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itself, (see figure 4) we should at once say that just j 
above that level where condensation of' vapour 
commences, the rising column is suddenly checked, 
flows over in a fountain-like head, and then sinks 
on every side in downward currents beneath the 
level of precipitation. All changes take place, of 
course, in the opposite direction, when air is 
descending, instead of ascending. The sensible 
temperature rises, and though the actual elastic 
force of the aqueous vapour increases, the possible 
force increases still more quickly, so that the pre¬ 
cipitated watery particles begin to evaporate again, 
and when the air reaches the same level where 
precipitation began in the upward motion, all 
cloudiness disappears in the downward motion. 
45. The general form and the internal motions 
of the cumulus arc shewn in fig. 4, and may be 
illustrated with some approach to accuracy by a 
simple experiment in the section-glass, being, in 
fact, a fountain-like jet of liquid, of which the 
weight or gravity is reduced to an exceedingly 
small amount, relatively to the surrounding liquid. 
(See fig. 5.) 
Fill the section-glass with water containing only 
a trace of common salt, and then introduce from 
the centre of the lower side a slow vertical current 
of distilled water, of the same (ordinary) tem¬ 
perature, and containing a trace of nitrate of silver, 
and 1 part in 5000 of its weight of sugar in 
solution. The specific gravity of the latter liquid 
is very slightly greater than that of the first, 
owing to the minute quantity of sugar, but the 
original momentum of the jet causes it to ascend 
slowly, forcing its way upward in a peculiar 
fountain-like form. Now the action of gravity, 
however slight its- force, gradually retards its 
velocity, and at last checks its upward motion 
altogether, just as in the case of a stone thrown 
upwards into the air. The jet, then, after collecting 
in a somewhat globular head, commences descending 
either side in a regular and graceful fountain-like 
form, which is afterwards maintained, essentially 
unchanged. 
These motions, which, it. is evident, are the 
simple effects of gravity acting against momentum, 
are, 1 believe, more or less closely analogous to 
the internal motions of a pure cumulose cloud, 
although the latter are even more sluggish, but the 
analogy only holds above the vapour-plane at 
which the existence of watery cloud-particles 
begins, and ends in the atmospheric cloud. 
46. But we musl now take into account the 
reaction of tire aqueous vapours, or the changes of 
temperature and density which may be occasioned 
in the air by its changes of condition. 
The large amount of latent heat which aqueous 
vapour gives out when condensed into watery 
particles, must be communicated to the air. The 
precipitation of cloud particles will, therefore, tend 
to prevent the temperature of the air from sinking 
as rapidly as it would otherwise do; but as the 
column of air continues ascending through colder 
and colder layers of surrounding air, the effect of 
the vapour, as it becomes condensed, is actually to 
increase the relative warmth of the ascending air, 
and to increase consequently its buoyant force. 
Indeed, if this were its only effect, it is sufficiently 
evident that precipitation once commenced would 
increase and perpetuate the cause which occasioned 
it, viz., the ascensional motion. 
47. The explanation of the cumulus so far, 
contains little that was not understood before, but 
I now come to a point, a most simple and evident 
one, 1 should imagine, which nevertheless seems to 
have been overlooked by meteorologists, or, at all 
events, its direct effects neglected. 
It should not be forgotten that precipitation of 
watery particles, while it has the effect of warming 
the air, whether in the cumulus or in any other 
form of cloud, has also the contrary effect in 
another and more direct manner of increasing the 
density of the whole. A certain portion of gaseous 
vapour which possessed only six-tenths of the 
specific gravity of common air is reduced to liquid 
water, which is '815 times as dense as air. It is 
true that this water is in the form of minute sus¬ 
pended particles or vesicles which are not at rest, 
and must always tend to subside, but it does not 
follow- that their weight must not he added to the 
weight of the air to obtain the weight of the whole 
mass. Indeed, it is an almost self-evident me¬ 
chanical proposition that the weight of any given 
body will not be affected by any motions merely 
internal and relative. Thus it is plain that a given 
quantity of moist air will possess the same absolute 
weight, whether the moisture it contains be a gas 
or a liquid; but it is also plain that as vapour fills 
nearly 1000 times the space of the water from 
which it is derived, the total volume of the moist 
air will be greater—the pressure and temperature 
being supposed unchanged when the water is gaseous 
than when it is liquid. In short, the simple fact I refer 
to is, that a mixture of water and air must have a 
greater specific gravity than air alone. 
48. While, therefore, the beat given ont in the pre¬ 
cipitation of watery particles tends to expand the air, a 
counter-effect is also produced by tho diminished 
volume of the aqueous part. It is not possible to de¬ 
termine whether precipitation will really decrease or 
increase the density of tho air and accelerate or arrest 
the upward motion of tho cumulose current, except by 
exactly calculating out tho amounts of tho two opposite 
effects, and thus discovering in which way the resultant 
tends. The data upon which such a calculation must 
rest are very numerous and complex, and with regard 
especially to the specific heat of gases, are very uncer¬ 
tain, if at all obtainable. Some rough results which I 
have obtained tend to a conclusion quite the reverse of 
wliat this theory requires; thus I find that, with the 
temperature and dow point both at 00 deg., and tho 
barometer at .‘10-0 inches, if the whole vapour contained 
bo suddenlv condensed, the latent beat evolved will ex¬ 
pand tho air in the ratio of 1 to l'059- Tho obstruction 
of vapours will at the same time diminish tho volume 
(according to Glaisher's Hygrometrical Tables) in the 
ratio of 10173 to 1, or the air will actually be expanded 
in the ratio of 1 to 1-041: 
49. But tho point is not to bo decided in tills rough 
manner, and I give very little credit to this calculation. 
I have, indeed, failed to provo from the known and 
already explained properties of the atmosphere whether 
a cumulose ascending current will bo checked by the 
precipitation of vapours; but if we regard the cloud 
itself in tho sky, we can have no possible doubt that 
some cause must exist to chock it, for the simple reason 
that it is checked. The pure cumulus is usually of a 
very small vertical height, and is as definitely termi¬ 
nated above as the fountain-like bead seen in the sec¬ 
tion glass (fig* v.) It is indeed not altogether neces¬ 
sary to the theory to suppose that the precipitation of 
vapour actually increases the density of tho air, as it 
will bo shown at a later part of this paper (67-71) that 
it must increase tho density of the cumulose current 
relatively to surrounding air. 
50. The theory of the cumulus u hich I adopt is this-- 
* Since the above was written I have noticed a very 
remarkable confirmation of the opinion expressed. During 
