TRANSLATIONS AND SELECTED ARTICLES. Al 
each other, such friction being produced by the rapidly entering 
masses of air. The motion visible in the clouds is not an effect of 
the electricity, but rather the cause. The rain is often seen to fall 
heavily after each flash of lightning, which is explained by supposing 
the rapid entrance of a cold mass of air, the deposition of water and 
friction of the particles, thus causing evolution of electricity, visible 
in the flash a considerable time before the consequent thunder is 
heard, and long before the rain can fall. 
Electrical storms are well known to be more dangerous in winter 
and spring than in summer, which may be explained by the fact that 
the quantity of vapourised water is small on the whole in winter, and 
only considerable near the surface of the earth. Hence condensation 
and production of a vacuum can only take place here, and hence the 
ease with which the electricity may pass from the cloud to the earth. 
The cooling effect of a storm is explained by the mingling of the 
colder upper air with the warmer on the surface. When this cool- 
ing is considerable, another storm may not be expected, inasmuch as 
a greater uniformity has been produced in the atmosphere. Another 
storm will only occur after a continuance of heat. If no cool- 
ing effect is observed, it is probable that another storm will occur 
shortly. 
Mohr thus attempts to explain the fluctuations of the barometer 
by the condensation of water, and as this explanation seems open to 
question, the passage is given in his own words :— 
‘© A balloon floating over my barometer presses on it as if it were 
filled with air of the same density as that which surrounds it. A 
drop of water floating over the barometer presses on it with the same 
force as an equally small volume of air. When water passes from 
the state of vapour to that of liquid, it loses almost entirely its 
action on the barometer. Hence every condensation must cause a 
lightening of the barometer, the mercury must fall, and not only in 
the immediate vicinity of the storm but also at a distance, inasmuch 
as masses of air are drawn away to fill up the vacuum formed at the 
spot where the storm is raging.” \ 
H. CG. 
