De la Rive on the Aurora Borealis. 363 
it alike at a height of 6 to 7 miles. On the 2nd of April, at the 
most northerly station a brilliant are was seen 10° above the ho- 
rizon; at the other station, it was not visible. The 6th of Au- 
gust the aurora was at the zenith at one station, and 9° in height 
at the other. On the 7th of April it was again in the zenith at 
the first station, and 9° to 11° in height at the second. 
Again, Hansteen, and after him, MM. Lottin and Bravais, were 
led to believe as a consequence of their observations, that the arc 
of the aurora is a luminous ring whose different parts are sensibly 
equidistant from the earth, and which is centered around the 
magnetic pole so as to cut at a right angle all the magnetic me- 
tidians which converge towards this pole. Such a ring is the au- 
roral arch and its apparent summit is necessarily in the maguetic 
meridian of the place. M. Bravais also observes that the arc seems 
tohave a kind of movement of rotation from the west to the east 
passing by the south. From this description the phenomenon is 
quite similar to the result of the experiment described above, and 
the direction of the rotation in the luminous ring is precisely that 
which ought to take place according to the laws governing the 
mutual action of currents, if it be the, positive electricity which 
passes from the atmosphere to the surface of the earth, thence to 
penetrate about the north magnetic pole, reunite with the nega- 
live electricity, and thus constitute the current. 
The diameter of the luminons ring will be greater, as the mag- 
netic pole is more distant from the earth’s surface, since this pole 
ought to be found in the intersection of the plane of the ring 
with the axis of the terrestrial globe. 
It hence results that each observer sees the summit of the au- 
toral are in his own magnetic meridian ; and hence only those on 
the same magnetic meridian see the same summit, and can take 
simultaneous observations for ascertaining the height. 
the summit of the are pass the zenith of the observer, he is 
surrounded on all sides by the matter of the aurora, or the auro- 
ral influences which proceed from the earth, and then, if at all, 
the crackling sound which has been alluded to should be heard. 
If it does not reach the zenith, the observer is then outside of the 
region; and the aurora is more or less distant according to Its 
altitude. The noise may be produced by the action of a powerful 
Magnetic pole on luminous electric jets very near this pole, as I 
have proved by experiment ; I have succeeded in producing a sim- 
ilar sound by bringing a piece of iron, strongly magnetised, to the 
luminous arch formed between the poles of a voltaic battery. — 
As to the sulphurous odor, it proceeds like that which accom- 
panies lightning, from the conversion of the oxygen of the air 
Into ozone by electric discharges. 
The light of the aurora is not polarized, as was remarked by 
Biot in 1817, from his observations at the Shetland Islands. This 
