474 POPULAR SCIENCE MONTHLY. 



during auroras. Andree, in 1882-'83, at Spitzbergen, with a Mas- 

 cart electrometer, found that before an aurora appeared the positive 

 electric potential of the air diminished rapidly, and even became 

 negative, as usually happens when it rains; but as soon as the aurora 

 appears the potential takes as before a high positive value. But it 

 must be remembered that the electric potential of the atmosphere, 

 especially at a short distance from the earth, varies constantly in an 

 abrupt and irregular manner; so that it would be imprudent to 

 assume any coincidence, even during an aurora. 



Auroras seem to depend for their form and position in space on 

 the distribution of magnetism on the surface of the globe. The 

 appearances of auroras coincide in the majority of cases with mag- 

 netic disturbances on the earth's surface. The auroral arc generally 

 has its summit near the magnetic meridian, where the compass needle 

 points. The force which governs the auroras appears to be the 

 same which the magnetic needle obeys. In general, the point where 

 the rays converge in auroral crowns, the center of the crown, is near 

 the magnetic zenith. The variation, in fact, in such instances is 

 but one degree, whi(;h may be allowed as the amount of probable 

 error in observations. 



The conclusion is, therefore, that the earth's magnetic forces cer- 

 tainly play the most important part in the auroral display. The arcs 

 or " bands " arc nearly perpendicular to the magnetic meridian, and 

 the direction of the rays parallel to the magnetic needle. The devia- 

 tions from these rules are due to atmospheric conditions of tempera- 

 ture and humidity. 'J'he electric discharges which constitute the 

 aurora borealis encount(!r all sorts of conditions of the atmospheric 

 strata which are unequally conductive; hence lack of symmetry in 

 the auroral displays. Meteorological conditions very likely influ- 

 ence not the production of the aurora, but its form and position. 

 There is an intimate relation, as we have already pointed out, be- 

 tween the weather and these magnetic disturbances. 



The magnetic needle sometimes begins to be agitated an hour be- 

 fore the appearance of an aurora. The magnetic disturbances last a 

 long while, often for twenty-four hours. Motionless arcs and faint 

 auroras do not affect the magnetic needle sensibly, while, during an 

 active aurora, which is apparently nearer, the needle is greatly agi- 

 tated, especially wlicu the great red and green rays flash suddenly 

 like lightning. 



Arago has shown that if the aurora seems to be absent during 

 magnetic disturbances, it is often because it is too distant, or below 

 our horizon, and visible only in more northern latitudes. Tn the 

 arctic and antarctic regions there is small deviation of the magnetic 

 needle, but this is considered due to the groat height of these auroras, 



