able, in the physical laboratory, to reproduce these conditions closely enough to feel assured that we fully understand the mechanism of the thundercloud.
As to the origin of the electricity or the electrification, it is quite plausible that there may be some minor processes at work, such as friction, the discharge of vapors from volcanoes, chemical activity, induction, etc., but our attention is at present directed principally toward two sources: (a) The bombardment and ionization of the outer atmosphere by electrified corpuscles or electrons issuing at great velocities from the sun. This theory has been developed by Strömer, Birkeland, Arrhenius, and others, and has much in its favor. (b) The electrical separation supposed to be produced by the breaking of large raindrops into smaller ones by an uprushing current of air. This theory, advanced by Simpson,1 is supported by observations and numerous laboratory experiments, and deserves careful study.
In regard to the electric phenomena of the atmosphere, it is not safe to hazard definite statements, but possibly auroras are due to earth-captured solar electrons, while the lightning of a thunderstorm owes its origin, chiefly, at least, to the electrical separation produced by the action of wind on raindrops.
ABBE CLEVELAND, "The Mechanics of the Earth's Atmosphere, a Collection of Translations," Washington, 1891. (Smithsonian miscellaneous collections, 843.) Translations of papers by Hagen, Helmholtz, Kirchhoff, Oberbeck, Hertz, Bezold, and Margules, and reprints of papers by Rayleigh and Ferrel.
BIGELOW FRANK II., "Report on the International Cloud Observations," Washington, 1900. ( United States Weather Bureau, Report of the Chief, 1898- 99, vol. ii.) Includes comparative study of the contributions of Ferrel, Oberbeck, Sprung, Hertz, Bezold, etc., and a proposed uniform system of fundamental constants and formulæ.
BRILLOUIN MARCEL, "Mémoires originaux sur la circulation générale de l'atmosphère," Paris, 1900. Comprises translations and abstracts of the principal contributions of Halley, Hadley, Maury, Ferrel, Werner Siemens, M. Möller, Oberbeck, and H. von Helmholtz.____________________