METEOROLOGY. 
parts of the horizon, generate in a particu- 
Jar spot, and imperceptibly increase till the 
■whole expanse is obscured. It is singular 
that clouds collect and spread at a consi- 
derable height in the atmosphere, where 
the air is drier than in the lower strata, 
which are generally overcharged with mois- 
ture. “ It is equally remarkable,” says a 
late writer, “ that the part of the atmos- 
phere at which they form has not arrived at 
the point of extreme moisture, nor near that 
point, even a moment before their forma- 
tion.” Thus it appears, that their forma- 
tion does not proceed from a greater quan- 
tity of vapour accumtilating than could re- 
main in the atmosphere without passing its 
maximum. M. De Luc asserts, that the 
heat of clouds exceeds that of the surround- 
ing air in some particular instances ; hence 
their formation cannot arise from the capa- 
city of air for combining with moisture be- 
ing decreased by cold, as clouds may fre- 
quently be observed, which, after floating 
through the atmosphere during the heat of 
the day, disappear at night when the heat 
diminisltes : thus we might proceed to prove 
that clouds do not originate in the way sup- 
posed by many observers, and that we are 
still ignorant in what manner vapour is dis- 
posed of after it enters the atmosphere j and 
why it rejects its assumed form, returns 
again to vapour, and falls in rain ; and why 
evaporation should prevail during very hot 
and dry seasons, without visibly saturating 
the whole atmosphere. Theories in this in- 
stance are of very little use, as tlie subject 
is evidently placed too far out of our reach 
for experiment, in this state of uncertainty 
we must have recourse to facts. 
The quantity of rain, taken at an annual 
mean, is the greatest at the equator, and it 
lessens gradually to the poles j but there 
are fewer days of rain there, the number of 
which increase in proportion to the distance 
from it. The Journal de. Physique contains 
the following observations : “ From north 
latitude 12° to 43°, the mean number of 
rainy days is 78 ; from 43° to 46°, the mean 
number is 103 ; from 46° to .50°, 134 ; and 
from 51“ to 60°, 161.” Winter often pro- 
duces a greater number of rainy days than 
summer, though the quantity of rain is more 
considerable in the latter than in the former 
season : at Petersburgh rain and snow falls 
on an average 84 days of the winter, and 
the quantity amounts to about five inches ; 
on the contrary the summer produces eleven 
inches in about the same number of days. 
Mountainous districts are subject to great 
falls of rain, among the Andes particularly 
it rains almost incessantly, wliile the flat 
country of Egypt is consumed by endless 
drought. The rain guage affords reason to 
suppose, that a greater quantity of rain falls 
in the lower strata of the atmosphere than 
in those above, which may be accounted for 
by the drops attracting’ vapour in their near 
approach to the earth, though it must ba 
admitted, that Mr. Copland, of Dumfries, 
discovered the rain collected in the lower 
guage was greatest when it continued fall- 
ing for some time, and that the greatest 
quantity was collected in the higher during 
short rains, or at the conclusion of length- 
ened ones. 
As rain is known to fall at all hours of the 
day and night, and at every season of the 
year, it is apparent that it is caused by ope- 
rations which prevail eternally, and without 
defined interruption, M. Toaldo seems to 
think that a greater quantity descends in 
the night than the day ; and it is certain 
that a south wind produces more rain than 
any others, though it falls during the preva- 
lence of every wind : heavy falls also occur 
in the most complete calms. M. Cotte 
published a paper in the Journal de Phy- 
sique, from which it appears that the mean 
quantity of rain descending at 147 places, 
between latitude 11° and 60° north, is 34.7 
inches. “ Let us suppose then,” observes Dr. 
Thomson, “ (which cannot be very far from 
the truth) that the mean annual quantity of 
rain for the whole globe is 34 inches. The 
superfices of the globe consists of 170,981,012 
square miles, or 686,401,498,471,475,200 
square inches: the quantity of rain, 
therefore, falling annually will amount to 
23,337,650,812,030,156,800 cubic inches, 
or somewhat more than 91,751 cubic miles 
of water.” 
There are 52,745,253 square miles of dry 
land on the globe j consequently the annual 
amount of the quantity of rain descending 
upon it will be 30,960 cubic miles. The 
sea is supposed to receive 13,140 cubic 
miles of water, which flows into it annually; 
therefore it must supply an equal quantity 
by evaporation, or the land would be com- 
pletely drained of every particle of moisture. 
Mr. Dalton estimates the quantity of rain 
falling in England at 31 inches. 
Exclusive of the general appearance of va- 
pour when condensed into clouds, there are 
other forms in which the existence of mois- 
ture in the atmosphere is observable, particu- 
larly the halo, a luminous circle appearing 
under certain circumstances round the sun, 
moon, and stars. This has been almost uni- 
versally ascribed to the rays of light issuing 
