consequence of the dissipation of the heat, by the 
rarefaction arising from which they are sup- 
ported, they often reach strata of the atmosphere 
comparatively dry, and of higher temperature 
than they themselves possess. In such a case, 
the vapour may be again taken up, and the cloud 
dissipated. Thus clouds are frequently seen to 
roll down the sides of the mountains, and to dis- 
appear at a certain level; this is a proof of a dry 
state of the air beneath, and is therefore con- 
sidered by the inhabitants of mountainous coun- 
| tries, as a prognostic of fair weather. When a 
cloud, on the other hand, formed in high and 
| cold regions, passes in its descent through strata 
_ saturated with moisture, or nearly so, it may cool 
' them until precipitation ensue; the precipitated 
| moisture, uniting itself to the descending cloud, 
_ will augment the intensity of the rain it causes. 
_ Thus the same rain will be more copious in valleys, 
_ than upon the neighbouring mountains; and the 
_ difference is so sensible in this respect, that it 
_ has been detected by means of the rain-gauges 
_ at the observatory of Paris, one of which is upon 
| the ground, the other upon the terraced roof of 
| the building. 
| Snow.—When the precipitation of vapour en- 
' sues at temperatures below the freezing point, 
_ snow is formed; the particles of the condensed 
| aqueous matter being free to move in any direc- 
_ tion, arrange themselves under the action of their 
| mutual attraction, in the manner of crystals. 
_ These crystals have usually the figure of six- 
| pointed stars; and the aggregation of broken 
crystals of this shape forms flakes of snow. 
Hail— Hail is a phenomenon that is not com- 
pletely explained ; the best theory on the subject, 
although not absolutely satisfactory, is as follows: 
—It is known that when water is frozen in a tor- 
ricellian vacuum, it granulates and assumes the 
form of hail; hail also reaches the ground with a 
very great velocity : hence we may conclude, that 
it is formed in very rare air, and in a high region 
of the atmosphere. The decomposition of organic 
substances, is constantly giving out hydrogen 
gas, and this, from its specific levity, rises to the 
higher regions of the atmosphere; hence, as no 
gas can remain long over another unmixed, it 
mingles with atmospheric air, and’ becomes sus- 
ceptible of being inflamed by electricity. Should 
it be thus acted upon, it forms water, will be con- 
densed into a space much less than it formerly 
occupied, and would leave a vacuum, did not the 
adjacent portions of air rush in to fill the void. 
The sudden rarefaction of this air will produce 
an intense cold; the newly formed water will be 
frozen, and under circumstances that will cause 
it to granulate; descending from a lofty region, 
it will have great velocity ; formed from hydrogen 
gas, and by the electric discharge, it will occur 
most frequently during the summer months, and 
accompany lightning. 
Dew.—tThe earth retains a constant mean tem- 
perature, under the joint action of solar and ter- 
ATMOSPHERE. 
restrial radiation; but the rate of these is un- 
equal, not only at different seasons, but from 
hour to hour ; the former ceases altogether at the 
setting of the sun, while the latter continues for 
a time undiminished. Hence the surface of the 
earth cools rapidly after sunset, and may speedily 
reach the dew-point of the air in contact with it. | 
So soon as this is the case, moisture begins to be 
precipitated, and a cloud is formed, the descent 
of the water of which this is composed, forms the 
deposit, that we call dew. The cooling will be 
propagated slowly upwards, and the cloud will 
appear to rise ; notwithstanding which, the mois- 
ture of which it is composed, actually falls. After 
some hours, the earth and air will assume the 
same temperature, and the cloud will disappear. 
The first morning rays of the sun, passing hori- 
zontally through the air, will heat it, long before 
their influence can be felt upon the ground. The 
air will therefore acquire a greater capacity for 
moisture ; if there be any water in the vicinity, 
vapour will rise, and propagate itself through the 
mass; but as the ground still remains colder, a | | 
new precipitation will ensue ; thus dew willagain | 
be formed, and moisture occur in the morning. 
When the surface of the ground, or of any other 
substance, is cooled by radiation to the tempera- 
ture of 32°, the dew is frozen, and takes the form || 
of white or hoar frost; this may often be de- | 
posited, when the temperature both of the air || 
and of the ground at avery small depth, is above | 
that of freezing. 
When clouds exist in the atmosphere, the ra- 
diation is impeded, and dew will not be formed. 
Thus a want of dew is usually a prognostic of 
rain. When the air is still, dew is most copious, 
and thus it falls in greatest abundance in shel- | 
tered situations, and frosts will continue later in 
the spring, and begin earlier in autumn, in valleys 
than on the open hills in the vicinity. The mo- 
tion of air mixes the portion cooled by contact 
with the earth, with that which is not, and brings 
new masses into contact ; hence, although the loss 
of heat by radiation, may be as great or even | 
greater, the ground will receive heat from the 
air, and the change of temperature will be less. 
In conformity, heavy dews do not fall during the 
prevalence of high winds, and hoar frosts rarely 
occur while they blow. Surfaces that radiate 
well, will be most'cooled, and will in consequence 
receive the greatest quantity of dew; and thus 
of land frequently tilled, and that which is left 
undisturbed, the latter will derive most moisture 
from the atmosphere in this form. 
The moisture of the air plays an important 
part in the economy of the earth. To it is due 
the constant irrigation of its surface, keeping up 
a constant circulation by evaporating from the 
sea and other great reservoirs, being carried in- 
land by the wind and other commotions of the 
air, and there again condensing on mountains 
and other high and colder places, collecting into. 
springs, creeks, &c., or depositing itself more 
ate ; 
