitude is least; and inconsiderable wherever 

 the direction of the wind is either comparatively constant 

 throughout the year, or where it changes in the contrary sense 

 to that above described. At the last-named class of places the 

 rate of decrease 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 every- 

 where 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 variation in 

 Australia and the Indian Ocean, we shall be satisfied that some- 

 thing 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 hemisphere, 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 mcreases 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 tend- 

 ing to compensate the pressure 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 consider- 

 able, 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 pro- 

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



