214 AURORA BOREALIS. 



great velocity. It is pomotimes observed simultaneously in a large number of 

 neigbboring beams. When a beam rise^ and falls alternately without any con- 

 siderable change of length, it is said to dance. This is a common occurrence in 

 high latitudes, where it is known by the name of the merry dancers. 



The corona. — When the atmosphere is filled with a large number of separate 

 beams, all parallel to each other and to the direction of the dipping needle, ac- 

 cording to the rules of perspectiv^e these beams will seem to converge to one 

 point, viz : the magnetic zenith. Hence results the appearance of a corona or 

 crown of rays, whose centre is generally (but not always) dark. The observers 

 at Bossekop made 43 measurements of the position of this corona, the mean of 

 which differed less than one degree from the magnetic zenith. In one instance 

 the position of the corona differed 15^, and in two other instances it differed 12^ 

 from the magnetic zenith. A portion of these differences may be ascribed to 

 the difhculty of makiug sucli observations with precision; but it seems necessary 

 to conclude that the auroral beams are not always rigorously parallel to the 

 direction of the dipping needle. 



Observations made in other parts of Europe as well as in the United States* 

 show that the centre of the corona is always very near the magnetic zenith, but 

 not always exactly coincident with it. From a series of careful measurements 

 of tlie aurora of November 17, 1848, Professor Challis found that the corona 

 had aim >st exactly the same altitude as the magnetic zenith, but was situated 

 1° or 2"^ more to the west. The observations, however, showed considerable 

 discordances, which seemed to indicate that the centre of the corona was con- 

 tinually shifting its position. t 



The corona is sometimes incomplete, sectors of greater or less extent being 

 deficient. At Bossekop these incomplete coronaj generally occupied the north- 

 ern part of the visible hemisphere. It follows from this that the beams which 

 form a corona, although covering at times a very large region, are nevertheless 

 limited, and frequently did not extend south of Bossekop. The passage of a 

 striated arch over the magnetic zenith frequently presents the appearance of a 

 corona. If the arch advances fi-om north to south, before reacbing the magnetic 

 zenith it forms a half crown on the northern side; at the instant of passing the 

 magnetic zenitji we have a complete corona of an elliptic form, whose rays de- 

 scend nearly to the horizon on the eastern and western sides; after the arch has 

 passed the magnetic zenith, there is formed a half crown on the southern side. 

 » Auroral cloudrS. — When an aurora becomes less active, its beams become 

 more feeble, their edges more diffuse, their length diminishes and their breadth 

 increases, and they assume the appearance of luminous clouds. Their outline 

 is rounded and a little less brilliant than the centre. Sometimes they exhibit a 

 fibi'ous structure, and present a strong resemblance to cirrus clouds. These 

 auroral clouds generally correspond to an hour of the night more advanced than 

 arches and beams. According to observations made at Bossekop on 37 nights 

 upon which the three forms of arches, beams, and auroral clouds were all ob- 

 served on the same night, the average time of first appearance was as follows : 



Uours. Miuuteg. 



Auroral arches ^ . . 7 52 



Auroral beams 8 26 



Auroral clouds 11 18 ♦ 



The average hour of disappearance was 14 hours 3 minutes. , 



Auroral vapor. — Frequently, during the exhibition of a brilliant aurora, there 

 is an appearance of general nebulosity or luminous vapor covering large por- 



* American Journal of Science, n. s., v. 40, p. 23(>. 

 tCauibriJije riiil. Traus., v. 8, p. G28. 



