92 PRACTICAL LESSONS IN SCIENCE. 



filings be sprinkled over the paper, on tapping the paper gently 

 the filings will arrange themselves in beautiful curves about the 

 poles of the magnet, showing that a magnet makes magnets 

 without contact, and all these iron filings, as secondary magnets, 

 are arranged pole to pole in regular order according to the laws 

 mentioned above. The most important use of the magnet is in 

 the construction of the compass. A slender bar magnet bal- 

 anced on a pivot, is called a magnetic needle. The compass con- 

 sists of a circular box, usually of brass, with a glass cover, in 

 which a magnetic needle is balanced over a card on which the 

 different directions are marked. The mariner's compass is so 

 mounted that whatever the motion of the ship the compass is 

 always horizontal. With it the mariner, aided by observations 

 of the sun and stars, finds his way across the trackless sea. 



In general, the needle varies more or less from the true 

 north. The line of no variation passes near Cape Lookout, 

 Cleveland, Ohio, thence across Hudson's Bay to the pole, enter- 

 ing Europe just east of the White Sea, thence across the Caspian 

 Sea, and through Western Australia to the south pole, thence 

 northward through eastern South America. West of this line the 

 needle varies west, east of it the needle varies east. This line itself 

 varies east and west. In 1580, the variation of the needle, at 

 Paris was 11 30' east, showing that the line of no variation was 

 west of Paris. In 1663 this line was at Paris, while in 1825 the 

 variation was 22 22' west, showing that the line of no variation 

 was far to the east. Besides this slow variation there are annual 

 and daily variations, and variations due to volcanic eruptions 

 and earthquakes, and variations attended upon marked displays 

 of the Aurora borealis, and disturbances in the sun as shown by 

 sun spots. These latter variations are said to be due to mag- 

 netic storms or disturbances. 



The magnetic needle does not remain in a horizontal position, 

 but north of the equator the north end dips, while south of the 

 equator the south end dips, so that in practice one end must be 

 weighted to keep the needle horizontal. Here again we have no 

 rational explanation of the phenomena. 



