478 MAGNETISM AND ELECTRICITY 



hemisphere it is customary to make the south end of the 

 needle a little heavier so that it will stay in a horizontal 

 position. At the magnetic pole the needle would stand 

 vertical. If a needle is accurately balanced on a horizontal 

 axis and then magnetized, it will show the angle of dip in 

 any locality. Such a needle is called a dipping needle 

 (Figure 149). 



The Mariner's Compass. In the ordinary mariner's 

 compass (Figure 150) a magnetic needle is arranged so that 

 it will swing freely in a horizontal plane. A circular card is 

 divided into four equal parts, the divid- 

 ing lines of which are marked with the 

 cardinal points of the compass, the inter- 

 vening spaces being divided into eight 

 equal divisions. The card is attached to 

 the needle and inclosed in a box called 

 the binnacle. This box is arranged so 

 FIGURE 150 that ^ wn "l always remain horizontal. 

 A fixed line on the binnacle shows the 

 direction of the keel of the ship. The card being attached 

 to the needle always has its " north " pointing toward the 

 north. To determine the direction of the ship it is only 

 necessary to notice on the card in what direction the keel 

 line is pointing. The mariner of course must know the 

 declination at the place where he is and make the proper 

 corrections. The different governments furnish tables and 

 charts showing these corrections. 



Theory of Magnetism. Experiment 161. Heat a No. 20 

 knitting needle red hot and plunge it quickly intp cold water. This 

 tempers the needle so that it will break readily. Magnetize the 

 needle as was done in Experiment 8. When it has become well 

 magnetized, break it in the middle. Test each half with a sus- 



