TRANSLATIONS AND SELECTED ARTICLES. 37 
evaporate, freeze, and cause a further deposition of water on their 
surface, which is continuously converted into ice. The same objec- 
tions, but in a greater degree, apply to this theory as to that of 
Volta. Water falling from the upper regions, and passing into 
warmer and moister strata, cannot possibly evaporate so rapidly as 
to freeze. 
‘“As long as it was believed that dew produced cold, the phe- 
nomenon remained inexplicable. Wells inverted the axiom and the 
problem was solved.’ — Dove's Meteorology. 
Mohr does not attempt to account for the production of cold, but 
assumes it as existing, viz-, in the upper regions of the atmosphere. 
Barral and Bixio found a gradual diminution of temperature up to 
21,060 feet, at which elevation the thermometer indicated 39 below 
zero. (Very recently Glaisher has observed the same, but the 
change of temperature was not by any means uniform or in one 
direction.) 
in a state of repose the lower strata of air will always be the 
warmer, heavier, and more saturated with moisture, while the higher 
strata will be the colder, lighter, and drier. A mixture of two 
strata may cause the temperature to sink below the dew point, thus 
causing a deposition of water, and herein, according to Mohr, lies 
the cause of a much more important disturbance. 
(Mohr then proceeds to account for the formation of an enormous 
vacuum by the sudden conversion of vapour into water; but while 
it is quite true that a vacuum is produced by the condensation of 
steam, it seems somewhat doubtful whether the enormous vacuum 
required by the theory, can be produced by the condensation of the 
vapour of water contained in air, the volume of which does not 
depend on the amount of moisture; hence the translator prefers 
to give the German philosopher’s explanation in his own words.) 
«« A volume of water when converted into vapour at a temperature 
of 100° C and 760 mm pressure, acquires a volume about 1700 
times greater. At lower temperatures the increase is still greater, 
at 0° 182,323, and at 20° 58224. Aqueous vapour expands ac- 
cording to Mariotte’s law under diminished pressure, but under 
increased pressure a portion of water is formed and the density 
remains the same. At a height where the stand of the barometer is 
only 380 m m, 2.¢., about 18500 feet, aqueous vapour must have 
double the expansion above mentioned, viz., 3400 at 100°, at 0° 
