AUEORA BOKEALIS. 213 



the movement of rotation was from east to west by the south, and seven in 

 which the motion was from west to east. 



The rate of motion of arches is very variable. The angular motion of trans- 

 lation som(^times amounts to 17° per minute, and frequently amounts to 5° per 

 minute. With a vortical elevation of 125 miles above the earth, the last rate 

 of motion would imply an actual velocity of 1,000 feet per second. We shall 

 iind, hereafter, that the movement of auroral beams is still more rapid than that 

 of auroral arches. 



Light of auroral arrlics. — The light of auroral arches is generally of a yel- 

 lowish white; the lower edge is better defined than the upper; the latter is 

 usually very indefinite and blends with the general tint of the sky. This dif- 

 ference in the sharpness ef the two edges is less noticeable in southern arches. 



The greater distinctness of the lower edge of the arch may in part be ex- 

 plained by its greater distance from the observer; but it seems probable that 

 the substance of the upper part of the arch is really less dense and more difi~use 

 than the under part of the arch. 



Structure of auroral arches. — Auroral arches generally tend to divide into 

 short rays running in the direction of the breadth of the arch, and converging 

 toward the magnetic zenith. They frequently seem to be formed of transverse 

 fibres, terminating abruptly in a regular curve which forms the lower edge of 

 the arch. Arches entirely nebulous and homogeneous are not the most frequent; 

 arcs composed of rays, or striated arcs, are very common, and they present 

 every intermediate shade between those two extremes. Frequently a nebulous 

 arc resolves itself into a striated arc, without changing its general form. Some- 

 times the rays are distinct and isolated. In this case, the arch generally 

 increases in breadth, extending on the side of the zenith. Sometimes auroral 

 beams arrange themselves in the form of an arch, which is subsequently replaced 

 by an arch of nebulous matter. If the rays of the arch are broader than the 

 dark intervening spaces, and their light is imiform, we have the singular ap- 

 pearance of dark rays, or blade stria? perpendicular to the arch, and projected 

 upon a luminous surface. This fibrous constitution of auroral arches is most 

 noticeable when they pass near the zenith. On the evening of April 9, 1863, 

 there was noticed at New Plaven an auroral arch spanning the heavens, and 

 formed of short streamers parallel to each other. Most of them were from 10° 

 to 15^ in length, and for some time presented the appearance of a row of comets' 

 tails, all parallel to each other.* 



Auroral beams. — Auroral beams present every variety of length from 2^ or 

 ■3° up to 90° or more. Their breadth varies from 10' up to 2° or 3°. The 

 most brilliant beams have their edges sharply defined. Sometimes by the side 

 of such a beam the sky appears darker than elsewhere, the effect, probably, of 

 contrast. The lower part of a beam is generally better defined than the upper 

 part. Stars are frequently visible through the substance of the beams. 



Motion of auroral beams. — This motion is either longitudinal, in virtue of 

 which the beam extends towards the zeniih or the horizon, or it is a lateral 

 movement which displaces the beam parallel to itself, either from right to left, 

 or from left to right. Both of these motions may be very rapid. A beam has 

 been seen to move over an angular space of 90° in 27 seconds. t Beams 

 -advance either from north to south, or from south to north ; but the former mo- 

 tion is the most common. They sometimes move laterally from east to west, 

 and sometimes from west to east; but in the United States, the former motion 

 13 the most common.^ Frequently a beam extends suddenly either upward £)r 

 downward. This motion is most common downward, and sometimes with very 



* American Journal of Science, n. s., v. 35, p. 461. 



t Voyages en Scandinavie, p. 498. 



X American Joiuual of Science, n. s., v. 34, p. 45. 



