Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 13, MAT-MIC

708
Gravity of
the Air.
*3
Atmo-
fphere
forms two
inclined
planes
meeting at
the equa¬
tor.
M E T E O
10. They are greater alfo in the daytime than durino-
, the mo-ht. °
mmJ
11. The higher the fun rifes above the horizon, the
Ids are the ofcillations j they increafe as he approaches
the weftern fide of the horizon, and are exceedinofy
great when he comes oppofite to the eaftern part of the
horizon.
12. They are, to a certain degree, independent of
the changes of temperature.
13. The mercury generally rifes between the new
and the full moon, and falls between the latter and the
new moon.
. I*- rj^es more in the apogee than the perigee:
it ufually rifes between the northern luniftice and the
louthern, and falls between the fouthern luniftice and
the northern.
15. In general, a comparifon of the variations of
the mercury with the pofitions of the moon gives nothing
certain, j the refults of N* 13. and 14. are the moft
conftant.
16. In the neighbourhood of Paris the barometer ne¬
ver continues 24 hours without changing.
- *7- The barometers in the weftern diftrifts rife and
tall looner than thofe in the more eaftern.
18* When the fun pafles the meridian, the mercury
if falling, continues to fall, and its fall is often haftened. ’
. *9- When the mercury at the fame period is rifing
it falls, remains ftationary, or rifes more flowly.
. 2°- When the mercury, under the fame circumftances
is ftationary, it falls, unlefs before or after it becomes
.ftationary, it has been in the a& of rifing.
21. The above changes commonly take place between
2 x in the morning and 1 in the afternoon, but oftener
before than after noon.
22. Before high tides there is almoft alw'ays a great
fall of the mercury j this takes place oftener at the full
than the new moon.
. Sucl1 is a general view of the variations in the gra¬
vity of the air, as far as they have been obferved by the
barometer ; and we (hall now endeavour to give fome
plaufible theory of them.
It is evident that the denfity of the atmofphere is
of J , n . .1 , 1 „
E ° L O G Y. Chap. 1.
lefs inclined j while that of the fouthern hemifphere, Gravity 0f
^ Tn'/b
Tranf. vol.
h- p. 43,
&c.
leaft at the equator, and greateft at the poles ; for at
the equator the centrifugal force, the diftance from
the centre of the earth, and the heat (all of which
tend to diminifh the denfity of the air), are at their
maximum, while at the poles they are at their mini¬
mum. The mean height of the barometer at the level
of the fea, all over the globe, is 30 inches ; the weight
of the atmofphere, therefore, is‘the fame all over the
globe. This weight depends on the denfity and height
of the air; where the denfity is greateft, its height
muft be leaft ; and on the contrary, where its denfity is
leaft, its height muft be. greateft. The height of the
atmolphere, therefore, muft be greateft at the equator,
and leaft at the poles ; and it muft decreale gradually
between the equator and the poles, fo that its upper
furface will refemble tw7o inclined planes, meeting above
the equator their higheft part*.
During fummer, when the fun is in our hemifphere,
the mean heat between the equator and the pole does
not differ fo much as in winter. Hence the rarity of
the atmofphere at the pole, and confequently its height,
will be increafed. The upper furface of the atmo¬
fphere, therefore, in the northern hemifphere, will be
from contrary^ caufes, will be much more inclined. the Air.
-Lhe reverfe will take place during our winter. 1 v—
Ihe denfity of the atmofphere depends in a frreat
meafure on the preffure of the fuperincumbent column,
and therefore decreafes according to the height, as the
preiiure of the fuperincumbent column conftantly de¬
creafes.. But the denfity of the atmofphere in the torrid
zone will not decreafe.fo faft as in the temperate and
ingid zones, becaufe its column is larger, and becaufe
there is a greater proportion of air in the higher part
ivr ^column. This accounts for the obfervation of
Mr Caffon, that the barometer finks only half as much
for every 200 feet of elevation in the torrid as in the
temperate zones. The denfity of the atmofphere at
the equator, therefore, though at the furface of the earth
it is lefs, muft at a certain height equal, and at a Hill
greater muft exceed, the denfity of the atmofphere in
the temperate zones and at the poles.
. faa1} Patently endeavour to prove, that a quan-t,r,
tity of air is. conftantly afoending at the equator, andmerLr/i*
that pait of it at leaft reaches and continues in the high-higheft in
er parts of the atmofphere. From the fluidity of air, w‘nter in
it is evident that it cannot accumulate above the equa,n0lt^ern
tor, but muft roll down the inclined plane which thelatitudGS*
upper furface of the atmofphere affumes towards the
poles. As the furface of the atmofphere of the
northern hemifphere is more inclined during our winter
than that of the fouthern hemifphere, a greater quan¬
tity of the equatorial current of air muft flow over upon
the northern than upon the fouthern hemifphere; fo
that the quantity of our atmofphere will be greater du¬
ring ivinter than that of the fouthern hemifphere; but
during fummer the reverfe will take place. Hence
the gieateft mercurial heights take place during winter,
and the range of the barometer is lefs in fummer than
m Avinter.
I he denfity of the atmofphere is in a great meafure
legmated by the heat of the place ; AA'herever the cold
is greateft, there the denfity of the atmofphere will be
greateft, and its column fliorteft. High countries, and
ranges of lofty mountains, the tops of Avhich are covered
with fnoAV the greateft part of the year, muft be much
colder, than other places fituated in the fame degree
of latitude, and confequently the column of air over
them much fliorter. The current of fuperior air Avill
linger and accumulate over thefe places in its paffage
towards the poles, and thus occafion an irregularity in
its motion, which will produce a fimilar irregularity in
the barometer. Such accumulations Avill be formed
over the north-weftern parts of Afia, and over North
America; hence the barometer ufually Hands higher,
aiid varies lefs there, than in Europe. Accumulations
alfo are formed upon the Pyrenees, the Alps, the
mountains of Africa, Turkey in Europe, Tartary, and
1 ibet.. When thefe accumulations have gone on for
fome time, the denfity of the air becomes too great to
be balanced by.the furrounding atmofphere; it nifties
down on the neighbouring countries, and produces cold
winds which raife the barometer. Hence the rife of
the barometer which generally attends north-eaft winds
in Europe, as they proceed from accumulations in the
north-Aveft of.Afia, or about the pole; hence, too, the
north-weft Avind from the mountains of Tibet raifes the
barometer at Calcutta.
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