METEOROLOGY. 
must be least at the poles, and greatest at 
tlie equator, decreasing gradually in the 
interval, and thus forming the resemblance 
of two inclined planes, meeting at the high* 
est part above the equator. 
The difference of the mean heat between 
the pole and the equator, wlien the sun is 
in our hemisphere in the summer, does not 
vary so much as in the winter, as the heat 
at that period in northern countries equals 
that of the torrid zone ; hence the thermo- 
meter rises to 85" in Russia during the 
months of July and August, of necessity the 
rarity of the atmosphere and its height in- 
creases ; in consequence, the upper part in 
the northern hemisphere inclines less, but 
that of the southern, from different causes, 
must be much more inclined ; during our 
winter the exact reverse takes place. 
The pressure of the superincumbent co- 
lumn in a great measure causes the density 
of the atmosphere, and therefore decreases 
in proportion to the height as the pressure 
of the column constantly decreases, yet the 
density in the torrid zone does not decrease 
so rapidly as'in the temperate and frigid, 
as the column is longer, and because there 
is a larger proportion of air in the upper 
part of it. This fact agrees with the assertion 
of M. Cassan, “ that the barometer only 
sinks half as much for every two hundred 
feet of elevation in the torrid, as in the 
temperate zones.” The density at the 
equator, though less at the surface of the 
eartli, must equal at a certain height, and 
Still higher exceed the density in the tem- 
perate zones, and at the poles. 
It is ascertained that a current of air 
constantly ascends at the equator, part at 
least of which reaches to and remains in 
the highest parts of tlie atmosphere; the 
fludity of that body prevents it from ac- 
cumulating above the equator, and hence 
it must descend the declined plane before 
mentioned. The surface of the atmosphere 
being more inclined in the northern hemis- 
phere during our winter than that of the 
soulhern, more of the current must flow 
on the northern than on the southern, from 
which cause the quantity of one atinos- 
))here is greater in winter than that of the 
soulhern hemisphere ; in the summer it is 
just the conti'ary; consequently the range 
of the barometer is less in summer than 
in winter, and the greatest mercurial 
heights occur during winter. 
The heat of any given place in a great 
measure influences the density of its atmos- 
phere ; that density will be most consider- 
able where it is coldest, and its column 
shortest. Chains of mountains, the summits 
of which are covered with snow great part 
of the year, and highlands, must be colder 
than places less elevated in the same lati- 
tude, and the column of air over them 
much shorter. The current of air above must 
be impeded and accumulate while on its 
passage over these places towards the poles, 
which causing an agitation, it will be com- 
municated to, and indicated by, the baro- 
meter in a singular manner. These accu- 
mulations occur over the north-w'est parts of 
Asia and North America, and this raises 
the barometer, and causes less variation 
in it there than in Europe. It is precisely 
so on the Pyrenees, the Alps, and the 
mountains in Africa, Turkey in Europe, 
Tartary, and Tibet. After the accumula- 
tions have existed some time the sur- 
rounding atmosphere becomes incapable of 
balancing the density of the air, when it 
descends with violence, and occasions cold 
winds, which raise the barometer ; it is to 
this that we are to attribute the rise of the 
barometer, almost always attending north- 
east winds in Europe, which is the effect 
of accumulations near the pole, or in the 
north-west parts of Asia ; it is thus besides 
that the north-west wind from tlie moun- 
tains of Tibet raises the barometer at Cal- 
cutta. It may be supposed that in the 
polar regions large quantities of air are 
casually compressed ; when this is the case 
the southern atmosphere must rush in to 
replace it, which occasions south-west gales 
and the fall of the barometer. 
The mean heat of our hemisphere vary- 
ing in successive years, the density of the 
atmosphere, and necessarily, the quantity 
of eqnatorical air passing towards the poles, 
cannot be otlierwise than variable, hence 
occurs the different ranges of the barometiu- 
in successive years; at some particular 
periods, more considerable accumulations 
take place in the highest parts of Asia, and 
the south of Europe, than at others, which 
may be produced by early falls of snow, or 
the interruption of the sun’s rays by long 
continued fogs ; at such times the atmos- 
phere in the polar regions becomes propor- 
tionably lighter, and this causes the pre- 
valence of southerly winds in some winters 
more than in others. The heat of the 
torrid zone never greatly valuing, the 
height and density of the atmosphere un- 
dergoes but few changes, thence arises the 
comparatively small range of the barometer 
witliiii tlie tropics, which gradually in- 
