MAGNETISM. 401 



substances are attracted; also soot, and the dust 

 which floats in the atmosphere, are often attracted 

 by the magnet. 



Cavallo discovered, that if* most specimens of 

 brass which show no attraction towards the magnet, 

 be hammered, they will in that hardened state 

 (produced by the hammering) be attracted. The 

 same piece of brass will no longer be attracted, 

 after being softened in the fire ; a second hammer- 

 ing will again render it attractable, and so on 

 repeatedly. Most of the native grains of platina 

 have the same property. 



Of the Polarity of the Magnet. 



Every magnet has a south and a north pole, 

 which are at opposite ends ; and a line drawn from 

 one to the other, passes through the centre of the 

 magnet. Here it must not be understood, that 

 the polarity of a magnet resides only in two points 

 of its surface, for in reality, it is the one half of 

 the magnet that is possessed of one kind of 

 polarity, and the other half of the other kind of 

 polarity : the poles, then, are those points in which 

 that power is the strongest. 



The line drawn from one pole to the other, is 

 called the axis of the magnet. 



It is the polarity of the magnet that renders it 

 so useful to navigators. When a magnet is kept 

 suspended freely, so that it may turn north and 

 south, the pilot, by looking at the position of it, 

 can steer his course in any required direction. 

 Thus, if a vessel is steered towards a certain place 

 which lies exactly westward of that from which it 

 set' out, the navigator must direct it so, that its 



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