THE AURORA. 



645 



through the forest. Many instances are on record concerning the luminosity 

 of pointed sticks, and even of the tails and manes of horses in certain conditions 

 of the atmosphere, and of the 'universal power of electricity and its pervading ( 

 influence in nature. The benefits conferred by thunderstorms in purifying 

 the air, and in the production of ozone and nitric acid, are very great, and r 

 apart from the magnificent phenomena exhibited, are well worth our atten- 

 tion, though beyond our reach. 





Fig. 726. Aurora Borealis. 



Terrestrial magnetism, however, is still more puzzling in its action than is 

 electricity, and the study of the needle, its destination, inclination, and 

 intensity, which are marked upon charts, just as are the weather reports of the 

 Times, is an interesting one. These magnetic maps are termed the charts of 

 ISOCLINIC and ISODYNAMIC lines. The declination of the magnetic needle from 

 the true north is its deviation from that point, and the " inclination " is its dip 

 towards the horizon. The line of its direction being known as the magnetic 

 meridian, its divergence from this line constitutes its declination. There are 



