MAGAZINE OF SCIENCE AND ART. 
169 
40. But this double motion cannot take place uni¬ 
formly over the whole area, because two bodies of air 
cannot pass directly through each other. As must al¬ 
most certainly happen, some little circumstance will 
cause the warm and light air to accumulate at some 
particular spot more than over the surrounding surface. 
At that spot its buoyant tendency will be so far in¬ 
creased as to enable it to force its way upwards through 
the superincumbent stratum of cold air, and through 
this passage, once formed, other neighbouring portions 
of warm air, collecting themselves together by a lateral 
motion, will flow in a continuous ascending stream. 
Adjoining cold and dense air wilL at the same time 
descend in contrary distinct currents to the surface, 
where it will fill the space vacated by the former por¬ 
tions of waim air, hut will become itself gradually 
warmed by contact with that surface. Now these 
motions once originated will continue with more regu¬ 
larity than might }>e supposed, as long as the surface of 
the earth communicates neat to the air above it, and 
furnishes sufficient supplies of warm and buoyant air to 
the ascending currents. Such currents may force their 
way upwards through the atmosphere to a very great 
height, flowing between walls, as it were, of cold and 
denser air almost as they might flow up the chimney of 
a house. Similar currents, I should observe, may* be 
produced in any body of liquid by applying heat to the 
lower part; and if wo insert a glass tube across the 
lower part of the section-glass and pass hot water 
through it, the motions of the liquid within the section- 
glass will present a tolerable representation of the cir¬ 
culating action which goes on in the lower parts of the 
atmosphere during the day, resulting, except during 
very dry weather, in the production of cumulus. 
41. Now what are the changes which the in¬ 
ternal condition of the air must undergo, in 
ascending ? As a body of air rises, the column of 
the atmosphere pressing upon it becomes less and 
less; and therefore presses with a rapidly dimin¬ 
ishing weight. The elastic force of the ascending 
air, of course, causes it to expand, and to diminish 
in density at about the same uniform rate which 
the density of the atmosphere exhibits in every 
other locality. Expansion, as has before been 
shewn, (14) occasions the absorption of heat, so 
that a volume of warm air rising upwards in the 
atmosphere must slowly sink in temperature as it 
swells in volume. The rate of this decrease of 
temperature has been rougly ascertained for a long 
time, and is said to be about one degree for every 
100 yards. But inasmuch as a corresponding 
expansion of volume and diminution of density 
takes place in every part of the atmosphere, a 
uniform corresponding fall of temperature must 
also be everywhere found. Thus the walls of air 
which surround the ascending column, and were 
cold with regard to it at the surface, must sutler, 
in every part of its course, an equal diminution of 
temperature, proportioned to the height at which 
we test it. In fact, however high the column of 
air ascerqi, it will fall continuously in temperature, 
but still remain warm with respect to the sur¬ 
rounding air on the same level. Its motion, sup¬ 
posing it not to become broken up and dispersed, 
might thus be perpetuated up to the limiting 
surface of the atmosphere, but for unforeseen 
causes, or for the necessary interference of the con¬ 
tained aqueous vapour, which we must now 
consider. 
42. It is evident that the aqueous vapour, which 
is always entangled in greater or less quantity with 
the air, must experience exactly corresponding 
changes of volume and density. When the air is 
expanded into twice its former volume, or reduced 
to half its former elastic force, the gaseous vapour, 
uniformly and inextricably (diffused through it, 
must also be spread through twice the space ; and, 
supposing the temperature to remain unchanged, 
its actual elastic force must be reduced to half its 
former amount. In the same proportion, therefore, 
that the air expands in rising, does the aqueous 
vapours it contains diminish in actual elastic force. 
But we have seen that the temperature of the air 
falls at the same time at the rate of one degree per 
100 yards ; and though it is very doubtful whether 
this rate is continued at any great elevation, we 
may at all events assume that the temperature falls 
in the same ratio as the volume of air expands. 
And we must remember, too, that the possible 
elastic force which is the greatest which vapour can 
support at any given temperature, decreases or 
increases in some ratio much more rapid than that 
of the simple arithmetical scries expressing the. 
degrees of temperature by the thermometer. 
Although, therefore, the actual elastic-force of the 
vapour does diminish, under all circumstances, 
when air rises in the atmosphere and expands, it 
does not diminish as rapidly as the regular fall of 
the temperature and the rapid decrease of the 
possible elastic force require. There mnst always 
arrive a moment at which the actual elastic force of 
the contained vapours is exactly equal to the 
greatest possible force which can be sustained at 
that temperature, and if the motion he continued 
ever so short a distance higher, the possible will 
overtake the actual force, and the whole of the 
vapour can no longer be maintained in the gaseous 
state. A small part will be condensed into watery 
particles: the remainder of the vapour thus 
relieved of the surplus pressure being enabled to 
retain its gaseous form. A further increase of 
elevation, however small, will, of course, necessitate 
a repetition of this action, and the condensation of 
a further portion of the contained vapour. 
43. Now what will he the form assumed by this 
condensed vapour ? If a column of uniform 
heated air ascend, the condensation ought to com¬ 
mence throughout its area at exactly the same 
elevation, or we should expect, a priori, that the 
base of the cloud would be horizontal, as it is 
always seen to be, more or less, in nature. The 
cumulus will also present the circular form in a 
horizontal section, and the columnar form, when 
viewed in elevation, will most probably represent 
the form of an ascending current in the atmosphere. 
But if it is asked to what height the cloud will 
extend, it is not easy to return an a priori 
answer. 
44. I have shewn (41) that disregarding the 
contained aqueous vapour, there is no apparent 
essential cause to prevent a column of warm air 
rising to the utmost limit of the atmosphere, sup¬ 
posing, of course, that it is not dissipated by 
gradual mixture with surrounding air. In spite of 
all regular changes of temperature and elastic 
force, it w r ill always maintain its superiority of 
temperature and its consequent buoyancy, just as 
in a body of liquid which does not possess the 
property of elasticity, any part which is warmer 
and of less specific gravity than the rest, must 
continue rising till it reaches the surface. 
Yet it is evident that the uprising column of air 
is actually terminated for the reason that the 
Cumulus, as continually occurring in the atmosphere, 
is always of a moderate elevation, and terminates 
in a sharply defined spherical or ma-meUoid head. 
To judge solely from the appearance of the cloud 
