MEMOIR OF AUGUSTE BEAVAIS. 155 



and luminous, that is to say, toward the extreme limits of the terres- 

 trial atmosphere, the extent of which had long been supposed to be less 

 considerable. 



The color of the arcs is usually of a uniform yellowish white. They 

 are of sufficient transparence to allow the stars to be seen through 

 them, and while the radiance of the most brilliant arcs equals that of 

 stars of the first magnitude, the greater number are only comparable to 

 those of the second, "third, and fourth. The position of each arc does 

 not remain invariable during its whole duration ; on the contrary, it 

 varies with much rapidity, so as to compel the observer to operate with 

 great quickness, if he would give to the different parts ^f the same arc 

 positions exactly corresponding as regards one another. In their move- 

 ments the arcs sometimes approach the zenith and sometimes withdraw 

 from it, whether toward the north or toward the south. Their edge 

 nearest to the horizon is usually the best defined. They have not always 

 regular forms ; we see them assume a thousand fantastic configurations, 

 such as that of an undulating scarf, or even of a crook. They some- 

 times show, especially toward the end, a tendency to become decomposed 

 into short rays in a direction conformable to the width of the arc. 



After the arcs, at a rather more advanced hour, appear the rays prop- 

 erly so called, which form the second type to which the gleams of the 

 aurora borealis may be referred. The rays are luminous columns of 

 much greater length than breadth, the prolongation of which on high 

 would terminate at the magnetic zenith, the i^oiut of apparent concourse 

 of all the lines parallel to the needle of inclination, and situated, at 

 Bossekop, only lo'^ toward the south of the astronomical zenith. The 

 brilliancy of the rays is variable like that of the arcs, and generally 

 more vivid. They are susceptible of two movements ; one in virtue of 

 which the ray prolongs itself toward the zenith or toward the horizon, 

 the other by which it is displaced laterally and parallel to itself. These 

 movements are sometimes of an excessive rapidity, and it is not rare to 

 see the rays dart their light, with a vibratory movement, toward the 

 zenith, and still more frequently toward the horizon, with extreme viva- 

 city. When these movements are alternate, the ray seems to gambol 

 or dance ; hence, the caprce saltantes of old authors, the marionnettes of 

 the inhabitants of Newfoundland, the merry dancers of England. In 

 general the more rapid the movements the more brilliant become the 

 rays. The color of these is usually white or pale yellow, sometimes of a 

 reddish hue. When the vibratory movements of the rays become very 

 precii)itate, the brilliant yellow tint is concentrated in their middle part 

 and the op]iosite extremities take the color of violet-red and green, the 

 red alwaj's showing itself on the side to which the ray darts its light. 

 Occasionally the rays unite with one another at the magnetic zenith to 

 form a crown either complete or incomplete ; and when, in executing 

 this movement, they lose their usual yellowish tint and glow with an 

 intenser luster, passing into red and green, the crown presents the great- 

 est degree of magnificence which the aurora is capable of displaying. 

 At certain moments the vibratorj^ movements by which the rays are 

 animated change into a sort of general palpitation in which all the 

 gleams of the aurora are confounded, the arcs as well as rays. It is 

 the announcement of a diminution more or less proximate of this splen- 

 did meteor. 



The refulgence of the aurora borealis might seem to have been given 

 to the polar regions as a compensation for the absence of the sun ; for 

 these arctic lights, barely visible two or three times a year on the liori- 

 zou of PariSj illumine almost every evening the latitudes from which 



