34 M. Dove on the theory of the Variations 
appears generally applicable in the torrid zone: the annual varia- 
tion 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 says with spoken distance 
from a equator is more rapid than in the first cla 
all stations in Europe and Asia ‘the pressure of the dry 
air aulvenii from the colder to the warmer months, and every- 
where in the temperate zone has its minimum in the warmest 
month. 
_ If we compare the annual variation of the pressure of the 
dr ry 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 change 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 henriepbers 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 northwest 
coast of America, ‘the pressure of the dry air increases from win- 
ter 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 Atlan- 
tic 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 strata of the atmosphere from the region of the mon- 
soons to that of the trades 
A, From the combined action of the variations of the aqueous 
vapor and of the dry air we now derive immediately the peri- 
odical variations of the whole atmospheric peoeenre: As the dry 
air and the aqueous vapor mixed with it press in common on the 
ometer, oe that the upborne column of mercury consists of 
one. by the dry air, the other by the aqueous va- 
ll understand _— as with increasing — 
