1855.] 



ON THE CAUSE OF THE AUROEA BOREALIS. 



IGc 



brilliant aurora presents at night. Arago, on consulting old 

 records at the observatory, found that there were considerable 

 magnetic disturbances that day in the magnetic needle kept 

 for showing the diurnal variation, thus proving beyond question 

 that the phenomenon observed by Dr. Usher was a veritable 

 day aurora. 



1 find also in the account of the voyage of the Venus by M. 

 de Tessan, that M. Cornulier, an intelligent oflBeer in the 

 French Navy, often observed on the coast of New Holland a 

 particular direction in the cirrus clouds during the day, 

 from which he was enabled always to announce a fine aurora 

 australis at night. M. Cornulier, like M. Verdier, was con- 

 vinced, from a study of the arrangement of the cirrus clouds, 

 that in those reigons, auroras occur during nearly every day, 

 and that the variation is only as to their brightness ; they are 

 often hid from view by clouds and storms. This remark 

 agrees with the observations made under the direction of 

 Captain Lefroy in Canada, at 13 different stations, and with 

 others, collected by the Smithsonian Institution. It results 

 from all these observations, that the aurora was seen on almost 

 all clear nights, when the moon was not too bright, although 

 not at all the stations. This is especially true during the 

 months when the nights are longest. From October to March, 

 there is scarcely a night without a visible aurora ; and they 

 are most brilliant in the month of February. The tables show 

 that auroras were seen during 261 nights in 1850, and 207 in 

 1851. It is also remarkable and natural, that the auroras should 

 have been seen most frequently in the stations nearest the 

 magnetic pole. ' 



Recnn'ing to the coexistence of icy particles in the air with 

 the auroras, we find sti'iking proof on this point in the Canada 

 observations. The tables give with exactness the weather be- 

 fore and after the auroras. The aurora was almost always 

 preceded by a full of rain or snow ; it also often happened that 

 a full of one or the other succeeded the aurora. The appear- 

 ance of lunar halos, a common prelude to auroras, is a proof of 

 the presence in the atmosphere of these icy particles which 

 make up the network illuminated by the electric current. 



But the most important proof of the electrical origin of the 

 aurora is that derived from its action on the magnetic needle. 

 The observations by Arago at the observatory of Paris,* by 

 Forster, Farquharson, and by all voyagers, establish the 

 following conclusions : — 



1. During the day preceding the night on which an aurora 

 appears, the declination of the magnetic needle to the west is 

 always augmented 10, 20 or 30 minutes, or more. 



2 On the contrary, at the middle, and at the end of the 

 exhibition, the needle deviates from its normal state to the 

 east. 



3. Finally, the needle often undergoes irregular perturbations 

 during an aurora, aniountitig to several minutes. 



It happens ordinarily that the maximum deviation of the 

 needle during the day preceding the night of the aurora, is at 

 noon, or half an hour after noon; and the deviation due to the 

 disturbance may be 5 to 30 minutes or more, beyond that of 

 the days before or following. Sometimes tlie maximum 

 western deviation is at other hours in the morning, and it is 

 probable that in such cases ihere is an aurora during the day. 

 Arago cites several ca.ses of this kind. Thus, on the 17th of 

 August, I82S, the declination from 85h. .\.m. till noon was 5' 



* Ann. (leCh. ct de Pbys., x. 120; xxx, 423; xxxvi, 398; xxxix, 

 869; xlii, .Sol ; xlv, 403. 



above the mean of the month for the same hours ; and on the 

 same day, at lOh. p.m., Messrs. Coldstream and Foggo per- 

 ceived feeble traces of an aurora which was probably the end 

 of a day aurora. During the evening the needle was in its 

 ordinary position. 



The magnetic observation made in the regions near the 

 pole confirm the influence on the needle. Thus at Reykinwik 

 (64° S' 15" N.) MM. Lottin and Bravais, having made num- 

 erous observ'ations on the diurnal variation of the needle 

 parallel with similar observations at Paris and Cherbourg, were 

 struck with the almost continual disturbance of the needle. 

 They at first attributed it to some movement in the earth : but 

 afterwards, remarking the concordance of their obser\'ations 

 with those of M. de Lowenorn made in 1786, 50 years before, 

 they satisfied themselves that the effect was due to auroras 

 invisible to them because of the continued presence of the sun 

 above the horizon. 31. Ginge, a Danish Missionaiy, made ob- 

 servations in 1786, 1787, continued through the 2-1 hours, 

 which showed that the western declination was ordinarily 

 strongest from 9 to 10 in the evening, and least at 9 to 10 in 

 the morning, a fact which he attributed without hesitation to 

 the aurora. This conclusion it confirmed by the very numer- 

 ous and excellent observations of MM. Lottin and Bravais. 



We thus see, that for a long period observations near the 

 pole have shown that auroras must be more frequent than was 

 supposed, and this is confirmed by the facts observed in 

 Canada and the United States. 



We therefore conclude, that the production of auroras, 

 northern and southern, is the normal mode of neutralising the 

 positive electricity of the atmosphere with the negative of the 

 earth. This neutralisation should not t;ike place in a manner 

 very uniform or regular. It is evident that the variations in 

 the mists or conducting capabilities of the atmosphere will be 

 attended by variations in the facility of this neutralisation. 



These difi'erences will be evinced by the deviations or dis- 

 turbances of the magnetic needle, which will be sensible at 

 great distances from the poles, as in the temperate zone where 

 they are often obser\-ed. The western deviation which 

 in the middle latitudes usuall}- precedes an aurora, indi- 

 cates a large accumulation of electricity, due to a powerful 

 condensation of vapours in the polar regions, which by 

 facilitating the reunion of the two electricities, augment, the 

 intensity of the terrestrial current passing in our hemisphere 

 from the equator to the north, and consequentl3- carries the 

 needle more to the west. When the aurora is once visible, 

 the current becomes less strong, because the light it-self of 

 the aurora is proof of the resisteuee (probably due to the con- 

 gelation of the particles of water suspended in the air that 

 constitutes the mist) which the reunion of the two electricities 

 encounters ;* the needle will then retrograde to the east, as 

 actually takes place. 



In the higher latitudes, the disturbances of the needle are 

 continual, because the slightest differences in the intensity of 

 the electric discharges that take place in the polar regions 

 should be there perceived. As to the observations of MM. 

 Ginge, Lowenorn and Lottin, that the maximum deviation of 

 the needle takes place from 8 to 10 o'clock in the evening, and 

 the minimum at fl to 10 in the morning, they were made only 

 during some weeks in summer, and they ])rove only that at 

 this season of the year, the greatest amount of conden.sation of 



* It is clear tliat the mi.«t when first formed should he a tetter con- 

 ductor than when, nftcrwiinl^, it consi.^ts only of icy particles. 



