2 Prof. E. Edlund on the Electrical 



radiation which could be produced by the variable number and 

 size of the sun-spots. It is henceforth difficult to doubt that 

 the aurora? boreales owe their production to electric currents 

 passing through the terrestrial atmosphere ; but there has 

 sometimes been observed an auroral light originating at a 

 height so considerable above the earth's surface that the tenuity 

 of the air at that altitude must far exceed that which it is pos- 

 sible to produce by means of the best apparatus of our labora- 

 tories. This auroral light shows that the rarefied air of those 

 high regions must necessarily be conductive, seeing that it 

 transmits electric currents, although the experiments of our 

 laboratories lead to the admission that air of corresponding 

 density is a nonconductor of electricity*. The question 

 whether electricity needs or does not need any ordinary pon- 

 derable matter in order to propagate itself from one place to 

 another has been much discussed, and is of great importance 

 when viewed from the standpoint of theory. If a satisfactory 

 answer could be made to it, the question of the electrical 

 resistance of vacuum would also, of course, be found solved. 

 It is, unfortunately, impossible to produce an absolute vacuum, 

 so as to solve the said question by a simple experiment ; 



* Warren De La Rue and H.W.Miiller could not, with a battery of 11,000 

 elements, make the current pass through hydrogen of which the pleasure 

 was O-OOOOoo millimetre of mercury. According to the calculations, tin- 

 press ure coi responds to the atmospheric pressure at a height of 81*47 

 English miles above the surface of the earth. As electricity passes with 

 more facility through hydrogen than through air, it would therefore fol- 

 low from those negative experiments that no electric discharge can take 

 place at that altitude (Nature, [5] xxii. p. 33, 1680). On the ground of 

 his experiments, Pliicker carries to 9 geographical miles the maximum 

 height ahove the earth at which aurora? boreales can be produced (Pogg. 

 Ann. cxvi. p. 53, 1862). According to the calculations of Looinis. the 

 intense aurora borealis which appeared on the 28th of August 1869 ex- 

 tended to 534, and that which appeared on the 2nd of September follow- 

 ing to 49o English miles above the surface of the earth (American Journal 

 of Science and Arts, n. s. vol. xxxii. p. 31^, 1801). From observations 

 made upon 28 auroras boreales Professor Newton has calculated that their 

 altitude varied between 33 and 281 English miles, and their mean 

 height above the earth amounted to 130 miles (Nature, voL xxii. 

 p. 291, I860). The calculation of observations made simultaneously at 

 Berlin and Christiansand (Norway) upon the intensely luminous aurora 

 borealis of Jan. 7, 1831, permitted Hansteen to estimate its height above 

 the earth's surface at 26 geographical miles (J. Midler, Lehrbtich der ko$- 

 nuschen Fhysik, Brunswick, 18G1). That the aurora in question, seen at 

 Upsal with its crown in the magnetic zenith (Pong. Ann. xxii.), must 

 have presented a considerable height above the earth, appears to follow 

 from the fact that it was simultaneously visible at Madrid ( Vevzeichniz* 

 beobaohteter NordUchter, von Fritz, Vienna, 1873). Lemstrom (CEfo-rt-. 

 Jcongl. Vetensk.-Akad. ForhondL 1809), however, has shown that auroral 

 rays may also appear, principally in the polar regions, at a small distance 

 above the surface of the earth. 



