40 TRANSLATIONS AND SELECTED ARTICLES. 
nomenon, depending on peculiar circumstances. A storm may be 
formed in the immediate neighbourhood, or may approach from a 
distance ; the latter kind is most violent, as the vacuum has been 
prolonged to the surface of the earth through the warmest and 
moistest strata, while in the former it has been only partially pro- 
duced in the upper and drier regions. 
When the atmosphere is strongly warmed and laden with mois- 
ture, after a continuance of sultry weather, the first indication of the 
formation of a storm appears as a veil (flor) overspreading a portion 
of the heavens, and in which, after a time, darker spots are formed 
assuming the appearanee of clouds. Gradually the dark-grey or 
black colour of the cloud increases, shewing the great elevation at 
which the condensation is beimg effected. A fiash of lightning passes 
through the darkening mass, and streaks of rain are almost imme- 
diately perceptible. Now the storm begins to move, and takes a 
eertain direction quite independent of the wind. The rain as it falls 
to the ground, is accompanied by a strong eurrent of descending air, 
which is clearly shown by the bending down of the flexible branches 
of poplars, by the flattened appearance of well foliaged fruit trees, 
and by the whirling rise of leaves and dust from the surface of the 
ground. A sultry calm precedes the storm, while immediately after 
its passage a similar stillness ordinarily prevails. The tempest can- 
not have been brought by tke wind, but must have brought or pro- 
duce it. The condensation of water, sufficiently rapid to produce 
a tempest, ean only take place in still weather, during prevalence of 
wind only general showers can be formed. While the storm is 
gathering the motion is only in the clouds, it is brought into the 
lower regiuns by the descent of the rain. Most storms occur in the 
afternoon, when the sun is in the south or south-west, hence the 
shadow of the cloud falls north or north-east. The shadow thus 
formed has a cooling and hence condensing effect on that side, and 
therefore produces a tendency in the storm to move in that direc- 
tion, when the rain falls this is enormously increased, and thus the 
path of the tempest is established without reference to the direction 
of the wind, if there be any. Various slight circumstances may 
affect the initial direction, and it is not uncommon to see storms 
crossing each other at right angles.” 
Mohr assumes that the electrical phenomena usually accompanying 
a storm, are caused by the friction of the particles of water against 
