De la Rive on the Aurora Borealis. 367 
This neutralisation should not take place ina manner very uniform 
or regular. It is evident that the variations in the mists or con- 
ducting capabilities of the atmosphere will be attended by varia- 
tions in the facility of this neutralisation. 
These differences will be evinced by the deviations or disturb- 
ances of the magnetic needle, which will be sensible at great 
distances from the poles, as in the temperate zone where they are 
often observed. The western deviation which in the middle lat- 
itudes usually precedes an aurora, indicates a large accumulation 
of electricity, due to a powerful condensation of vapors in the 
polar regions, which by facilitating the reunion of the two elec- 
tricities, augments the intensity of the terrestrial current passing 
in our hemisphere from the equator to the north, and consequent- 
ly carries the needle more to the west. When the aurora is once 
visible, the current becomes less strong, because the light itself 
of the aurora is proof of the resistance (probably due to the con- 
gelation of the particles of water suspended in the air that con- 
stitutes the mist) which the reunion of the two electricities en- 
counters ;* the needle will then retrograde to the east, as actually 
takes place. ; , 
In the higher latitudes, the disturbances of the needle are con- 
tinual, because the slightest differences in the intensity of the 
electric discharges that take place in the polar regions should be 
there perceived. As to the observations of MM. Ginge, Léwe- 
nérn and Lottin, that the maximum deviation of the needle takes 
Place from 8 to 10 o’clock in the evening, and the minimum at 
9 to 10 in the morning, they were made only during some weeks 
Mi summer, and they prove only that at this season of the year, 
the greatest amount of condensation of moisture takes place, as 
should be the case, at times just preceding and following the set- 
ting of the sun, and the least 7 or 8 hours after its rising. In 
the observations of Lieutenant Hood, made in the voyage of 
Captain Franklin, between the Ist of February and the 31st of 
ay, the greatest declination took place at 8 and 9 o’clock in the 
morning, and the least at an hour after noon. Thus, as is seen, 
the times of the maxima and minima are widely variable in those 
high latitudes, where there are great differences in the length of 
1¢ day, and also in temperature, and therefore considerable electric 
disturbances of the air. 
_, itis a singular fact, sometimes noticed, that when an observer is 
in the midst of an aurora, so to speak, the action on the needle may 
null, This was remarked by Mr. Forster, at Port Bowen, be- 
yond 65° N., the latitude of Forts Franklin and Enterprise, where 
Dr. Richardson had on the contrary observed the action of the 
needle. In fact, a needle in the interior of the circle formed by 
* It is clear that the mist when first formed should be a better conductor than 
when, afterwards, it consists only of icy particles. . 
