168 
MAGAZINE OF SCIENCE AND ART. 
essentially new in what X have said about the 
stratus. The explanation of this cloud has long 
been given, and it is only introduced here for 
the sake ot showing that the section-glass can 
illustrate a known fact, beinre trusting to its 
guidance in unexplored parts of the subject. I 
now proceed to a cloud of greater beauty and in¬ 
terest, namely, the Cirrus. 
Cirrosb Cloud. 
32. The serond case in which Dr. Hutton’s theory of 
precipitation Uv mixture comes into play is when’the 
upper of two strata happens to have the greater specific 
gravity. It is here evident that equilibrium cannot 
continue for any length of time t the upper stratum 
must, from the effect of mere gravity, descend through 
and displace the lower one, causing at least partial 
mixture of the two. The appearances which will result 
are conclusively ascertained from the following experi¬ 
ment, (See fig. iii.) 
33. A liquid is prepared consisting of pure water 
with a trace of commonsalt, and 1 part in 1000 of su"ar 
dissolved in it. After this has been heated to the tem¬ 
perature of about 100 deg. Fahrenheit, it is placed in a 
chemical beaker (or the section-glass), and by means 
of a tube funnel, a second portion of liquid is carefully 
introduced at the bottom ot the glass, being pure water, 
at the ordinary temperature and containing only a trace 
of nitrate of silver in solution. 
31. A white precipitate of chloride of silver, of pe¬ 
culiar appearance, will at once begin to form. Small 
streams in tile shape of threads or curious hands will 
be seen passing from one stratum into the other, and 
often curving about in the most complicated and beauti¬ 
ful manner. After a time the middle of the glass will 
be filled by a dense and confused, but still fibrous. ma«s 
of cloud, which will probably soon extend itself to the 
bottom ; but there will now also be seen with the creat¬ 
es 4 distinctness numbers of these small parallel threads 
ascending and reaching; nearly to the surface of the 
upper stratum, of considerable length, and ending in 
evanescent points. The slightest circular motion or 
disturbance communicated to the strata will cause these 
fibres to assume all sorts of curved and flexuous forms 
winch, however, in general still maintain their parallel¬ 
ism. Any poison whs has remarked the common ap¬ 
pearance of tho Cirrus in the atmosphere cannot help 
recognising^ I should think, a striking resemblance to it 
in this miniature cloud, upon which resemblance the 
truth of the theory which I am explaining must ho al¬ 
lowed almost entirely to rest. 
35. If we compare this miniature cirrus with the 
miniature strains (fig. ii. 29) before explained, tho 
contrast is most striking -, yet, as the only difference of 
conditions consists in the inversion of the light and 
dense strata, we are at once led to the conclusion that 
different portions of liquids may from the effects of very 
slight differences of specific gravity alone he caused to 
mix and pass into each other in the form of minute 
cirroso streamlets. In the experiment this happens in 
the following manner The density of the fust added 
liquid is not so much increased by the addition of sugar 
as it is diminished by being warmed to 100 dee "of 
temperature, and it therefore lies at first in a tranquil 
horizontal stratum above the cold liquid. Bat it can¬ 
not long remain so, for wherever the two strata are in 
■contact they must communicate their heat and tend to 
•assume a mean temperature; and it is evident that 
whenever this is the case, the portions of liquid con¬ 
taining sugar must always be slightly denser than those 
which are tree from it, and must consequently sink 
below and displace the latter. We shall thus have 
parts of the upper stratum continually sinking into the 
lower, and corresponding portions of the lower rising 
through the upper. Thus are produced peculiar small 
liquid threads or streamlets, which represent, I believe, 
the distinct fibres of cirrose cloud. The action may- 
best be described as the inter-filtration of strata. 
36. It should be clearly understood that the differ¬ 
ence of temperature of the strata in this experiment is 
not a material point, being simply a means or device 
employed to enable us to lay one’stratum temporarily 
upon another of a slightly greater density, so that wo 
may afterwards observe the mixing process and change 
of position in the most gradual manner possible. Caro 
must be taken not to follow out the analogy between 
tho atmospheric cirrus and its experimental imitation 
beyond its narrow and legitimate limits. The motions 
and the forces producing those motions must be the 
same in both, hut thore is not, except by chance, the 
least analogy between the origin of tho forces in either 
case. 
37. To translate, indeed, the conditions of tho experi¬ 
ment into their corresponding atmospheric terms, we 
aro driven to the single supposition of two moist strata 
of air, of which the higher is slightly the colder. Inter¬ 
filtration of the two will then undoubtedly ensue, and 
will produce cirrose fibres of cloud according to the 
principle of precipitation by mixture , discovered bv Dr 
Hutton. It might be thought that this theory of the" 
cirrus should also include the cause or variety of causes 
which may dispose a body of air with the colder and 
denser part upmost, a condition of things which it is, I 
acknowledge, no easy thing to comprehend or explain. 
I am at a loss to.offer bvcu a conjecture as to such ulti¬ 
mate cause in the case of the pure and isolated cirri 
which often appear in very elevated [arts of the at¬ 
mosphere in the infinitoly varied forms of fibrous tufts, 
whitpij branches, scrolls, &c. But when the cirrus 
appears in connection with other clouds, and forms only 
a part of a general organization, such os is seen in a 
thundercloud, I hope to offer some reasonable sugges¬ 
tions as to the actual conditions of its production. 
I ought to notice that 1 published this theory, in the 
first place, in the “ Philosophical Magazine," of London, 
for July 1357, where it is supported by arguments much 
the same as those given above. I have unfortunately 
had no opportunity of learning what opinion, if anv, 
has been expressed upon it in the scientific circles of 
England but from the simple and natural principles 
on which it is founded, I feet so much confidence in its 
substantial truth, as not only to bring it forward again, 
but to follow out the same lino of argument into more 
extensive and difficult cases. 
Cumulus, 
38. We now come to the form of cloud named Cumu¬ 
lus, or “heap-cloud,” which occurs in the sky more 
frequently than any other kind, and, from its dense 
structure and usually largo volume, composes a veri- 
large proportion of the total mass of clouds. Its general 
nature and causes have been understood for some times 
but as tho explanations of it- which aro given ia 
meteorological words are, 1 believe, more or less er¬ 
roneous, and always incomplete, I think it worth while, 
on the present occasion, to enter fully into the subject,’ 
especially as, without a full acquaintance with ‘ the 
cumulus, we cannot possibly undeniaad the cumulo- 
stiatus or tho thundercloud. 
39. Tho pure cumulus is a simple roundish or hemis¬ 
pherical heap of clond-matter extending itself upwards 
from a horizontal base, and appearing mysteriously 
suspended quite apart from all other clouds at a mode¬ 
rate distance from the surface of the earth. In this 
pure form it occurs only during the day, and this fact 
leads to the supposition, well supported by the observa¬ 
tion of its usual form, and the curious internal motions 
which it often exhibits, that it is occasioned by upward 
currents of air originating at the surface of tin, earth. 
Tile air receives from the surface with which it is in 
contact heat and moisture, vhich hare both the effect of 
reducing its density ; the first, of course, by expansion, 
the second by reason of the low specific gravity of 
aqueous vapour. This warm air at tho surface will, 
therefore, tend to rise upwards through the cold and 
denser stratum immediately above it, and cold air from 
aloft must simultaneously sink downwards towards the 
surface to supply the place of the warm air just re¬ 
moved. 
