156 MEMOIR OF AUGUSTE BRAVAIS. 



the star of day is withdrawn. They are no longer observed there dur- 

 ing the uninterrupted day of summer ; it is at the end of August, and 

 especially at the period of the autumnal equinox, that their number is 

 multiplied in Lapland, and their frequency diminishes at the vewial 

 equinox, and still more toward the end of April, During this interval 

 of more than six months very few nights are destitute of the auroral 

 dis])lay. 



The apparition of the auroras is therefore subject to the course of the 

 seasons, and it is not less remarkable that even during the hibernal 

 night the hours of their commencement and of their different phases 

 maintain a consltant relation to tlie hour of the passage at the meridian 

 of the sun, which has become invisible. Their appearance always takes 

 place during the hours which correspond to the night of our tem]>erate 

 zones. It is generally between ten and eleven in the evening that they 

 assume the effulgent colors by which some of them are distinguished, 

 and, in all, their greatest brilliancy corresponds to the same period of 

 the night. The meteor usually disappears toward morning. 



M. Bravais states that by the light of a brilliant aurora he could read 

 a page printed in small type almost as easily as by the light of the full 

 moon. When the sun no longer rises, the moon, which at its full is in 

 opposition with the sun, is seen almost constantly on the horizon, and 

 the double eft'ulgence of that planet and of the aurora greatly dimin- 

 ishes the obscurity of the polar night. Irregular as are these lights, 

 they suffice to enable the Lapps, the Samoieds, and the Esquimaux to 

 traverse in sleds the limitless snows which cover their country ; and 

 when the absence of the sun would tend to dull their minds, the fantas- 

 tic images presented by a fitful illumination serve to arouse their imag- 

 ination and afford a pabulum on which it is marvellously exercised. 



Notwithstanding the movements wdth which the arcs and rays of tlie 

 aurora are endowed, it is evident that they follow the movement of ro- 

 tation of the earth. The aurora borealis is therefore an atmospheric 

 and not a cosmical phenomenon. Canton, M. Becquerel, and other phy- 

 sicists, have pointed out the resemblance which exists between the vio- 

 let-red tints of this meteor and those which electricity displays when 

 moving in a vacuum. This circumstance, added to the action of the 

 aurora on the magnetic needle, has led physicists to class it among elec- 

 tric phenomena. M. Bravais gives his adhesion to this opinion, the 

 verification of which has been recently corroborated by a remarkable 

 experiment of our distinguished colleague, M. De La Eive. 



After a sojourn of seven months the commission quitted Bossekop, 

 April 1839, and returned to Hammerfest in order to execute sundry 

 labors and await the corvette which was to convey them a second time 

 to Spitzbergeu. Vegetation was renewing and develoi)ing itself with 

 that astonishing rapidity which, from the commencement of May, an 

 almost continual day gives to it in Lapland. M. Bravais could not resist 

 his passion for herborizing, but unfortunately, in attempting to gather 

 a i)lant springing from the crevice of a rock, he sustained a violent fall 

 and fractured" a knee, so that when the corvette bore off" his companions 

 he found himself under the necessity of remaining at Hammerfest until 

 the end of the polar summer should bring back the other members of 

 the commission. Far from being discouraged, how^ever, by so vexatious 

 a mishap, he continued the series of meteorological and magnetic obser- 

 vations, and as soon as the injury permitted him to walk, labored at the 

 sompletion of two memoirs commenced during his stay at Bossekop, 

 one on the tides, and the other on the lines of the ancient level of the sea. 



A sojouru of more than a year had enabled him to perfect his obser- 



