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

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7H
Tempera¬
ture ef
the Air.
,.33
Of iflands.
2*
C-'otfe’s axi
om . re-
fpefting
tempera¬
ture .
ftandard by about lo° or i 2°, as appears from Ame¬
rican meteorological tables. The caufes of this re-
mJ roarkable difference are many. The highefl: jiart of
North America lies between 40° and 50° of north lati¬
tude, and ioo° and iio° of longitude weft from Lon¬
don, for there the greateft rivers originate. The very
height, therefore, makes this fpot colder than it would
otherwife be. It is covered with immenfe forefts, and
abounds with large fwamps and morafles, which render
it incapable of receiving any great degree of heat ; fo
that the rigour of winter is much lefs tempered by the
heat of the earth than in the old continent. To the
eaft lie a number of very large lakes, and farther north,
Hudfon’s bay ; about 50 miles on the fouth of which
there is a range of mountains which prevent its receiv¬
ing any heat from that quarter. This bay is bounded
on the eaft by the mountainous country of Labrador and
by a number of iflands. Hence the coldnefs of the
north-weft winds and the lownefs of the temperature.
But as the cultivated parts of North America are now
much warmer than formerly, there is reafon to expect
that the climate will become ftill milder when the coun¬
try is better cleared of woods, though perhaps it will
never equal the temperature of the old continent.
Iflands are warmer than continents in the fame de¬
gree of latitude ; and countries lying to the windward
of extenfive mountains or forefts are warmer than thofe
lying to the leeward. Stones or fand have a lefs capa¬
city for heat than earth has, which is always fomewhat
moift ; they heat or cool, therefore, more rapidly and
to a greater degree. Hence the violent heat of Arabia
and Africa, and the intenfe cold of Terra del Fuego.
Living vegetables alter their temperature very {lowly,
but their evaporation is great; and if they be tall and
clofe, as in forefts, thev exclude the fun’s ravs from
the earth, and {belter the winter fnow from the wind
and the fun. Woody countries, therefore, are much
colder than thofe which are cultivated.
We fhall conclude this chapter with a feries of meteo¬
rological axioms refpefting the temperature of the air,
by M. Cotte.
. 1. The extreme degrees of heat are almoft every¬
where the fame ; this, however, is not the cafe in re¬
gard to the extreme degrees of cold.
2. The thermometer rifes to its extreme height
oftener in the temperate zones than in the torrid zone.
3. It changes verv little between the tropics *, its
variations, like thofe of the barometer, are greater
the more one proceeds from the equator towards the
poles.
4. It rifes higher in the plains than on mountains.
5. It does not fall fo much in the neighbourhood of
the fea as in Inland parts.
6. The wind has no influence on its motions.
METEOROLOGY. Chap. Hi.
7. Moifture has a peculiar influence on it, if follow- Evapora¬
tion and
Rain.
ed by a wind which difperfes it.
8. The greateft heat, and the greateft cold, take
place about fix weeks after the northern or fouthern —J
folftice.
g. I he thermometer changes more in fummer than
in winter.
10. The coldeft period of the day is before fun-
rife.
11. The greateft heat in the fun and the {hade fcl-
dom takes place on the fame day.
12. The heat decreafes with far more rapidity from
September and Oftober, than it increafed from July to
September.
13. It is not true, that a very cold winter is the
prognoftic of a very hot fummer.
Chap. III. Of the Changes which take place in the
Air with refpeB to Evaporation and Rain.
There feems no reafon to doubt that water exifts in Qualities of
the atmofphere in an intermediate ftate between that ofvai)0ur.
a fluid and that of abfolute fteam. This is the ftate of
vapour, of the qualities of which it is proper that we
fliould here take a general view.
We are indebted to the experiments of Sauflure and
de Luc for much of our knowledge of the qualities of
vapour. It is an elaftic invilible fluid like common
air, but lighter j being to common air, according to
Sauflure, as 10 to 14, or, according to Kirwan, as 10 to
1 2$ it cannot pafs beyond a certain maximum of denfity,
otherwife the particles of water which compofe it unite
together, and form fmall, hollow, vifible veficles, cal-
ed vejicular vapour; which is of the fame fpecific
gravity with atmofpherical air. It is of this vapour
that clouds and fogs are compofed. This maximum
increafes with the temperature ; and at the heat of boil¬
ing water is fo great, that fteam can refift the whole
preflure of the air, and exift in the atmofphere in any
quantity.
After what has been ftated under Chemistry with
refpeft to the nature and properties of vapour, wre have
nothing here to add on that fubjedt, except to give the
refult of obfervations that have been made on the ftate
of vapour in the atmofphere. ^
It is found that the evaporation of water into the airEvapora-
is confined entirely to the furface, and hence it is alwaystior -onfin.
proportional to the furface expofed to the adlion of the ed t0 the
air. Accordingly, obfervation {hows that in maritimefuiface'
countries, and in marfliy fituations in the neighbour¬
hood of lakes, rivers, &c. the evaporation is much
greater than in inland countries, and dry fituations.
It is found that evaporation is greateft in hot weather; Proportion-
whence it muft depend, m fome degree, on the tempera- ai to the
ture tempera¬
ture of the
the temperature required. An example will make this rule fufficiently obvious. In latitude j6° the heat below
being 94°^ required the temperature of the air at the height of 803 feet?
Here ^= 54, /=5533,
22 , h
-^-^=0404=^ and cX —=0.404x8.03 = 3.24 = 0?, and m — d —
100
54—--50.75. Hence we fee that the temperature of the air at the height of 803 feet above the furface
1550°.75.