646 



SCIENTIFIC RECREATIONS. 



places where it does not deviate, and these, in direction north and south, 

 are called lines of " no variation." There are also places in the equatorial 

 regions where the needle does not " dip." The line connecting such places 

 is termed the Magnetic Equator, and north or south of this the needle dips 

 respectively to north or south in degrees coinciding with the distance from 

 the equator. 



The earth, then, acts as a magnet, and attracts the needle, but the mag- 

 netic poles are not identical with the terrestrial poles. The north magnetic 

 pole was reached in 1831 by Sir James Ross, when the dip was only one 

 minute less than 90, and the south magnetic pole was very nearly reached 

 also by him in 1840. The magnetic equator passes between these two 

 points. 



i-ig. 727. Paraselense, or mock moons. 



It is to magnetic atmospherical disturbance that the aurora is due. 

 These northern (or southern) phenomena are extremely brilliant and diversi- 

 fied. In temperate regions the aurora does not present such grand forms as 

 in the extreme north. There the spectacle is astonishingly beautiful. The 

 sky at first clouds over, and mist is developed. Humboldt has eloquently 

 described the aurora borealis, and the beautiful changes of light, the constant 

 movement, flashes, etc., denoting a " magnetic " storm, as electrical dis- 

 charges indicate an electric storm, although the area affected by the former 

 is far more extensive than that of the latter, and there is no thunder accom- 

 panying the magnetic storm, with the production of which the electricity of 

 the earth is unassociated. To the continuous flow of this electricity the 

 aurora is due, and the flashes are only the electric current descending 



