METEOROLOGY. 243 
sometimes negative ; more frequently the latter. The intensity also is in 
proportion to the amount of condensation and the water falling in a given 
time. The direction of the wind is also of great influence: in north winds 
the rains are most frequently positively, and in south winds negatively 
electrified. In positive deposits the electricity is generally more intense 
than in negative, this electricity being usually far greater in amount in 
summer than in winter. 
The most magnificent and at the same time most complex exhibition of 
electricity is furnished by thunder and lightning. Thunder clouds are 
generally small at first, but increase very rapidly, and soon cover the previ- 
ousiy clear blue sky. Their color is in some places dark grey; under 
certain circumstances, however, exhibiting brilliant colors, particularly when, 
situated in a western sky, a declining sun tinges them with a yellow 
passing gradually into grey or blue. Generally a slow continuous falling of 
the barometer is observed previous to the formation of the cloud, accom- 
panied in summer by an oppressive sultriness and a calm condition of the 
atmosphere. Should a thunder-cloud have formed in the vicinity of the 
zenith, a brisk wind rises at its approach, which blows in every direction 
from the cloud: in the cloud itself motions more or less lively are exhibited, 
and the electricity of the atmosphere quickly increases. The height 
of thunder clouds is sometimes very great, amounting occasionally to more 
than 20,000 feet. Should the charge of electricity in the cloud be suffi- 
ciently heavy, a flash of lightning will take place. Arago distinguished 
three kinds of lightning: 1. That consisting of a very fine and well defined 
luminous line, generally serpentine or zigzag. 2. That which illumines a 
great surface of the heavens at once; sometimes, however, only the outlines 
of those clouds from which it comes. 3. Lightning of greater duration than 
several seconds, of a well defined, generally spherical form ; in this respect 
similar to the fire balls already described. The most frequent lightning is 
that of the second class. The color of lightning is sometimes white, some- 
times bluish, violet, or red. A very deep red frequently characterizes 
lightning of the second class, whose light at any rate is generally less white 
and lively than that of the first. The duration of lightning of the first or 
second class is exceedingly short, not amounting even in the most brilliant 
and extended flashes to the thousandth part of a second. A division of a 
zigzag flash of lightning into two branches, very rarely into three, is some 
times observed. In many cases the discharge takes place between different 
strata of cloud, in others between a cloud and the surface of the earth: as a 
general rule, the flash comes from the cloud to the earth; sometimes, how- 
ever, it passes from the earth to the cloud. . 
A noise of greater or less intensity, called thunder, generally accompanies 
the lightning. It differs greatly in its duration and character at different 
times, and appears to be much modified by the echo of terrestrial objects, 
and especially of mountains. When lightning strikes in our immediate 
vicinity, we hear first a sharp crack, and then a rattling sound in the 
distance. In other cases, particularly when the discharge takes place 
between the clouds themselves, there is heard a dull rolling or rumbling, 
ICONOGRAPHIC ENCYCLOP2DIA.—VOL. I. 27 417 
