MAGAZINE OF SCIENCE AND ART. 
171 
A body of air ascending in the atmosphere by virtue of 
its comparative warmth and buoyancy wi 11, if it force 
its way to a sufficient height, reach a plane above which 
it cannot pass without a part of the aqueous vapour it 
contains becoming condensed into cloud-particles. Some 
cause exists, simultaneously with the precipitation of 
the vapour, -by which the ctensitv of the rising air is 
sensibly increased in comparison with the surrounding 
air on tho same level; thus the ascending current is 
not only checked, but usually caused to re-descend 
below the plane at which precipitation commenced. 
Thus is prod need the regular detached pure cumulus, 
that roundish, well-defined, but ever changing cloud 
which usually dots the sky during the pleasantest days. 
Its pure and brilliant upper surface, reflecting tho. direct 
rays of the sun, is strongly contrasted with its dark 
base, and both with the clear blue of tho intervals of 
sky beyond, giving a cheerful yet lialf-shady aspect to 
the scene. But w e must pass on to consider how tho 
same action which in fine weather yields the simple 
cumulus may, if proceeding to an excessive extent, 
cover the sky by an unbroken and gloomy sheet of 
cloud, or when modified by other simple causes, produce 
all the phenomena of storm-clouds. 
Comolose Sheets op Cloud. 
51. Though meteorologists are somewhat acquainted 
with two or three of the more distinct and regular forms 
of cloud, it must he acknowledged they have scarcely 
ventured to approach tho subject of the enormous masses 
of confused cloud, which in reality yield us the greater 
part of our supply of rain. I allude at present to those 
gloomy unbroken coverings, of cloud which produce 
what we should term a rainy day. But following out 
the above discussion as to the changes which the specific 
gravity of a body of air may undergo during precipita¬ 
tion, &c., we mavj meet, 1 think, with some slight clue 
to the explanation even of such phenomena. 
For instance, supposing the above theory of the 
cumulus to be correct, cases may occur in which the 
warmth, moisture, and consequent buoyancy of the as¬ 
cending air are so great that the first precipitation of 
w atery particles does not increase its specific gravity 
above that of the surrounding air at the same level; its 
upward motion will therefore continue until further 
precipitation produces this effect, and the air overflows 
in a fountain-like head and descends. But it will not 
now descend below that plane at which precipitation 
commenced, but -will, on the cent may, be able to retain 
a part of its moisture in the form*of cloud-particles, 
and yet remain suspended stationarily at a determinate 
elevation in the atmosphere. The pure rounded cumu¬ 
lus will now appear surrounded by liattish projections, 
extending from its lower part and resting upon tho same 
horizontal base. 
52. If copious supplies of warm and moist air con¬ 
tinue to ascend from below, these flat cumuli will so 
increase in size as to join each other, and we shall have 
a continued cloudy stratum, terminated below by a 
rough dark horizontal surface, but presenting above 
many large rounded or mamolloid projections which 
prove the cumulose action to be still going on through¬ 
out the whole mass, though it be. not apparent from 
below. 
Thus M. Arago learned from some French officers 
engaged in surveying at great elevations in the Pyre¬ 
nees, that when a stratum of cloud appears perfectly 
smooth and uniform oil its under surface, its upper snr- 
the severe hot wind of December 24th, 1857, a large 
column of smoke ( probably from a bush fire) was seen 
rising into the atmosphere. After attaining a certain ele¬ 
vation this was sharply terminated by a nearly level plane 
beneath which the smoke seemed to spread out on all sides. 
But in several places small bodies of cumulose cloud pro¬ 
jected up above this plane so as to present almost as dis¬ 
tinct an illustration of the mode in which it was formed , 
as the imaginary drawing (fig, iv) could do. 
face consists wholly of high protuberances and deep 
cavities. (Essavpn Thunder and Lightning, page 8.) 
bo. Again, a widely extended stratum of loose cloud 
might be produced in another and a slightly different 
manner. W hen the air which has ascended and pro* 
duced a cumulus sinks again by reason of its increased 
density below the piano of precipitation the cloud par¬ 
ticles indeed evaporate, but tho air, though clear and 
transparent, retains the warmth and moisture which 
originally caused its ascent. It will, therefore, leave 
no tendency to descend lower, but will spread out and 
accumulate iu successive layers just beneath the plane 
of precipitation. \r'e may eouceive, in fact, that the 
part of the atmosphere between the surface of the'earth 
ami the plane of precipitation forms a distinct stratum, 
in which, as in any other stratum of gas or liquid, the 
warmest and lightest pails will occupy the highest 
position. Xow, if any general cause, such as a fall of 
the. barometer occasioned by changes in the loftier 
regions of the aticosphere, or an access of cold and dense 
air beneath, tends to raise this stratum, bodily, the 
higher layers w ill expand and sink in temperature An 
such a manner as to produce precipitation over their 
whole area at once, and a. thick uniform stratum of 
cloud will be the result. 
51. It is, indeed, general changes of the barometrical 
pressure occasioning the, ascent of largo masses of the 
atmosphere at once which must produce in general the 
extensive strata of cloud we are now considering. It is 
quite certain that such upward and downward move¬ 
ments must go on, since from them alone can originate 
those horizontal movements which we continually ex¬ 
perience as wind at the surfhee. Whenever there Is up¬ 
ward motion, there wo shall assuredly have, in general, 
the production of clouds, which must then cither termi¬ 
nate iu rain, or evaporate again by reason of a contrary 
downward motion. 
55. I would remark, that the cause of much bad success 
in such subjects as the present,Is, I think, the wrong idea 
which is generally entertained of the nature of a cloud. 
So long as the watery particles of which it is composed 
do not descend as rain, they are suspended in a very 
minute state nf division In the air, and are inseparable 
from it, except by the slow-process of subsidence. This 
is not In the least inconceivable or anomalous ; for in¬ 
stance, precipitated gold (specific gravity, 19‘5J is often 
in such a minute state of division, tliat it remains 
suspended in a glass of water for many days, merely 
communicating- to it a peculiar bluish* colour. The 
mechanical properties of such a fluid arc not in the 
least altered by the suspended solid, except tliat.its 
specific gravity, as a whole, is sllghriy increased. In 
the same manner, although the density of air in a cloud 
is undoubtedly increased by the weight of the watery 
particles included in it, it is to all intents and purposes 
a gas merely rendered opaque by an extraneous sub¬ 
stance. A stratum of cloud is mcrflly a stratum of air 
in a peculiar condition, and all difficulties as to the 
forms and suspensions of clouds arc resolved into prob¬ 
lem of arid motions depending on changes of'tempera-' 
turc, density, precipitation, evaporation, &c. 
Comulo-St hat us. 
55. \Vb have always hitherto considered the watery 
particles, of clouds as inextricably mingled with the air, 
so that a cloud merely resembled opaque air of a 
• lightly increased specific gravity. The supposition 
I that the cloud particles increase iu size, so as to subside 
* through the air, or eventually become amalgamated 
together into large drops of rain which then descend 
rapidly to the surface of tho earth, will now lead to 
many conclusions of interest. 
It is evident th&t the immediate effect will be to 
diminish specific gravity of the cloud as a whole, for 
the aerial part must become lighter and more buoyant 
when the heavy watery particles, which before weired 
it down, subside and .are separated off. It is scarcely 
possible that I am the first to notice that the fall of rain 
must relieve the atmosphere, or some particular portion 
of the atmosphere, of weight, and thus cause changes 
