172 ELEMENTS OF SCIENCE 



it constantly points towards the magnetic north pole or 

 rather its long axis always coincides with the line join- 

 ing the magnetic poles unless it be made to deviate by 

 some local magnet, or mass of iron in its vicinity. But 

 as the magnetic poles do not coincide with the earth's 

 poles of rotation, and as the imaginary circles or 

 " meridians " (real as drawn on maps) which mark 

 degrees of longitude, all pass through the poles of 

 rotation (the geographical north and south poles), the 

 long axis of the magnet will not coincide with such 

 " meridians " along any great circle. Theoretically, 

 there ought to be one such circle of coincidence, but, 

 owing to the fact that in the globe (as in other natural 

 magnets) magnetism is, more or less, irregularly dis- 

 tributed, the line of coincidence is also irregular. This 

 circle extends from near Hudson's Bay, through the 

 United States, Cuba, Jamaica, and Brazil, to the South 

 Magnetic Pole, whence it is continued on through 

 Australia, China, and Siberia to the North Magnetic 

 Pole, near Hudson's Bay, whence we started. The 

 further the magnetic needle may diverge from this line, 

 the greater, of course, will be its divergence from a 

 geographical meridian, and such divergence is called its 

 variation. The magnetic needle, while constantly point- 

 ing towards its pole, undergoes slight changes, daily, 

 monthly, and yearly, while every now and then it 

 undergoes sudden and irregular disturbances, indicating 

 what are called magnetic storms, and these seem to 

 extend over the whole earth. 



The magnetic needles may be exactly balanced 

 horizontally, and yet able to lower one end, or, as it is 

 technically termed, to dip. The nearer such a needle 

 is brought towards either magnetic pole, the more it will 

 dip, and at either such pole it will be vertical, while 



