XI.] THE AKRIVAL OF MIGRATORY BIRDS. 429 



double or multiple arcs are seen, generally lying in about the 

 same plane and with a common centre, and rays are cast between 

 the different arcs. Ai'cs are seldom seen which lie irregularly to 

 or cross each other. 



The area in which the common arc is visible is bounded by 

 two circles draw^n upon the earth's surface, with the aurora-pole 

 for a centre and radii of 8° and 28° measured on the circumference 

 of the globe. It touches only to a limited extent countries 

 inhabited by races of European origin (the northernmost part of 

 Scandinavia, Iceland, Danish Greenland), and even in the middle 

 of this area there is a belt passing over middle Greenland, South 

 Spitzbergen, and Franz Josef Land, where the common arc forms 

 only a faint, very widely extended, lumiiious veil in the zenith, 

 which perhaps is only perceptible by the winter darkness being 

 there considerably diminished. This belt divides the regions 

 where these luminous arcs are seen principally to the south from 

 those in which they mainly appear on the northern horizon. In 

 the area next the aurora-pole only the smaller, in middle 

 Scandinavia only the larger, more irregularly formed luminous 

 crowns are seen. But in the latter region, as in southern 

 British America, aurora storms and ray and drapery auroras are 

 instead common, and these appear to lie nearer the surface of the 

 earth than the arc aurora. Most of the Polar expeditions have 

 wintered so near the aurora-pole that the common aurora arc 

 there lay under or quite near the horizon, and as the ray aurora 

 appears to occur seldom within this circle, the reason is easily 

 explained why the winter night was so seldom illuminated by 

 the aurora at the winter quarters of these expeditions, and why 

 the description of this phenomenon plays so small a part in their 

 sketches of travel. 



Long before the ground became bare and mild weather 

 commenced, migratory birds began to arrive : first the snow- 

 bunting on the 23rd April, then large flocks of geese, eiders, long- 

 tailed ducks, gulls, and several kinds of waders and song-birds. 

 First among the latter was the little elegant Sylvia Etvers- 

 manni, which in the middle of June settled in great flocks 

 on the only dark spot which was yet to be seen in the quarter 

 — the black deck of the Vega. All were evidently much 

 exhavisted, and the first the poor things did was to look out 

 convenient sleeping places, of which there is abundance in the 

 rifforinor of a vessel when small birds are concerned. I need 

 scarcely add that our new guests, the forerunners of spring, were 

 disturbed on board as little as possible. 



We now began industriously to collect material for a know- 

 ledge of the avi- and mammal-fauna of the region. The 

 collections, when this is being written, are not yet worked out. 



