180 



AUEORAS. 



nates in the rings : then, sometimes at one pole 

 of the sphere, usually at both, a luminous jet 

 from the pole to the surrounding ring appears. 

 Let the poles of the sphere be now at the same 

 time magnetized : the jet at once spreads around 

 the ends of the soft-iron poles, very soon forming 

 two distinct colored rings, concentric with it 

 and between it and the iron ring the inner of 

 a rose-red, the outer violet with a dark annu- 

 lar space between ; and both these rings rotate 

 around the pole. But the course of the cur- 

 rent is thus far the reverse of that in the natu- 

 ral phenomenon. Now let the positive pole 

 of the electrical apparatus be the rings, and the 

 negative be the two soft-iron poles of the sphere 

 the air in this experiment requiring to be some- 

 what more dense than before : the conditions 

 then represent those in nature. Having ob- 

 tained the jet of light showing passage of the 

 current, again magnetize the poles of the sphere, 

 and at once the jet again begins to spread out 

 and rotate ; but it now invariably extends it- 

 self laterally by throwing off a multitude of 

 minute brilliant jets (the auroral streamers), 

 which extend like the spokes of a wheel from 

 the poles toward the rings, and turn with 

 greater or less rapidity. Here, then, is an ex- 

 tremely faithful reproduction of the aurora; 

 and it is particularly worthy of notice that this 

 appearance is only to be obtained when the 

 positive current flows from the rings toward 

 the poles of the sphere ; and that, however ap- 

 parently identical the arrangement and condi- 

 tions of the two poles of the sphere, the dis- 

 charge will still through slight and inexplicable 

 changes of condition, but in exact imitation of 

 the natural phenomenon, begin at one pole first 

 on one occasion, and again at the other, or dur- 

 ing its progress will suddenly cease at one pole, 

 or fly from one to the other. The phenomena 

 of the current in the meridian conducting band, 

 at least so far as the principal deflection of the 

 needle is concerned, are illustrated by passing 

 the same current as that in the apparatus 

 through a surface of mercury over which a 

 delicate needle is suspended, or by the galva- 

 nometer ; though in these experiments the sec- 

 ondary induced currents and their effects do 

 not come in. 



In the communication of Prof. Loomis, above 

 first referred to, he calls attention to the fact 

 of his having shown, in his eighth article in the 

 same journal, the existence of a stream of elec- 

 tricity drifting across Central Europe in a di- 

 rection from about N. 28 E. to S. 28 W. 

 After collecting during several months materi- 

 als drawn from observations on the magnetic 

 needle made at Toronto, Cambridge, and Phil- 

 adelphia, from May, 1840, to Dec. 1842, and 

 during a portion of this time at Washington 

 also, he is led by a comparison of the times of 

 the daily maximum and minimum deviations 

 of the needle at the places named, to conclude 

 that the maximum deviation of the needle ad- 

 vances like a wave over the earth's surface 

 the direction of its motion being from N. 68 



E. to S. 68 "W. The observations, secondly, 

 of the minimum deviation of the needle indi- 

 cate a wave from K 69 E. to S. 69 TV. In 

 view of their close approximation, he assumes 

 the former as the true direction. The average 

 of the velocities of the two waves, as deduced 

 from comparison of the Cambridge and Toronto 

 observations, is 134 miles per minute ; by the 

 Philadelphia and Toronto observations, 89 

 miles per minute. Comparing the observations, 

 further, with the notices (often nearly or quite 

 simultaneous) of occurrence of auroral displays, 

 the conclusion is that for the eastern part of 

 the United States the irregular deflections of 

 the needle, whether attended or not with au- 

 roras, are as the rule propagated in the direc- 

 tion already stated, and with an average veloci- 

 ty of 112 miles per minute. A summary and 

 comparison of notices from 1820 to 1850 in- 

 dicating the fact and direction of lateral dis- 

 placement of the auroral beams, shows 36 .cases 

 of movement of the arches or beams from N. 

 to S., only three in which it was from S. to N., 

 and a few instances of change from one course 

 . to the other ; also, 31 cases of movement from E. 

 to W., and 15 from W. to E. Taking into ac- 

 count the effect of apparent motion, in parts 

 lateral to the observer, the general conclusions 

 are, that the actual motion of the streamers is 

 from about N. N. E. to S. S. "W. ; and hence, 

 that there is a general correspondence between 

 the direction of the electric currents which tra- 

 verse the earth's surface during displays of au- 

 rora and that of the movement of the auroral 

 beams the former, in the United States, being 

 from about N. 68 E. to S. 68 W., the latter, 

 from about N. 30 E. to S. 30 W. 



A remarkable aurora visible in the region of 

 the city of New York, at very near the hour 

 of 12 on the night of August 4, 1862, was ob- 

 served for -a short time by the writer of this 

 article. No notice of the phenomenon, save 

 that of a daily newspaper, having been met 

 with, the commencement and duration of the 

 display cannot be given. In this aurora, the 

 light was at least in most parts of the heav- 

 ens covered by it not that of a somewhat 

 fixed arch ; on the contrary, it appeared and dis- 

 appeared over considerable portions of the entire 

 northwestern and northern sky in a rapid suc- 

 cession of almost instantaneous pulses or flashes, 

 comparable in this respect to the appearance of 

 very quickly moving puffs of steam, but of a 

 pale reddish color. The general direction of 

 the waves of light appeared to be from N. N. 

 W., and they reached quite to the zenith. It 

 soon became evident, on carefully observing the 

 light, that the successive flashes continued to 

 become visible and to disappear over nearly or 

 precisely the same patches of sky. Such, it 

 can readily be imagined, would be the case 

 with electric discharges passing through suc- 

 cessive bodies of air of differing conductive 

 power ; at all events, it appears difficult to re- 

 concile the actual and peculiar character of 

 such an aurora with any theory of its existing 



