MAGAZINE OF SCIENCE AND ART. 
167 
Stratose Cloud. 
21. Having now concluded a statement of those 
properties of the atmosphere which form the fun¬ 
damental data of Meteorology, I might attempt to 
explain in a systematic and d priori manner the 
scries of phenomena which must ensue. To draw 
out, in fact, a sort of history of the movement and 
changes which a particle of water usually under¬ 
goes, or might undergo, from its iirst evaporation 
at the surface of the earth or sea, to its final pre¬ 
cipitation as rain or dew, in a high or low region 
of the air, a higher or lower latitude of the earth’s 
surface, or in a multitude of varying circumstances. 
But, for obvious reasons, I must content myself, 
to-night, with describing a few phases of this 
physical history, with regard to which I have some 
suggestions, derived from experiments with the 
section glass to offer. 
25. Of the immediate causes of the precipitation 
of aqueous vapour into watery particles or cloud 
(for the sake of that great desideratum in a com¬ 
plicated subject like the present, namely, distinctness 
of [expression, I shall always use the adjective, 
aqueous, when speaking of water in the gaseous, 
and watery, when in the liquid state) one of lire 
most important is that first explained long since by 
Dr. Hutton, namely, that when two portions of 
air, saturated or nearly saturated with vapour, hut 
of different temperatures, are mixed together, the 
whole of the vapour can no longer be maintained 
in the gaseous state. 
26. It has been stated' (17) that the possible 
elastic force of aqueous vapour increases with the 
temperature, but at a much, more rapid pace than 
the latter. If, then, a cubic foot of aqueous 
vapour at 60 degrees he mixed with a cubic foot of 
vapour at 40 degrees, the mixed vapour will, we 
should suppose, possess the elastic force which is 
the mean of the separate elasticities, hut as the 
force at the highest temperature is disproportionately 
great, the mean elastic force cannot correspond to 
the mean temperature (50), hut to a temperature 
nearly two degrees higher: thus— 
Elastic force at.60 deg. '51S In. Mer. 
„ .40 „ ’247 „ 
Mean elastic force (51'6) „ '3S3 „ 
Elas. force atm. temp. (50) „ "361 ,, 
Difference (vapour con.) '022 „ 
If the vapour were to remain entirely uncon¬ 
densed, it would behave with regard to temperaturo 
like any other gas. The mixed gas would assume 
the mean temperature (50 deg.) of the two equal 
component portions. We have here, then, this 
incompatibility, that the elastic force of the mixed 
vapour corresponds to the full elastic force of the 
temperature of 51'6 degrees, while the actual 
temperature of the mixture is only 50'0 degrees. 
Under these circumstances, a certain proportion of 
the vapour must necessarily be condensed into 
watery particles. The elastic force of the remaining 
vapour is thus relieved, and its sensible tempera¬ 
ture is at the same time raised by the latent heat 
given out in condensation, and the two discordant 
amounts, meeting each other half-way, so to speak, 
soon come to agreement. 
27. But tliis is not exactly the case, if the two 
cubic feet of vapour be mixed or entangled, as I 
have said, (28) with two cubic feet of air, at the 
temperatures of 60 and 40 degrees. The mixed 
air, of course, assumes the mean temperature of 
50 degrees, and the vapour in contact with it must 
have the same temperature. In this ease the con¬ 
densation of a part will have much less effect in 
warming the whole mixture of air and vapour than 
it had before in warming the vapour alone, or w c 
may say, with very little error, that the elastic 
force of the vapour in the mixed air must be 
reduced till it equals that due to the mean tem¬ 
perature, which the air alone would naturally 
assume. Tliis reduction will be seen above to 
equal '022-inch of Mercurial pressure (about l-200th 
part of the whole) which represents the qurutity of 
vapour condensed into cloudy matter. 
28. Having now a distinct physical cause for 
the production of cloud, it remains for experiments 
with the section-glass to show what form or forms 
such cloud may assume. Mixture cannot go on 
without motion, and every form thus depending 
upon dynamical considerations alone may un¬ 
doubtedly he initiated with liquids. 
Now the only two distinct cases of mixture are 
as follows :—• 
1st. A light stratum of fluid moving over a 
dense one. 
2nd. A dense stratum placed above a lower 
one of less specific gravity. 
29. The first case is represented in fig. II. A 
circular glass vessel (employed merely instead of 
the section-glass) contains two strata of water, of 
which the upper one, containing only a trace of 
common salt in solution, waa first poured in; the 
second stratum, which contains about 2 parts in 
10,000 of sugar, in addition to a trace of nitrate 
of silver, being then carefully introduced beneath 
it, by means of a tube fuunel properly adjusted. 
30. The quantity of sugar, though minute, was 
sufficient to produce a difference of specific 
gravity in favour of the stratum _ last added, 
which, therefore, lies in a tranquil horizontal 
layer at the bottom of the glass. But it is im¬ 
possible that the two strata should not become 
more or less mechanically mixed, in the opera¬ 
tion of pouring in; and the mixed liquid, 
possessing, of course, intermediate composition 
and intermediate specific gravity, will settle 
itself into a horizontal intermediate stratum. 
The white precipitate of chloride of silver, which 
is necessarily extended through it, form a hori¬ 
zontal stratose or slieet-like cloud. The dynamic il 
conditions of this form of cloud, we may therefore 
announce to be two strata, such as will cause 
cloud by their intermixture, arranged in statile 
equilibrium, but caused to pine at their hori¬ 
zontal dividing surface by friction or other me¬ 
chanical means. 
31. This miniature cloud represents what 
meteorologists call by Howard’s oxprossive name 
of stratus. Mists, which lie in their horizontal 
sheets above the ground at night are all that 
are usually comprehended under the term, as 
far as I can understand, and these are produced 
by air, after being cooled by powerful terrestrial 
radiation from tho tops of the hills, flowing 
down into tho vallie3 by virtue of its increased 
specific gravity, and there mixing with other 
moist but less cooled portions of air. But as 
clouds very frequently appear at great elevations 
in the atmosphere, possessing the same sheet¬ 
like form, and invariably horizontal position, I 
do not hesitate to ascribo them to the same 
producing cause, viz., the frictional mixture of 
currents, and to include them under the term 
stratus. 
it will be understood that there is nothing 
