METEOROLOGY. 
fliat the eastern parts of North America has 
a much colder air than the opposite Euro- 
pean coast, and falls short of the standard by 
about ten or twelve degrees. There are 
several causes which produce this consider- 
able difference. The greatest elevation in 
North America is between the 40th and 
50th degree of north latitude, and the 100th 
and noth of longitude west from London, 
and there the most considerable rivers have 
their origin. The height alone is sufficient 
to make this tract colder than it would 
otherwise be ; but there are other causes, 
and those are most extensive forests, and 
large swamps and morasses, each of which 
exclude heat from the earth, and conse- 
quently prevent it from ameliorating the 
rigour of winter. Many extensive lakes lie 
to the east, and Hudson’s Bay more to the 
north ; a chain of mountains extend on the 
south of the latter, and those equally pre- 
vent the accumulation of heat; besides, 
this bay is bounded on the east by the 
mountainous country of Labrador, and has 
many islands ; from all which circumstances 
arise the lowness of the temperature, and 
the piercing cold of the north-west winds. 
The annual decrease of the forests for the 
purpose of clearing the ground, and the con- 
sumption 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 of the European 
and American continents to something like 
a level. 
Continents have a colder atmosphere 
than islands situated in the same degree of 
latitude ; and countries lying to the wind- 
ward of the superior classes of mountains, or 
forests, are warmer 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 are heated 
and cooled with more rapidity : it is from 
this circumstance that the intense heats of 
Africa and Arabia, and the cold of Terra 
del Fuego, are derived. The temperature 
of growing vegetables changes very gra- 
dually ; but there is a considerable evapora- 
tion from them ; if these exist in great num- 
bers, and congregated, or in foiests, their 
foliage preventing the rays of the sun from 
reaching the earth, it is perfectly natural 
tliat the immediate atmosphere must be 
greatly affected by the ascent of chilled 
vapours. 
Gurnext object is the ascent and descent 
of water. The first-mentioned operation of 
this fluid has been noticed already. See 
Evaporation. 
Dews, the effect of the same cause, are 
variously accounted for by different obser* 
vers of nature ; the general result, however, 
seems to be, that they are the last feeble, 
efforts of evaporation, which deprived of 
their warm stimulus by the approach of 
night, fall through the chill of the air in ex- 
tremely small and distinct globules, cover- 
ing every substance with that trembling 
and brilliant lustre which rain is incapable 
of affording through the weight of each drop. 
According to Hales, 3.28 inches of dew an- 
nually falls on the earth ; but Dalton asserts, 
that the quantity is about 5 inches in the 
same period. M. Prevost made some cu- 
rious experiments to ascertain why dew 
should be deposited on glass, when it did 
not adhere to metal almost in contact : 
plates of metal fixed on glass are sometimes 
covered by dew, and at others the case is 
reversed ; in the latter instance they are 
bounded by a dry zone : if the other surface 
of the glass is exposed, the part opposed to 
the metal remains perfectly dry, and if the 
metal is applied again it will not prevent 
tlie deposition. The experiment may be 
made at a window, when moisture attaches 
to either side. M. Prevost observes, that 
glass is covered externally, even when the 
air is warmest within the house, and that 
metal fixed internally receives more mois- 
ture than the glass. After pursuing the 
subject to its utmost limits, this gentleman 
concludes that the phenomena are entirely 
the effect of the action of heat. That des- 
cription of dew known by the name of 
honey-dew is attributed to insects. 
The strata of air near tlie surface of the 
earth unquestionably contains more mois- 
ture, or vapour, than the higher parts of tlie 
atmosphere. The regions above the sum- 
mits of mountains are probably very dry ; 
and De Luc and Saussure say, tlie air on 
those they explored was less impregnated 
with vapour in the night than during the 
day; for as the stratum next the earth con- 
denses and cools at the former period, tliere 
can be no doubt that each stratum descends, 
yet as clouds are seen to tower far above the 
most elevated peaks, vapour must at parti- 
cular times rise to an amazing height. 
Rain never descends till the transparency 
of the air ceases, and the invisible vapours 
become vesicular, when clouds form, and at 
length the drops fall; clouds, instead of 
forming gradually at once throughout all' 
