Wlarvels of the Universe 987 
avocations quite comfortably. In 1872 a Kentucky negro, condemned to death for having murdered 
a fellow workman, was about to be executed. At the moment of his setting his foot upon the 
scaffold there was a terrible clap of thunder and the condemned man was struck dead by the lightning 
(Flammarion). The sheriff was so much moved by the occurrence that he resigned office. 
The appearance of the sky previous to a thunderstorm is usually very characteristic, the ‘‘ thunder- 
clouds” being well-known forms. The heavy, cumulus clouds with admixture of cirro-stratus 
above and “ cloud curtain of loose texture ’’ beneath produce intense darkness. The cirro-stratus 
incoming clouds are sometimes twenty to fifty miles in advance of the storm. Next come the 
cumulus clouds, “ thunder-heads,”’ under this the rain curtain and squall clouds below. The wind 
now changes in direction, the air becomes cooler and the barometer rises slightly. Then the rain 
or hail begins to fall, the lightning flashes and the thunder rolls, till the centre of the storm has 
passed. 
Mr. Shaw, in his recent work on “ Forecasting Weather,” states the conditions necessary for the 
causing of atmospheric instability. The undercutting of the warm Surface layer of southerly air 
by a more westerly or northerly one creates the instability necessary for violent showers, hail, 
and possibly thunder and lightning, the flow downwards of the air behind the squall front and the 
ascent of the displaced air. 
In general, two classes of storms are fairly well defined, (1) heat thunderstorms, and (2) cyclonic 
thunderstorms. The former are mainly confined to the Tropics, but are also experienced during the 
summer time in more temperate climates ; the latter may occur also in winter, are characteristic 
of our own Atlantic coasts, and are so named from their connection with cyclonic disturbances. They 
Photo by] [L. B.A. 
ELECTRIC SPARK. 
This photograph of an electric spark from an induction coil shows on a small scale the character of the electric discharge 
in Nature—the flashes of lightning. 
