The same flashes photographed with (_a_) a stationary camera, and (_b_) a camera revolving on a vertical axis. One of the flashes is seen to have consisted of several successive discharges along an identical path Courtesy of U. S. Bureau of Standards and Popular Science Monthly.] Barometric pressure not only varies somewhat regularly with altitude--diminishing as we ascend--but also less regularly from place to place in a horizontal direction, and from time to time at a given place. In studying the weather meteorologists frequently wish to compare the barometric pressures prevailing at a certain time at a number of places lying in the same horizontal plane. Given a system of meteorological stations scattered over a certain territory, the first step is to secure simultaneous readings of the barometers at these stations. Then, if the stations are at various altitudes, as they commonly are, corrections must be applied to the readings to reduce all to a common plane; the plane adopted for this purpose is sea-level. Since most stations are _above_ sea-level, and since atmospheric pressure diminishes with altitude, reduction to sea-level generally involves applying an _additive_ correction. THE WEATHER MAP Now please attend carefully to what follows; because I am going to attempt to put into a minimum number of words the essential facts concerning the _weather map_, the best clue to weather mysteries yet devised by man. At about 200 stations of the Weather Bureau, distributed over the United States, the barometer and other meteorological instruments are read twice a day; viz., at 8 A. M. and 8 P. M., eastern standard time. The readings are promptly telegraphed in cipher to Washington, where they are entered on a map. The barometer readings at the different stations, reduced to sea-level as just explained, will vary, say, from 29 to 31 inches. Lines, called _isobars_, are now drawn through places having the same pressure; the intervals between the lines corresponding to differences in pressure of one-tenth of an inch. Lines (_isotherms_) are also drawn to connect places having the same temperature, a little arrow at each station shows the direction of the wind at that point, and various other symbols are used to facilitate the interpretation of the map; but the isobars are more important than anything else. [Illustration: CIRRO-STRATUS The appearance of this cloud precedes by a day or so the arrival of rainy and stormy weather] [Illustration: ALTO-CUMULUS] [Illustration: FAIR WEATHER CUMULUS This cloud marks the summit of an ascending air current, and appears toward midday or early afternoon in the warm season. When the air rises powerfully to great heights, cumulus is built up in mountainous masses and may become cumulo-nimbus, the thundercloud.]