: i
with clouds more dew wUl appeal- than if it were entirely uncovered. Dew probably
bcgms, in the country, to appear upon grass in places shaded from tbe sun durinn
clear and cabn weather, soon after tbe heat of tho atmosphere has declined and cou
tnmes to be deposited tlu-ough the wholc night, and for a littlo after sun’rise Its
quantity wiU depend, in some measure, on the proportion of moisture in the atmosphcrc-
and IS, consequently, greater after rain than after a long tract of dry weather - and ii!
Europe, with -routhcily and westerly winds, than with those wliich blow from tho north
and the cast. The^ direction of the sea determines this relation of the winds to dew -
for, ill E g ^ t , dew IS scarcely ever obson'ed, except while the northerly or Etesian wind!
prevail. lÿ n c e , also, dew is generally more abundant in spring and autumn than in
siunmcr. I t is always vciy copious on thoso clear nights which are foUowed by misty
mornmgs, which show the air to bo loaded with moisture ; and a clear morning, follow^
mg a cloudy night, determines a plentiful deposition of the retained vapour. When
wmmth of atmosphere is compatible with clearness, as is the case in southern latitudes,
though seldom m our country, the dew becomes much more copious because the
air then contains more moistm-c. Dew continues to form with increased copiousness as
the mght advances, from the increased reft-igoration of the ground.
1385. Cause o f dew. Dew, according to Aristotle, is a species of rain foi-mcd in the
lower atmosphere, in consequence of its moistm-c being condensed by the cold of the
mght into minute drops Opinions of this kind, says Dr. Wells, are still entertained by
many persons, among whom is the very ingenious Professor LesKe. (Relat. o f Heat aud
M ^ ta r e , p 37. and 132.) A f t e , however, first taken notice of by G a r s Z X pèib-
ishod his F r e a t e m D em m 1773, proves them to be ciToncous ; for he found that
bodies, a little elevated m the air, often become moist with dew ; while similar bodies
X X l f ? ’ “ T l necessardy, from Üieir position, as hable to b!
wetted, by whatever M s from the heavens, as the fomor. The above notion is perfectly
lefiited ^ by the fact, that mctalhc sm-faces, exposed to the air in a horizontal
position, remain dry while eveiy tiling around them is covered with dew. After a lonir
period of drought, -when the air was veiy still, and the sky serene. Dr. Wells exposed to
the sky, twenty-eight minutes before sunset, previously weighed pai-cels of wool and
swandown, upon a smooth, unpamtcd and perfectly dry fir table, five feet long three
broad, and neai-ly tlu-ee m height, which had been placed, an hour before, in fhe sunfa
'fl? ' f t ? , ®'‘ft be 14 colder than the air, and to have acquire‘d" 'fnto*™ w eight. Th"eft “sw' andown, the fqouiranndt ittyo
ttheaLn ttbh e au-, anTd iv as Xalso “w"'-i tIh o™ut any adftd®i t‘i*o‘fnt al weig'™ht®. ’ In “twfte nty mi‘n*u™te>s ‘m3°ore the
swandown was colder than the neighbouring ah-, and was stiU Without any incLas^
of Its weight. A t the same tune tho grass was 15° colder than the air, fom-feet above
the ground. Dr. Wells, by a copious induction of facts, derived from observation and
experiment, establishes the proposition, that bodies become colder than the neiglihourin!?
iui, before they are dewed. The cold, therefore, which Dr. Wüson and M. Six con?
jectured to be the effect of dew, now appears to be its cause. But what makes the
terrestrial surface colder than the atmosphere ? The radiation or projection of heat into
free space. Now, the reseai-ehes of H-ofcssor Leslie and Count Bnmford have demonstrated
that different bodies project heat with very different degrees of force. In tho
operation of this principle, therefore, conjoined with the power of a concave mirror of
‘o r o f l ? “ • ffo-ow down again those caloric emanations
p X i X of d o t “ " ™ " 'ft*'“ '™ ft® “ ft ” ft®‘
1386. Rum. Wlien the vapour whicli exists in the atmosphere has been precipitated
by a sudden reduction of tcmperatni-o or other causes, it foi-ms a number of “ hoUow
vesicles or bladders, the coatings of which ai-o inconceivably thin and similar in sti-ucture
to those usually blown from soap-suds.” These vesicles seem to be all charged with
the same kind of electricity, which causes them to repel each other, and as long as this
f t ? ‘',™ f be the case they continue to float in the atmosphere. When the air in
which these vesicles float is dry, the vesicles are so much lighter than the afr that they
nse to a great height, and are dispersed. “ Acoordingly,” as Dr. Thomson observes
we find that when clouds rise m the atmosphere, they speedily diminish in size
and at last vanish away ; being gradually converted again into vapour. I f the air
within the vesicles were in tlic same state with respect to moisture as the air in which
the cloud iloats, the vesicles should be heavier than afr, and constitute what we distino-uish
by the name ol fogs. {Heat ami Electricity, p. 274.) If, however, the atmospher! becomes
satm-ated with moisture, the vesicles which form tho clouds are forced closer
together by the compression of the air, and additional vesicles are formed till at last
the atmosphere becomes so loaded with vesicles that, as Hutchison obseiwcs, any
fui-tlior precipitation of moisture will cause the vesicles to run together, and thus the
vcsiculai- fonn wiU be destroyed, and the increased gravity which each integiant
particle of moisture acquires will cause it to descend rapidly iu the foim of a drop of
rain. When the air is still and the precipitation of liumiditj' into the vesicular form
continues to go on slowly and regulaily, what is called a drizzling rain will be produced
; but “ every increase in the rapidity with which the precipitation of moisture
into the vesicular form goes on, by correspondingly accelerating the running together
of the vesicles, will augment the size of the drops of rain, and the amount that falls in a
given time. During windy weather, rain can never assume the drizzling form.
The agitation of the atmosphere in such circumstances favours the uniting of the integrant
particles of moisture so much, that they never can reach the surface of the earth
except in drops of considerable size.” (Meteorological Phenomena, p. 176.)
1387. The cause o f rain is thus accounted for by Hutton and Dalton. I f two masses
of air, of unequal temperatures, are, when saturated with vapour, intermixed by the
ordinary currents of the winds, a precipitation ensues. I f the masses are under saturation,
then less precipitation takes place, or none at idl, according to the dcgi-ee. Also,
the wanner the air, the greater is the quantity of vapoui* precipitated in like circumstances.
Hence the reason why rains are heavier in summer tlian in winter, and in wann
countries than in cold. “ Upon rellccting on the different degrees of rapidity with
which rain falls at different times, and in different climates,” observes Hutchison, “ I
am disposed to think that the cajiacity of the atmosphere for suspending aqueous vesicles
is limited, and varies with its temperature. And irom the greater density of clouds
in warm climates, as well as the greater amount of rain which falls from them in a given
time, it seems probable that the capacity of the air fur suspending vesicles, like its
capacity for holMng water in invisible solution, increases with its temperature. Vesicles
of a given specific gravity, upon thcir formation, maybe supposed at a given temperature
relative to that of the afr, to have a tendency to descend to, and not below, a certain
altitude in the atmosphere; and owing to tlieir mutual repulsion, a given depth of atmosphere
must be loaded with them, before that degree of -s-esicular density and compression,
in which vesicular oversatm*atiou consists, tal^es place.” (Meteorological Phenomena,
p. 173.)
1388. The quantity o f rain, taken at an annual mean, is tlie greatest at the equator,
and it lessens gradually to the poles, at which tlierc ai*e fewer days of rain, the number
increasing in proportion to the distance from them. 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 produces a greater number
of rainy days than summer, though the quantity of riiiii is more considerable in the latter
than in the former season. A t St. Petersburgh, rain and snow fall, on an average, eighty-
four days of the w in te r; and the quantity amounts to about five inches : ou the contrary,
the summer produces eleven inches in about the same number of days. Mountainous
districts arc subject to great falls of rain ; among the Andes, pai-ticularly, it rains almost
incessantly; wliile the fiat country of Egypt is consumed by endless drought. Dalton
estimates the quantity of rain falling in England at thirty-one inches. The mean annual
quantity of rain for the wliole globe is thirty-foiu- inches.
1389. The mean monthly and annual quantities o f rain, at various places, deduced from
the average for many yeai-s, by Dalton, ai-e given iu the foUowing ta b le : —
i2sp
1 ”
l i
3 ”
lSi itto
s'"
i i
î i
I s
■h'2 l g J i
I s
I I
| ! l |
i t .2 g 1 ^
Inch. Inch. Inch. Inch. Inch. Inch. Inch. Inch. F r .I n . Fr. In . Inch.
January - 2-310 2-177 2-196 3-461 5-299 3 095 1-595 1-4G4 1-228 2-477 2-530
February - 2-568 1-847 1-652 2-995 5-126 2-837 1-741 1-250 1-232 1-700 2-295
March - - 2-098 1-523 1-322 1-753 3-151 2-164 1-184 1-172 1-190 1-927 1-748
April 2-010 2-104 2-078 2-180 2-986 2-017 0-979 1-279 1-185 2-686 1-950
May - - - 2-895 2-573 2-118 2-460 3-480 2-568 1-641 1-636 1-767 2-931 2-407
June - 2-502 2-816 2-286 2-512 2-722 2-974 1-343 1-738 1-697 2-562 2-315
Ju ly - 3-C97 3-663 3-OOG 4-140 4-959 3-256 2-303 2-418 1-800 1-882 3-115
August 3-665 3-311 2-435 4-581 5-089 3-199 2-746 1-807 1-900 2-347 3-103
September 3-281 3-654 2-289 3-751 4-874 4-350 1-G17 1-842 1-550 4-HO 3-135
October - 3-922 3-724 3-079 4-151 5-439 4-143 2-297 2-092 1-780 4-741 3-537
November 3-360 3-441 2-634 3-775 4-785 3-174 l-9('4 2-222 1-720 4-187 3-120
December 3-832 3-288 2-569 3-955 6-084 3-142 1-981 1-736 1-600 2-397 3-058
36-140 34-121 27-664 39-714 53-994 36-919 21-331 20-656 18-649 33-977
1390. The cause why less rain fa lls in the f r s t six months o f the year than in the last
six months is thus explained. The whole quantity of water in the atmosphere in January
is usually about three inches, as appears from the dew point, which is then about 32° ;
now, the force of vapours of that temperature is 0*2 of an inch of mercury, which is equal
to 2*8, or three inches uf water. The dew point in Ju ly is usuaUy about 58° or 59°,
coiTcspondiiig to 0*5 of an inch of mercury, which is equal to seven inches of water.
F F 4