SCIENTIFIC LITERATURE. 561 



of the popular interest which auroras have aroused is due to the varied and 

 often changing forms that they assume. Their protean shapes have caused 

 people of earlier times to see in them hloody flames, armies in the clash of 

 battle, or furiously riding Valkyries. They may appear as faint lights 

 without defined shape; if more distinct, they are seen to have the form of 

 rays converging upon some point in the sky, or of more or less clearly de- 

 fined arcs, or of hands which fold over on themselves like a curtain, and 

 are called draped auroras. M. Angot describes many variations of these 

 typical shapes, and presents plates on which some of the most interesting 

 are depicted. The vertical rays of which most auroras seem to be made up 

 move both horizontally and vertically, and as they are usually in constant 

 motion an aurora may readily change from one typical form to another. 

 While the light from most auroras is white, the rays are frequently tinged 

 with yellow, and are sometimes red at the lower extremity and green at 

 the upper. The nature of the auroral light is not established, although the 

 spectroscope and polariscope indicate that it is emitted by luminous gases. 

 It seems probable that a slight rustling or crackling sound accompanies 

 auroral displays, but our author finds no credible evidence of any odor. 

 While many auroras of small extent appear only as local phenomena in 

 high latitudes, others are visible to within twenty degrees of the equator. 

 It seems to be usual for an aurora australis to occur simultaneously with 

 an aurora borealis, notably on February 4, 1872, when the globe, with the 

 exception of an equatorial zone of about forty degrees, was enveloped in 

 polar lights. The periodicity of auroras has been studied with the result 

 of establishing a diurnal and an annual period, and a period of a little 

 more than eleven years. Less exactly determined are the periods of about 

 twenty-eight days, and of about fifty-five years and a half. Those of 

 twenty-eight days and eleven years seem to connect the auroras with 

 sun spots. The relations of the aurora with meteorological phenomena 

 and with terrestrial magnetism have also been investigated. The data 

 obtained from researches on the foregoing questions have given rise to 

 many theories as to the cause of auroras. Our author states several hy- 

 potheses that have been made clearly untenable by recent advances of 

 knowledge. One of these is the idea of Mairan that auroras occur when 

 the earth passes through the cloud of matter that produces the zodiacal 

 light, some of this matter falling into our atmosphere and becoming 

 ignited. The reflection of sunlight from particles of ice in the atmosphere 

 is another cause suggested, and still another regards the light as a sort of 

 fluorescence. Our author treats with more respect, although positively 

 rejecting it, the theory first definitely stated by Dalton, that the light is 

 given oflP from silent electric discharges between the upper and lower 

 strata of the atmosphere, these discharges being conducted through ferru- 

 ginous dust falling upon the earth from space. He gives also several elec- 

 tric theories, among which he regards that of Edlund as the most satisfac- 

 tory. Edlund starts from the phenomena of unipolar induction the 

 production of currents in a metallic sheath surrounding a magnet when 

 the sheath is rapidly revolved. The general phenomena of terrestrial 

 magnetism justify regarding the earth as such a sheath. Electricity, ac- 

 cording to this theory, is constantly being carried by molecules of air from 

 the equator to the poles, where it accumulates and from time to time re- 

 turns to the ground by slow discharges which produce auroras. M. 



VOL. Ll. 43 



