242 AUEORA BOEEALIS. 



electrometer soon indicates a diminislied ihtensity, which continues until towards 

 morning. 



The same causes which favor the escape of electricity from the upper atmos- 

 phere to the earth will produce an aurora whenever the electricity of the upper 

 air is sufBciently intense, and the conducting power of the air is favomble for 

 the slow transmission of an electric current. 



19. Cause of the annual inequality in the frequenry of auroras. — The unequal 

 frequency of auroras in the dift'erent mouths of the year appears to depend partly 

 upon the amount of electricity present in the upper air, and partly upon the 

 humidity of the air by which this electricity may be discharged. The supply 

 of electricity must be greatest when the evaporation is most rapid, that is, in 

 summer ; and this is probably the reason why in North America auroras are 

 more frequent in summer than in winter ; and it is not improbable that, were it 

 not for the longer continuance of daylight in summer, auroras would then be 

 more frequent than at any other period of the year. In Europe auroras are 

 seldom seen in midsummer, because, in those latitudes to Avhich auroras are 

 almost exclusively confined, twilight in midsummer continues all night. 



20. Cause oj" the secular inequality in the frequency of auroras. — The secular 

 inequality in the frequency of auroras seems to indicate the influence of distant 

 celestial bodies upon the electricity of our globe. The periods of auroras observe 

 laws which are very similar, if not absolutely identical, with those of at least 

 two other phenomena, viz : the mean diwrnal variation of the magnetic needle, 

 and the frequency of black spots upon the sun's surface. 



It is found that the north end of the magnetic needle has, in the morning, a 

 regular motion eastward amounting to from one to three minutes, when the de- 

 clination in New England is usually less than at any other hour of the day, 

 and may, therefore, be called the minimum. This minimum during the winter 

 is attained about nine o'clock, but during the summer months commonly as 

 early as seven. The needle then gradually deviates to the west, and attains its 

 greatest westerly bearing about two o'clock in the afternoon, when the declina- 

 tion is greater than at any other hour of the day, and may, therefore, be called 

 its maximum. From this time the needle again returns to the eastward, till it 

 attains its original bearing, about 10 o'clock. During the night another small 

 oscillation occurs, the north pole moving west until 3 a. m., and returning again 

 as before. The mean daily change of the magnetic needle not only varies with 

 the locality, but also varies from one year to another at the same locality ; and 

 these variations present a decided appearance of periodicity. In the following 

 table, column third shows the mean daily variation of the magnetic needle at 

 the stations named in the fourth column for the yeans mentioned in the first 

 column. These numbers are derived mostly from a table furnished by Pro- 

 fessor R. Wolf, and published in Poggendorflf's Auualen for 1862, v. 193, p. 503. 



Column second of the same table shows the relative frequency of the solar 

 spots, as determined by a collection of about 20,000 observations made by Pro- 

 fessor Wolf. 



