of 50 miles, more or less, from the thunder-storm, may be compared to the ground swell of the sea formed by a distant storm.
Electrical Conditions of the Air.—Humidity may or may not 
Ether wave indicator, Meteorological Laboratory. The wire at the upper right of the spark gap leads to the aerial. The lower binding post of the condenser leads to the ground. The coherer is shown above the bell hammer. affect the potential of the air materially, but it affects the conductivity greatly. Dry air is a poor conductor; and dust particles, unable to discharge their load of electricity, are strongly repellant and remain suspended in the air, sometimes for several days. The desert simoon is followed by a condition which keeps the air at a high potential, with a highly electrified dust, for several days.
At the trading-posts in the Colorado and Mohave Deserts, after a simoon has passed, metal containers on wooden shelves become condensers of a considerable capacity. Horses’ manes and tails stare like fright wigs, and sparks crackle to any ground conductor that may be touched. At such times, strong earth currents may be detected, and their influence may be felt many miles distant.
In a case of this sort the high potential is local—that is, it is confined to the mass of dry desert air, and this mass of air
mercury between two iron plugs within a glass tube makes an excellent coherer, when placed in a circuit. Lighting companies sometimes make use of such "storm indicators" to guide them in generating the additional current made necessary by the darkness accompanying summer storms.
