i f h
j)henoraena prove to demonstration that constant changes take place, the consequences
of new combinations and decompositions rapidly following each other.
1365. With respect to the changes in the weight o f the atmosphere, it is gcncraUy known
that the instmmcnt called the barometer shows tho weight of a body of air immediately
above it, extending to tlie extreme lioundaiy of the atmosphere, and the base of which is
eqnal to that of tlic mercury contained within it. As the level of the sea is the lowest
point of observation, tho column of air over a bai'omctcr placed at that level is the longest
that can be obtained.
1366. The variations q f the barometer between the tropics arc veiy trifling; they
increase gradually as the latitude advances towards the poles, till in the end they amount
to two or three inches. The following Table will explain this gvadual increase: —
02 O'
22 23
33 55
40 55
51 8
53 13
53 23
59 56
P eru -
Calcutta
Cape Town
Naples
Dover
Micldlewich
Liverpool -
St. Petersburgh
Range of tlio Baroniotcr.
0 20
0 77
0 89
1 80
1 94
1 96
2 77
1367. The range o f the barometer is considerably
less in North America than in the
corresponding latitudes o f Europe, pai'ticularly
in Virginia, where it never exceeds
IT. The range is more considerable at the
level of the sea than on mountains; and in
the same degi'ce of latitude it is in the inverse
ratio of the height of tho place above
the level of the sea. Cotte composed a table,
which has been published in the Journal de
Physique, from wliicli it appears extremely
probable, that the barometer has an invai-iablc tendency to rise between the morning and
the evening, and that this impulse is most considerable from two in the afternoon till nine
at night, when the greatest elevation is accomplished; but the elevation at nine differs from
that at two by four-twelfths, while that of two varies from the elevation of the morning
only by one-twelfth, and that in pai'ticulai' climates the greatest elevation is at two o’clock.
The obsciwations of Cotte confirm those of Luke Ilowai-d; and from them it is concluded,
that the barometer is infiucnced by some depressing cause at new and full moon,
and that some other makes it rise at the quarters. This coincidence is most considerable
in fair and calm weather; the depression in the interval between the quarters and
conjunctions amounts to one-tenth of an inch, and the rise from the conjunctions to the
quarters is to the same amount. The range of this instniment is found to be greater
in winter than in summer; for instance, She mean at York, during the months from
October to Mai-ch inclusive, in the year 1774, was 1'42, and in the six summer months
1*016.
1368. The more serene and settled the weather, the higher the barometer ranges: calm
weaüier, with a tendency to rain, depresses i t ; Iiigh winds have a similar effect on i t ;
and the gi'catest elevation occiu'S witli easterly and northerly winds; but the south produces
a dfrectly contrary effect.
1369. The variations in the temperature o f the air iu any particular place, exclusive of
the differences of seasons and climates, arc veiy considerable. These changes cannot
be produced by heat derived from the sun, as its rays concentrated have no kind of effect
on air; these, however, heat the surface of om* globe, from which heat is communicated
to tlie immediate atmosphere; it is through this fact that the temperature is highest where
tho place is so situated as to receive with most effect the rays of the sun, and that it
varies in each region with the season; it is also the cause why it decreases in proportion
to the height of the air above the surface of the earth. The most perpendicular rays
falling on the globe at the equator, there its heat is the greatest, and that heat decreases
gradually to the poles; of course the temperatiu-e of the air is in exact unison: from tliis
it appears that the air acquires the greatest degree of w'armth at the equator, whence it
becomes insensibly cooler till we airive at the poles; in the same manner the air
immediately above the equator cools gradually. Though the temperature sinks as it
approaches the pole, and is highest at the equator, yet as it vai-ies continually with the
seasons, it is impossible to form an accui-ate idea of the progression without forming a
mean temperature for a year, from that of the temperature of every degree of latitude for
every day of the year, wliich may be accomplished by adding together the wliole of the
observations and dividing by tlieir number, when the quotient will be the mean temperature
for the year. The “ diminution,” says Dr. Thomson, from the pole to the
equator takes place in arithmetical progression; or, to speak more properly, the annual
temperatures of all the latitudes are arithmetical means between the mean annual
temperature of the equator and the pole; and, as far as heat depends on the action of
solar rays, that of each month is as the mean altitude of the sun, or rather as the sine of
the sun’s altitude. La ter observations, however, have shown that all the formnlai for
calculating the mean temperatures of different latitudes, which are founded on Mayer’s
Empirical Equation, though tolerably accurate in the Northern Atlantic Ocean, to
latitude 60°, are totally iiTeconcilable with observations in very high latitudes; and
on the meridians, from 70° to 90° W. and E. of London. The results of late ai-ctic
voyages, and of Russian travels, have been satisfactoriiv shown by Sir David Brewstcr
(F Jm . Phil. Tr.), to prove the existence of two meridians o f greatest cold in the northern
hemisphere; and the mean temperature of pai-ticulaj- countries varies, not only according
to the parallels of latitude, but also according to thcir proximity to these two cold
meridians. (T .)
1370. Inconsiderable seas, in temperate and cold climates, are colder in winter and
warmer in summer than the main ocean, as they arc necessarily under the influence of
natural operations from the land. Thus, the Gulf of Bothnia is generally frozen in
winter, but the water is sometimes heated in the summer to seventy dcgi-ees, a state
which the opposite part of the Atlantic never acquh-es ; the German Sea is five degrees
wanner in summer than the Atlantic, and more than tliree colder in winter ; the IMcdi-
terraiiean is almost thi-oughout wai-mer hoth in winter and summer, which therefore
causes the Atlantic to flow into it ; and the Black Sea, being colder than the Mediterranean,
flows into the latter.
1371. The eastern parts o f North America, as it appears from meteorological tables,
have a much colder air than the opposite European coast, and fall short of the standard
by about ten or twelve degrees. There are several causes which produce tliis considerable
difference. The greatest elevation in North America is between the 40th and 50th
degree of north latitude, and the lOOtli and 110th of longitude west from London ; and
there the most considerable rivers have their origin. The height alone will partly exjilain
why this tract is colder than it would otherwise be ; but there are other causes, and those
arc most extensive forests, and large swamps and morasses, all of which exclude heat from
the earth, and consequently prevent it from ameliorating the rigom- of winter. Many
extensive lakes lie to the cast, and Hudson’s Bay more to the north ; a chain of mountains
extends on the south of the latter, and those equally prevent the accumulation of
heat ; besides, this bay is bounded on the east by the mountainous country of Labrador,
and has many islands ; from aU which circumstances arise the lowiiess of the temperature,
and the piercing cold of the north-west winds. The annual decrease of the forests for
the piu-posc of clearing the ground, and the consumption for building and fuel, is supposed
to have occasioned a considerable decrease of cold in the winter ; and if this should be
the result, much will yet be done towards bringing the temperature o fth e European and
American continents to something like a level.
1372. Continents have a colder atmosphere than islands situated in the same degree of
latitude ; and countries lying to tlie windward of the superior classes of mountains, or
forests, arc wanner than those which are to the leeward. Earth, always possessing a
certain degree of moisture, has a greater capacity to receive and retain heat than sand
or stones ; the latter, therefore, ai-e heated and cooled with more rapidity : it is from this
circumstance that the intense heats of Africa ancl Ai-abia, and the cold of Tcn-a del
Euego, ai-e derived. The temperature of growing vegetables changes very gradually ;
but there is a considerable evaporation from tliem : if they exist in great numbers, and
congregated, or in forests, thcir foliage preventing the rays of the sun from reaching the
earth, it is perfectly natural that the immediate atmosphere must be greatly afiected by
the ascent of chilled vapours.
1373. Our next object is the ascent and descent o f water : the principal appearances of
this element arc vapour, clouds, dew, rain, fi-ost, hail, snow, and ice.
1374. Vapour is water rarefied by heat, in consequence of which, becoming lightcr
than the atmosphere, it is raised considerably above the surface of the earth, and afterwards
by a partial condensation foims clouds. I t differs from exhalation, which is
properly a dispersion of diy particles from a body. Wlicn water is heated to 212° it
boils, and is rapidly converted into steam ; and the same change takes place in much
lower temperatures ; but in that case the evaporation is slower, and the elasticity of the
steam is smaller. As a veiy considerable proportion of the earth’s surface is covered with
water, ancl as this water is constantly evaporating and mixing with the atmosphere in the
state of vapour, a precise determination of the rate of evaporation must be of very great
importance in meteorology. Evaporation is confined entirely to the surface of the water ;
hence it is, in all cases, proportional to the surface of the water exposed to the atmosphere.
Much more vapour of coiu'so rises in mai-itirae countries, or those interspersed
with lakes, than in inland countries. Much more vapour rises during hot wcatlicr than
during cold : hence the quantity evaporated depends in some measure upon temperature.
The quantity of vapour which rises from water, even when the temperatm-e is the same,
varies according to cfrcumstanccs. I t is least of all in calm weather, greater when a
breeze blows, and greatest of all with a strong wind. Prom experiments, it apjieai-s, that
the quantity of vapour raised annually at Manchester is equal to about twenty-five inches
of rain. I f to this we add five inches for the dew, with Dalton, it will make the annual
evaporation thirty inches. Now, if wc consider the situation of England, and the gi-eatcr
quantity of vapour raised from water, it will not surely be considered as too great an
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