54 Meteorological Sketches. 
ted States, where the surface is often occupied in winter, for long 
periods, by an intensely cold stratum of air from the interior eleva- 
tions, the warm currents from lower latitudes appear to find their 
way at a superior elevation; and their presence in this position is 
often demonstrated by the phenomena which they induce. 
Clouds, Fogs and Rain. ‘ 
The atmosphere is always pervaded by water in the form of trans- 
parent or invisible vapor, and the process of evaporation is contin- 
ually carried on, except in cases where the thermometer is below 
what is called the dew point, or when the vapor is being condensed 
in the form of clouds, fogs, or rain. ‘* Clouds and fogs are the same 
thing, being an assemblage of small vesicles of water floating in the 
atmosphere. Ata distance in the atmosphere we see the whole as 
a cloud, but when the vapor sinks to the earth, or will not rise, and 
we are immersed in it, we call it a fog. Dew-fogs which hang over. 
fields, are stratus clouds ; and fogs which involve elevated objects, 
are cumulous clouds.” It is to circumstances of distribution, light, 
shade, distance, and perspective, that the great variety in the ap- 
pearance of the clouds is owing; and on this variety of appearance 
the following classification has been founded, by which the clouds 
have been considered as pertaining to seven classes : 
1. Like a lock of hair, or a feather, called cirrus. 
2. A cloud in conical or rounded heaps, called cumulus. 
3. A horizontal sheet, called stratus. 
4. A system of small fleecy or rounded clouds, called cirro-cu- 
mulus. 
5. The wavy or undulating stratus, called cirro-stratus. 
6. The cumulus and cirro-stratus mixed, called cumulo-stratus. 
7. A cumulus spreading out in cirrus, and raining beneath, called 
nimbus. 
The cirrus is usually the most elevated—sometimes as a gauze 
veil, or parallel threads. Its height is apparently from one to four 
miles. 
Dew is the condensation of aqueous vapor upon the surface of @ 
condensing body or substance. Clouds and fogs are watery particles 
condensed from aqueous vapor while floating in the atmosphere, 
where they continue to float till precipitated, or again dissolved. If 
by the concentration of these particles, or by any additional conden- 
