LAST VOYAGE OF CAPT. FOSS. 
158 
blood red, greenish red, orange red, and lake red. Some of the 
beams appear as if tinged with black, and resemble dense columns 
of smoke. The arches are sometimes nearly black passing into 
violet blue, grey, gold, yellow, or white, bounded by an edge 
of yellow; the colours are also sometimes vivid and prismatic. 
Early observers were disposed to assign to the aurora an im¬ 
mense elevation above the surface of the earth. The height of 
that seen in 1737, was computed at 825 miles ; Bergmann from a 
mean of thirty computations, forms an average estimate at 460 
miles. Euler gives the altitude of several thousand miles to the 
aurora, and Mairan fixes the elevation of the greatest number at 
600 miles at least, Dr. Blagden brought it down to 190, and 
Mr. Dalton could not assign a less elevation to the aurora seen in 
this country in 1828. But the result of the observations made 
by the several arctic expeditions seems to be, that the height of 
the aurora is different at different periods, it occurs at elevations 
much higher than the region of clouds ; though instances are 
mentioned by Capt. Frankling and Dr. Richardson, in which 
the aurora has been seen at a less elevation than that of dense 
clouds, the under surfaces of which they often saw illuminated 
by the meteor. 
The magnetic property of the aurora, or its power of agitating 
the magnetic needle, had long been suspected by philosophers, 
and though still doubted by some, and not confirmed by the 
observations of Captains Parry and Foster, seems now sufficiently 
established by the observations of Captain Franklin, Lieutenant 
Hood, and Dr. Richardson. During the first voyage of Capt. 
Ross, the ship was in a situation when the aurora appeared, that 
the electrometer could not be used ; nor in the last voyage were 
any decisive conclusions arrived at. At present, little more than 
the fact seems to be ascertained, as great obscurity still hangs 
over the cause from which this effect proceeds, and the mode 
of its operation; and it sometimes happens that one observation 
has a tendency to neutralize the conclusion to which another 
would lead. The aurora sometimes approached the zenity with¬ 
out producing the usual effect on the position of the needle. 
It is generally most active where it seems to have emerged from 
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