XL] THE AURORA AT THE VEGAS WINTER QUARTERS. 35 
the wind carries with it, is heated, and its relative humidity is 
slight, because a large portion of the water which it originally 
contained has been condensed in passing over the mountain 
heights. Accordingly when the dry fdlin winds prevail, a con¬ 
siderable evaporation of the snow takes place. The slight 
content of watery vapour in the atmosphere diminishes its 
power of absorbing the solar heat, and instead increases that 
portion of it which is found remaining when the sun’s rays 
penetrate to the snowdrifts, and there conduce, not to raise the 
temperature, but to convert the snow into water.^ 
The aurora is, as is well-known, a phenomenon at the same 
time cosmic and terrestrial, which on the one hand is confined 
within the atmosphere of our globe and stands in close con¬ 
nection with terrestrial magnetism, and on the other side is 
dependent on certain changes in the envelope of the sun, the 
1 In Lapland, too, the melting of the snow in spring is brought about in 
no inconsiderable degree by similar causes, i.e. by dry warm winds 
which come from the fells. On this point the governor of Norbotten lau, 
H. A. Widmark, has sent me the following interesting letter :— 
‘^However warm easterly and southerly winds may be in the parts of 
Swedish Lapland lying next the Kolen mountains, they are not able in any 
noteworthy degree to melt the masses of snow which fall in those regions 
during the winter months. On the other hand there comes every year, if 
we may rely on the statements of the Lapps, in the end of April or begin¬ 
ning of May, from the west {i.e. from the fells), a wind so strong and at the 
same time so warm, that in quite a short time—six to ten hours—it breaks 
up the snow-masses, makes them shrink together, forces the mountain sides 
from their snow covering, and changes the snow which lies on the ice of the 
great fell lakes to water. I have myself been out on the fells making measure¬ 
ments on two occasions when this wind came. On one occasion I was on 
the Great Lule water in the neighbourhood of the so-called Great Lake Fall. 
The night had been cold but the day became warm. Up to 1 o’clock 
P.M. it was calm, but immediately after the warm westerly wind began 
to blow, and by 6 o’clock p.m. all the snow on the ice was changed to 
water, in which we went wading to the knees. The Lapps in general 
await these warm westerly winds before they go to the fells in 
spring. Until these winds begin there is no pasture there for their rein¬ 
deer herds.” 
D 2 
