MAGNETISM. Ill 



MAGNETISM. 



THE substances endowed with magnetism exhibit that property by three 

 distinct effects : 



1. They attract iron and all ferruginous matter. 



2. Two bodies endowed with the property of magnetism will attract each 

 other at one part of their dimensions, and repel each other at another part. 

 These contrary effects, belonging to opposite sides or ends, are called mag- 

 netic polarity. 



3. When a magnet is placed on a vertical axis through its centre of gravity, 

 on which it is free to revolve, the axis being between its poles, it will oscillate 

 on each side of a certain determinate position, in which it will at length come 

 to rest. When in this position, a vertical plane passing through the axis and 

 the poles will be nearly, but not exactly, coincident with the plane of the me- 

 ridian of the place in which the magnet is situate. For all magnets similarly 

 supported, in the same situation, these planes will be parallel. This plane is 

 called the magnetic meridian. The angle which the magnetic meridian makes 

 with the terrestrial meridian is called the variation of the magnet. 



4. If a magnet be placed on a horizontal axis passing through its centre of 

 gravity at right angles to the magnetic meridian and between its poles, it will 

 oscillate on each side of a certain determinate position, in which it will at 

 length come to rest. When in this position, a plane passing through the axis 

 and the poles of the magnet will not be horizontal, but will make a certain 

 angle with a horizontal plane through the axis. This angle is called the dip 

 of the magnet. 



The power of the magnet, when placed on a vertical axis, to fix itself in the 

 magnetic meridian of any place to which it may be transported, is called its 

 directive power, and is the principle on which its application to navigation de- 

 pends. 



The attractive power of the magnet for iron was the property which was 

 first observed. This property was known to the ancients, who gave to the 



