178 



AURORAS. 



stances given, makes the lower limit of these at 

 a height varying from 46 to 50 miles above the 

 earth, the upper limit at heights of 495 to 534 

 miles. Many modern observers have, on the 

 other hand, concluded that the luminous space 

 is seldom higher than the clouds ; and some, as 

 Capt, Parry (1825), Sir Wm. Hooker, Gen. 

 Sabine, and Dr. Walker (1859), relate the ob- 

 serving of auroras, parts of which they decided 

 were very near the earth, in some of these in- 

 stances the lower portion appearing to be visi-, 

 ble (as in case of rainbows) between the ob- 

 server and an elevated shore or ground at some 

 distance. Dr. Walker observed the aurora in 

 the arctic regions occurring repeatedly in the 

 direction of open waters, from which evapora- 

 tion was going on, and at times when the air 

 showed minute spiculse of snow, or was gradu- 

 ally filling with mist, or where cirrous clouds 

 appeared in the sky. Biot proposed that the 

 light occurred during the imperfect conduction 

 of electricity between upper and lower atmos- 

 pheric strata, by a sort of clouds of extremely 

 subtile metallic particles, but of which it is very 

 difficult to admit the existence ; and Prof. Pot- 

 ter, of England, in the "Philos. Mag.," 1862, ob- 

 jecting to De la Rive's theory (yet to be named), 

 and holding to the view of a great altitude, 

 offers a hypothesis somewhat similar to the last, 

 to the effect that the light is due to the electro- 

 magnetic influence of the earth on masses of 

 very rare vapors, of a nature like that of me- 

 teorites or vaporous comets, moving under con- 

 trol of gravitation in the planetary spaces, and 

 affected in the manner supposed only when 

 nearly approximating to the earth. For a brief 

 mention of Mr. Marsh's theory, which like the 

 last two is cosmical rather than terrestrial in 

 character, and of an objection to that theory, 

 see Comets, under ASTEONOMICAL PHENOMENA, 

 &c. 



The experimental observations of Plucker, 

 Gassiot, and others, had already shown con- 

 siderable traits of resemblance between the be- 

 havior of the electric discharge in a partial 

 vacuum, and under the influence of magnetism, 

 and the appearance of the auroral light ; and 

 suggestions were offered by Faraday and Hum- 

 boldt pointing in the same direction. Hansteen 

 had generalized the fact that the luminous arc 

 of the aurora is concentric with the earth's 

 nearest magnetic pole that point, for the 

 strongest N". magnetic pole, now aboutlat. 60 K, 

 long. 81 W., toward which the magnetic meri- 

 dians converge, in other words, that the arc 

 stands at right angles to these meridians, It is 

 doubtful whether the theory of the aurora pro- 

 posed by Dr. Walker does full justice to some 

 of the facts and principles now presented ; al- 

 though the author claims for it a close agree- 

 ment with his own and other arctic observa- 

 tions of the phenomenon. He supposes that 

 the cause of the electrical disturbance is the 

 condensation and freezing of vapor, at any ele- 

 vations at which these changes may take place, 

 the result being an evolution of positive electri- 



city [query : that carried up by the vapor from 

 the evaporating surface ?] ; that the body of air 

 so affected being surrounded by air which does 

 not perfectly conduct, an inductive action takes 

 place between it and more external bodies of 

 air, as between the two coatings of a Leyden 

 jar ; but that the excess of positive electricity, 

 where liberated, being in some degree trans- 

 mitted from particle to particle of the air, ren- 

 ders the mass for the time luminous, the lower ' 

 edge of the arch showing where the change in 

 the vapor and in the electric equilibrium begins 

 to take place. The luminous tracts or stream- 

 ers, then, are portions of the air through which 

 the opposite electricities are making their way, 

 in the tendency to a restoring of the equili- 

 brium. If true, the theory would appear to 

 show why the aurora usually, if not invariably, 

 appears in close connection with clouds, usually 

 cirrous, or where at least the sky wears a 

 hazy aspect. Hence, too, the aurora can be 

 at any height at which the supposed conditions 

 exist ; and hence, it may be, the explanation 

 of the more frequent appearance of auroras of 

 late years, the winters having been in many 

 instances unusually severe. The theory of M. 

 De la Pave appears to be not only more clearly 

 in harmony with the great general facts now 

 known in relation to the aurora, but also to have 

 been followed out to greater particularity of de- 

 tail than those offered by other explorers; while 

 the experimental illustration which he has been 

 able through use of an apparatus constructed 

 under his direction by M. Eugene Schwerd, a 

 German artist in the establishment of Prof. 

 Thury to produce, is remarkable for the com- 

 plete and exact manner in which it imitates the 

 actual phenomenon in nature. Although the 

 germ of De la Rive's theory is conveyed in a 

 letter of his to Arago in 1849, and a more com- 

 plete development of it is given in his " Treatise 

 on Electricity" (London, 1858), yet his communi- 

 cation of the year 1862 is important as giving 

 the latest and fullest exposition, as well as the 

 more perfect arrangement for experimental il- 

 lustration, of the subject. The apparatus it- 

 self is of too complicated a nature to allow of 

 description in detail in this place : a general, 

 and it is hoped, intelligible account of both the 

 theory and its illustration can be briefly given. 

 M. De la Rive sets out with two principles 

 which he regards as definitively established ; 1, 

 that generally northern and southern auroras 

 coexist a fact which the series of observations 

 made at Hobart Town jn the southern, and 

 Christiana in the northern latitudes particularly 

 confirm ; 2, that the aurora is always an atmos- 

 pheric phenomenon. In confirmation of the 

 latter view he cites particularly the opinions of 

 Arago and Secchi. In reference to the source 

 of the electrical charge: the waters of the 

 ocean being at the surface continually in a posi- 

 tively electrified condition, the vapors arising 

 from them conduct this electricity to the upper 

 strata of the atmosphere ; and being then in a 

 considerable degree carried by the upper trade 



