which the original movement commenced, ■ 
After a continued gradual rise on the other 
hand, there usually occurs a similar depres- 
sion. Except on the eve of great storms, the 
rising movement is however the more rapid 
•f the two. 1 he undulations which are to 
be found in the curve corresponding to the 
intervals between the phases of the moon, 
often comprehend in their sweep some smaller 
ones, which appear to be due to occasional 
and less extensive causes. 
It happens also from some principle of the 
kind above stated, that these movements, 
which in fair and moderate weather proceed 
with considerable regularity, on being dis- 
turbed by storms, a. e not resumed suddenly 
but 'by degrees, and the interruption is per- 
ceptible for a considerable space afterwards. 
In long periods of wet weather, the baro- 
meter usually keeps about the mean altitude, 
rising and falling through a short space with 
little regularity. 
In serene and settled weather it is gene- 
rally high ; and low in calm weather, when 
the air is inclined to rain ; it sinks on high 
winds, rises highest on easterly and northerly 
winds, and sinks when the wind blows from 
the south. At Calcutta, it is always highest 
when the wind blows from the north-west and 
north, and lowest when it blows from the 
south-east. 
Such are the phenomena respecting the 
variations of the barometer, as tar as they 
can be reduced under general heads. Va- 
rious attempts have been made to explain 
them, but hitherto without any great degree 
of success. The theory of Mr. Kirwan ap- 
pears by far the most plausible, though it is 
not sufficient to explain all the facts. The fol- 
lowing observations may be considered as a 
kind of abstract of his theory, except in one 
or two instances. 
It is evident, that the density of the atmo- 
sphere is least at the equatoi>and greatest at 
the poles; for at the equator, the centrifugal 
force, the distance from the centre of the 
earth, and the heat, all of which tend to di- 
minish the density of the air, are at their 
maximum, while at the pole they are at their 
minimum. The mean height of the baro- 
meter at the level of the sea, all over the 
globe, is 30 inches ; the weight of the atmo- 
sphere, therefore, is the same all over the 
globe. The weight of the atmosphere de- 
pends on its density and height: where the 
density of . the atmosphere is greatest, its 
height must be least; and, on the contrary, 
where its density is least, its height must be 
the greatest. The height of the atmosphere, 
therefore, must be greatest at the equator, 
and least at the poles; and it must decrease 
gradually between the equator and the poles: 
so that its upper surface will resemble two 
inclined planes, meeting above the equator 
in their highest part. 
During summer, when the sun is in our 
hemisphere, the mean heat between the 
equator and the pole does not differ so much 
as in winter. Indeed, the heat of northern 
countries at that time equals the heat of the 
torrid zone: thus in Russia, during July and 
August, the thermometer rises to <S5°. Hence 
the rarity of the atmosphere at the pole, and 
consequently us height, will be increased. 
The upper surface of the atmosphere, there- 
fore, in the northern hemisphere, will be less 
METEOROLOGY. 
inclined, while that of the southern hemi- 
sphere, from contrary causes, will be. much 
more inclined. The very reverse will take 
place during our winter. 
The density of the atmosphere depends in 
a great measure on the pressure of the super- 
incumbent column ; and therefore decreases, 
according to the height, as the pressure of 
the superincumbent column constantly de- 
creases. But the density of the atmosphere 
in the torrid zone will not decrease so fast as 
in the temperate and frigid zones; because 
its column is longer, and because there is a 
greater proportion of air in the higher part 
of this column. This accounts for the obser- 
vation of Mr. Cassan, that the barometer 
only sinks half as much 'for every 200 feet of 
elevation in the torrid as in the temperate 
zones. The density of the atmosphere at the 
equator, therefore, though at the surface of 
the earth it is less, must at a certain height 
equal, and at ti still greater surpass, the den- 
sity of the atmosphere in the temperate zones 
and at the poles. 
A current of air is constantly' ascending at 
the equator, and part of it at least reaches 
and continues in the higher parts of the at- 
mosphere. From the fluidity of air, it is evi- 
dent, that it cannot accumulate aboVe the 
equator, but must roll down the inclined 
plane which the upper surface of the atmo- 
sphere assumes towards the poles. As the 
surface of the atmosphere of the northern 
is more inclined during our winter than that 
of the southern hemisphere, a greater quan- 
tity of the equatorial current of air must flow 
over upon the northern than upon the south- 
ern atmosphere ; so that the quantity of our 
atmosphere will be greater during winter 
than that of the southern hemisphere: but 
during summer the very reverse will take 
place. Hence the greatest mercurial heights 
take place during winter, and the range of 
the barometer is less in summer than in win- 
ter. 
As the heat in the torrid zone never differs 
much, the density, and consequently the 
height, of the atmosphere, will not vary 
much. Hence the range of the barometer 
within the tropics is comparatively small; 
and it increases gradually as we approach the 
poles, because the difference of the tempera- 
ture, and consequently of the density, of the 
atmosphere, increases with the latitude. 
The diurnal elevation of the barometer in 
the torrid zone corresponding to the tides, 
observed by Mr. Cassan and others, must be 
owing to the influence of the moon on the at- 
mosphere. This influence, notwithstanding 
the ingenious attempts of D’Alembert and 
several other philosophers, seems altogether 
inadequate to account for the various phe- 
nomena of the winds. It is not so easy to ac- 
count for the tendency which the barometer 
lias to rise as the day advances, which seems 
to be established by Mr. Cotte’s table. Per- 
haps it may be accounted for by the addi- 
tional quantity of vapour added to the atmo- 
sphere, which, by increasing the quantity of 
the atmosphere, may possibly be adequate to 
produce the effect. 
The falls of the barometer which precede, 
and the oscillations which accompany, vio- 
lent storms and hurricanes, shew us, that 
| these phenomena are produced by very great 
rarefactions, or perhaps destruction of air, in 
particular parts of the atmosphere. The fails 
1/1 
of the barometer, too, that accompany winds, 
proceed from the same cause. 
That the temperature of the air varies con- 
siderably, not only in different climates and 
in different seasons, but even in the same 
place and in the same season, must be ob- 
vious to the most careless observer. This 
perpetual variation cannot be ascribed to the 
direct heat of the sun; for the rays of that 
luminary seem to produce no efiect what- 
ever upon air, though ever so much concen- 
trated; but they warm the surface of the 
earth, which communicates its heat to the 
surrounding atmosphere. Hence it happens, 
that the temperature of the air is highest in 
those places which are so situated as to be most 
warmed by the sun’s rays, and that it varies 
in every region with the season of the year. 
Hence too the reason why it diminishes ac- 
cording to the height of the air above the 
surface of the earth. That portion of the 
earth which lies at the equator, is exposed to 
the most perpendicular ray;; of the sun. Of 
course it is hottest, and the heat of the earth 
diminishes gradually from the equator to the 
poles. The temperature of the air must fol- 
low the same order. The air, then, is hot- 
test over the equator ; and its temperature 
gradually diminishes from the equator to the 
poles, where it is coldest of all. It it hottest 
at the surface ; and it becomes gradually 
colder, according to its height above that 
surface. Let us examine the nature ef these 
two diminishing progressions of tempera- 
ture. 
1. Though the temperature of the air is 
highest at the equator, and gradually sinks as 
it approaches the pole, yet as hi every place 
the temperature of the air is constantly vary- 
ing with the season of the year, we cannot 
form any precise notion of the progression, 
without taking the temperature in every de- 
gree of latitude for every day of the year, 
and forming from each a mean* temperature 
for the whole year ; which is done by adding 
together the whole observations, and dividing 
by their number. The quotient gives the 
mean temperature for the year. The dimi- 
nution from the pole to the equator takes 
place in arithmetical progression ; or, to 
speak mote properly, the annual tempera- 
tures of all the latitudes are arithmetical 
means between the mean annual tempera- 
ture of the equator and the pole. This was 
first discovered by Mr. Mayer; and bv 
means of an equation which he founded on 
it, but rendered considerably plainer and sim- 
pler, Mr. Kirwan has calculated the mean 
annual temperature of every degree of lati- 
tude between the equator and the pole. He 
proceeded on the following principle: Let 
the mean annual heat at the equator be m, 
and at the pole m — n ; put f for any other 
latitude; the mean annual temperature of 
that latitude will be m — • n x sin. p 2 . If 
therefore the temperature of any two lati- 
tudes is known, the- value of m and n may 
be found. Now the temperature of north 
latitude 40° has been found by the best ob- 
servations to be 62.1°, and that of latitude 
50°, 52.9°. 'i He square of the sine of 40° is 
nearly 0.419, and the square of the sine of 
50* is nearly 0.586. Therefore, 
in — 0.4-1 n ~ 62. 1 , and 
in — 0.58 n — 52.9 ; therefore, 
62. 1 0.41 n — 52.9 -{- 0.58 n ; as each 
of them, from the two first equations, is equal 
