262 ELEMENTARY LESSONS ON [CHAP. iv. 



and two minima which in summer were at the hours of 

 ^p.m. and about midnight. He also found that in January 

 the electricity was about thirteen times as strong as in 

 June. Observations made by Prof. B. Stewart at Kew 

 show a maximum at 8 a.m. in summer at 10 a.m. in 

 winter, and a second minimum at 10 p.m. in summer 

 and 7 p.m. in winter. The maxima correspond fairly 

 with hours of changing temperature, the minima with 

 those of constant temperature. In Paris, M. Mascart 

 finds but one maximum just before midnight : at sun- 

 rise the electricity diminishes until about 3 p.m., when it 

 has reached a minimum, whence it rises till nightfall. 



Our knowledge of this important subject is still very 

 imperfect. We do not even know whether all the 

 changes of the earth's electrification relatively to the air 

 are due to causes operating above or below the earth's 

 surface. Simultaneous observations at different places 

 and at different levels are greatly wanted. 



3O9. The Aurora. In all the northern regions of 

 the earth the Aurora borealis, or " Northern Lights," is 

 an occasional phenomenon ; and within and near the 

 Arctic circle is of almost nightly occurrence. Similar 

 lights are seen in the south polar regions of the earth, 

 and are denominated Aurora australis. As seen in 

 European latitudes, the usual form assumed by the 

 aurora is that of a number of ill -defined streaks or j 

 streamers of a pale tint (sometimes tinged with red and | 

 other colours), either radiating in a fan -like form from I 

 the horizon in the direction of the (magnetic) north, or | 

 forming a sort of arch across that region of the sky, ofj 

 the general form shown in Fig. 112. A certain flicker- j 

 ing or streaming motion is often discernible in the i 

 streaks. Under very favourable circumstances the 

 aurora extends over the entire sky. The appearance of 

 an aurora is usually accompanied by a magnetic storm 

 (Art. 145), affecting the compass -needles over whole 

 regions of the globe. This fact, and the position of the 



