sharp, scissors-like mandibles, all the filaments of 
wool which occur on its path, devouring the 
shorter and finer ones as food, and weaving the 
longer and coarser ones, along with a silky mat- 
ter of its own spinning, into its mantle or dwell- 
ing. Many of the caterpillars also, as 1f to ascer- 
tain which side of the cloth is most suitable for 
their purpose, penetrate through the cloth, and, 
in consequence, not only shear but riddle it. All 
the portions of cloth actually shorn or pierced by 
|| the caterpillars are completely destroyed,—and 
i even adjacent portions are frequently rendered 
useless. See the article Moru. 
| CLOUD. ‘The clouds are aqueous vapours, 
| which hover at a considerable height above the 
| 
| 
| 
surface of the earth. They differ from fogs only 
by their height and less degree of transparency. 
The cause of the latter circumstance is the thin- 
ness of the atmosphere in its higher regions, 
where the particles of vapour become condensed. 
The varieties of clouds are numerous. Some cast 
| a shade which covers the sky, and, at times, pro- 
duces a considerable darkness; others resemble 
a light veil, and permit the rays of the sun and 
moon to pass through them. Clouds originate 
| like fogs. The watery evaporations which rise 
from seas, lakes, ponds, rivers, and, in fact, from 
the whole surface of the earth, ascend, on account 
of their elasticity and lightness, in the atmo- 
sphere, until the air becomes so cold and thin 
that they can rise no higher, but are condensed. 
Philosophers, however, are of very different opin- 
| ions respecting the way in which the condensa- 
tion and the whole formation of the clouds pro- 
ceed. De Luc, whose theory is considered the 
most probable, believes that the water, after its 
ascent in the form of vapours, and before it takes 
the shape of clouds, exists in a gaseous state, not 
affecting the hygrometer, which is the reason 
why the air, in the higher regions, is always dry. 
He explains the clouds to be collections of small 
vesicles, in the transformation of which from the 
gaseous state, he believes that caloric operates, 
in part at least, because, according to his opin- 
ion, clouds communicate a degree of heat to the 
body which they render damp. According to 
Hube, clouds are collections of precipitated bub- 
bles, and differ by their negative electricity from 
fogs, the electricity of which is generally posi- 
tive. If clouds and fogs lose their electricity, 
rains is produced. These explanations are, how- 
ever, by no means perfectly satisfactory. 
The change of winds contributes essentially to 
the formation of clouds and fogs. In countries 
where this change is small and infrequent, as 
between the tropics, these phenomena of humid- 
ity in the atmosphere must be comparatively 
rare, but, when they happen, the more violent, 
because a great quantity of vapour has had 
time to collect. The distance of the clouds from 
the surface of the earth is very different. Thin 
and light clouds are higher than the highest 
mountains; thick and heavy clouds, on the con- 
CLOUD. 
‘occupying an extent of 20 square miles, and their 
decreases and vanishes at sunset. 
trary, touch low mountains, steeples, and even 
trees. The average height of the clouds is cal- 
culated to be two miles and a half. Their size is 
likewise very different. Some have been found 
thickness, in some cases, has been ascertained, 
by travellers, who have ascended mountains, to 
be a thousand feet: others are very thin, and of 
small dimensions. 
The natural history of clouds, not as respects 
their chemical structure, but their forms, their 
application to meteorology, and a knowledge of 
the weather, has been well treated by Lucas 
Howard, in his Essay on Clouds. He distributes 
clouds into three essentially different formations. 
These formations are—l. cirrus, consisting of 
fibres which diverge in all directions; 2. cwmulus, 
convex and conical aggregates, which increase 
from a horizontal basis upwards; 3. stratus, lay- 
ers vastly extended, connected and horizontal. 
The clouds are generally assigned to three atmo- 
spherical regions, the upper, the middle, and the 
lower one, to which a fourth, the lowest, may be 
added. In the upper region, the atmosphere is 
in such a state, that it can receive and sustain 
aqueous matter dissolved into its integrant parts. 
This state of the atmosphere corresponds to the | 
highest state of the barometer. To this region 
belongs the czrrus, which has the least density, 
but the greatest height, and variety of shape and | 
direction. It is the first indication of serene | 
and settled weather, and first shows itself in a 
few fibres, spreading through the atmosphere. 
These fibres by degrees increase in length, and 
new fibres attach themselves to the sides. The 
duration of the errus is uncertain, from a few 
minutes to several hours. It lasts longer, if it ap- 
pears alone, and at a great height ; a shorter time, 
if it forms in the neighbourhood of other clouds. 
The middle region is the seat of ewmulus, which 
is generally the most condensed, and moves with | 
the stream of air nearest to the earth. This re- | 
| 
| 
a nh 
iui 
gion can receive much humidity, but not in per- | 
fect solution. The humidity becomes collected, | 
and shows itself in masses rising conically, and | 
resting on thé third region. The appearance, | 
increase, and disappearance of the cumulus, in — 
fine weather, are often periodical, and correspon- 
dent to the degree of heat. Generally, it forms | 
a few hours after sunrise, attains its highest de- || 
gree in the hottest hours of the afternoon, and || 
Great masses 
of cumulus, during high winds, in the quarter of 
the heavens towards which the wind blows, in- 
dicate approaching calm and rain. If the cwmu- 
lus does not disappear, but rises, a thunder-storm 
is to be expected during the night. If the upper 
region, with its drying power, predominates, the 
upper parts of the cumulus become cirrus. But, 
if the lower region predominates (into which the 
densest vapours are attracted and dissolved into 
drops), the basis of the cumulus sinks, and the 
cloud becomes stratvs, which is of moderate den- 
