174 
MAGAZINE OF SCIENCE AND ART. 
heat must everywhere have becu communicated to those | 
higher strata. Thus we may say with much confidence j 
that the temperature of any given portion of air lying 
above that plane at which precipitation commences (58) 
must have undergone a greater or less increase by the in¬ 
terference of aqueous vapour, tending, of course, to 
diminish' its specific gravity. 
69. Now, how is the case with air in which the aqueous 
vapour has been condensed, although it still remains sus¬ 
pended among the air in watery particles ? The same 
amount of latent heat will have been given out as in the 
former case; thus the temperature will be increased in the 
same degree, and the specific gravity will so far be di¬ 
minished. But the watery particles remaining suspended 
in it must Increase its specific gravity (47) as a whole ; a 
given volume of cloudy air must possess a greater weight 
than the same volume of air, differing only In the fact that 
the watery particles have been separated. 
A further difference is that the accession of warmtb is 
not permanent. If the air be depressed in the atmosphere 
below that plane at which precipitation commenced, the 
precipitated watery particles will necessarily evaporate 
again, and will absorb all the heat that was formerly given 
out in their condensation. 
70. The general result of the foregoing arguments will 
be seen, on due consideration, to be the following, viz.: 
1st. That on an average cloudy air will have a higher 
specific gravity than char air at the same horizontal plane 
of the atmosphere, and will therefore tend to sink until 
the cloudy particles evaporate; 2nd. When the cloudy 
particles separate as rain, this tendency is destroyed. 
71. in the theory of the cumulus (48, 49) 1 had con¬ 
fessed a Uifiioulty in explaining why an upward current of 
air producing a cumulus is checked jast above the level at 
which precipitation commences. We now meet, however, 
with a cause which will at all events tend to produce the 
effect observed, whether It be sufficient or not to account 
for it entirely. When a cumulus rises it must possess, on 
an avcrof/ei the same temperature as the surrounding 
char air at the same elevation, but, unlike this, is loaded 
with the weight of watery particles, which must sooner or 
later cause it to descend again. If the particles of water 
subside, the air which has ascended may possess precisely 
the same temperature and specific gravity as the surround¬ 
ing air, and the cumulo-stratus is produced. And lastly, 
if the formation of clouds and raiu goes on to an excessive 
extent, and the ascending air lias originally a superiority 
of temperature, it may eventually acquire a permanent 
superiority of warmth and buoyancy compared with tire 
surrounding strata of air, and will ascend to greater eleva¬ 
tions in the manner of a eirrose crest which distinguishes 
the Nimbus or thundercloud. 
Electrical Theory of tiie Thundercloud. 
72. It may have occasioned some surprise and mis¬ 
givings that, in discussing at considerable length the nature 
of clouds ending with the thundercloud, I have omitted 
even the mention of electricity, the grand and striking 
phenomena of which are among their most interesting and 
remarkable features. Nevertheless, it is upon this ap¬ 
parent omission in the theory that I am prepared to rest 
its main claim to notice. Among hundreds of theories 
which e3ch speculator has in his turn imagined to explain 
all phenomena of the thundercloud, it is perhaps the only 
one which dispenses altogether with the vague and almost 
unknown principles of electricity. If I have even in a 
feeble degree carried out my original plan, and produced 
any conviction in my hearers that the forms of clouds 
must be explained in dynamical principles only, and are 
the simple and direct effects of gravity, I shall feel my 
theory to rest on a more secure ground than the proofs of 
fifty electrical theories added together. I might oven 
reasonably refuse to enter into the subject of elcetrieity at 
all as incapable of affording either a proof or a refutation 
of what was capable of being proved or refuted according 
to principles so much simpler and better understood. 
Nevertheless, it may bring collateral support to the theory, 
aud may increase the Interest of the conclusions, if I show 
how the phenomena of electrical excitation which are 
familiar to iis all in thunderstorms may be made to har¬ 
monise with, or even to be deduced from the foregoing 
conclusions. 
73. Possessing, as I assume we do, a clear ami correct 
idea of the motions and changes which constitute a 
cumulus, and therefore a thundercloud, which is merely a 
cumulus of excessive dimensions, with a derivative and, 
aa it were, organized system of cirri and stratus, I see no 
difficulty in offering an explanation, at least plausible, of 
lightnings and of the amazing supplies of electric fluid 
which must ! e necessary to a cloud, in order to maintain 
its electric tension In spite of quickly repeated discharges. 
So simple, indeed, does this explanation appear that I 
shall only be surprised if it has hot before been proposed in 
works with which I am not acquainted, even independently 
of any theory of the form of the thunder-cloud. 
74. The thunder-cloud consists of a body of ascending 
air which, after undergoing the condensation and separation 
of Its contained aqueous vapour in the shape of rain, rises 
permanently to a higher level, and there it produces the 
accessory stratus, cirri, Alc. What if we now suppose 
these ascending streams of air to be electrically excited, or 
to contain, as is sometimes said, a considerable amount of 
electrical fluid ? 
Electricity probably exists In the atmosphere precisely 
as on the surface of the excited glass plate of the electrical 
machine. There arc large quantities of the electrical fluid 
or force distributed among the particles of air, but, as 
gases are entirely devoid of the power of conducting 
electricity, the fluid is in isolated portions, or, as it were, 
lost and inextricably mingled with the air, so that no large 
quantity can. unite at any one place to raise a high tension 
and discharge itself. 
Thus in the ordinary electrical machine, the whole 
surface of the glass cylinder or plate is uniformly excited 
by friction with the leather cushion, but glass being a 
perfect non-conductor, the electricity U obliged to remain 
thus diffused, and of low tension. But when the glass 
surface passes close to a series of metallic points which 
have the property of attracting the fluid, the whole charge 
collects and unites upon these points, and upon the surface 
of the conductor, Leyden jars, &C., which are metalically 
connected with them. Consequently the whole charge t f 
electricity which was formerly, supposing many revolutions 
of the machine to have been made, spread over an almost 
indefinite surface of glass, is available in an instant at any 
point, where its great intensity enables it to produce the 
disruptive discharge sfath the characteristic bright sparks 
and sharp report. 
75. Now water is an excellent conductor of electricity, 
and it is pretty evident that cloudy or watery particles will 
rapidly collect upon their surfaces, if anything can collect, 
all the electricity of the air in which they are immersed, at 
indefinitely small intervals. But it is the characteristic of 
a rain or storm cloud that the watery particles become 
separated from the air; they subside, sink, and coalesce 
into rain, while the air freed from their weight rises into 
higher strata. Surely this air will also be deprived of its 
electricity, as was the ease with the glass plate in passing 
near the metallic conductor. The watery cloud will act, 
in fact, like a vast conductor, collecting the whole elec¬ 
tricity of the current of air which feeds it from beneath, 
and, passing through it, is evolved at the summit. 
Now, as the particles of this cloud gradually coalesce 
into large spherical drops, the total superficial conducting 
surface is vastly diminished, and the tension of the elective 
charge raised in proportion, and finally to such a pitch that 
it must overcome the resistance of the non-conducting 
atmosphere, and discharge itself with the earth, which is 
in the opposite electric condition, or, in other terms, 
deficient in electric fluid. 
And it is plain that we have here an ample explanation 
of the continuous nature of the phenomenon, for as long 
as the cloud-action continues, as loug a3 moist electrified 
air enters beneath and passes out at the summit, dcsslcated 
and neutral, electricity must collect in the cloud; and as 
the latter continually subsides, coalesces, and falls away as 
rain, must be relieved of its increasing tension by repeated 
flashes of lightning. 
76. It might be concluded from the above theory that 
lightning would never be produced, except in cumulose 
