48 
POPULAR SCIENCE REVIEW. 
rises into a vacuum at any particular temperature — the same 
quantity rises in air. Thus the vapour which rises into a vacuum 
at 80° has a tension equal to one-thirtieth of the usual tension 
of the air. Or, if water at 80° be allowed to diffuse itself into 
dry air it increases its bulk by l*30th if the air is free to expand, 
or it increases its tension by l*30th if the air be confined. 
The spontaneous evaporatiou of water is therefore influenced by 
three circumstances. 1st. By the previous dr}mess of the air — 
for the air will only, under any circumstances, hold a given, and 
now’ well-determined, quantity of moisture. 2nd. By warmth — 
the higher the temperature the more considerable is the quantity 
of water-vapour which rises into any accessible space ; therefore 
humid hot air contains a much greater portion of moisture than 
humid cold air. 3rd. The evaporation of water is greatly acce- 
lerated by the constant removal of the air from its surface. 
Currents of air — Winds — are favourable to evaporation, because 
each portion of air takes its quantity of water-vapour ; it is re- 
moved, and another portion sweeps on to take its dose of humidity. 
The atmosphere may be regarded as a series of concentred zones 
of air, each one having, according to its distance from the Earth, 
its own density and its own temperature ; therefore each zone 
will possess its peculiar capacity for water-vapour. Air, for 
example, may become saturated with water-vapour wdthin a 
short distance of the surface of the land and sea, and remain 
perfectly transparent — free from cloud. This belt of air being 
warmer than the belt above it, rises ; and as it parts with its 
warmth, which it will do by expanding as the pressure is 
diminished, and also by its contact with colder air, some of the 
vapour is condensed and clouds are formed. 
It has been determined that over the land the cloud region 
varies from about three to five miles, but this is greatly influenced 
by the configuration of the land itself. Over the sea this region 
of vapour is more constant. Balloon ascents have shown that 
over England the cloud region has a thickness varying from 
1,500 to 3,000 feet, and that the temperature at the top is not 
lower than it is at the bottom, notwithstanding its thickness. 
The influences which effect — and are constautly disturbing the 
solution of water in air — its retention as invisible vapour, or its 
precipitation or condensation as visible vapour or cloud, are nu- 
merous ; but if it be distinctly understood that these influences 
are dependent upon solar and terrestrial radiation it will suffice 
for the present purpose. 
When a condensation of vapour takes place, if the temperature 
of the air be above 32°, the matter condensed is liquid, or in 
form of rain. If the drops of rain pass through a stratum of air, 
having a temperature below the freezing point of water, they are 
frozen into ice, and form hail. If a band of humid air having a 
