112 



ELEMENTARY LESSONS ON [CHAP. IT. 



LESSON XII. Terrestrial Magnetism. 



134. The Mariner's Compass. It was mentioned 

 in Art. 79 that the compass sold by opticians consists of 

 a magnetised steel needle balanced on a fine point above 

 a card marked out N, S, E, W, etc. The Mariner's 

 Compass is, however, somewhat differently arranged. 



In Fig. 65 one of the forms of a Mariner's Compass, 

 used for nautical observations, is shown. Here the 



Fig. 65. 



card, divided out into the 32 " points of the Compass," is 

 itself attached to the needle, and swings round with it so 

 that the point marked N on the card always points to 

 the north. In the newest and best ships' compasses 

 several magnetised needles are placed side by side, as it 

 is found that the indications of such a compound needle 

 are more reliable. The iron fittings of wooden vessels, 

 and, in the case of iron vessels, the ships themselves, 



