4 ON MAGNETISM. 



that direction, will return to it by a series of vibrations, 

 gradually diminishing in extent, from the effect of 

 atmospheric resistance, &c." As a general rule, the 

 material possessing this property to a degree admitting 

 of experimental examination must be steel. In some 

 exceptional cases however (to be hereafter mentioned) 

 the same properties may be given in a minor degree 

 to bars of iron. 



5. Size of magnets most convenient for experiments. 



For a few important experiments, to be mentioned 

 feelow, it is desirable to be provided with a large magnet, 

 perhaps one foot or two feet in length. But, generally, 

 the best magnets for experiment are small compass- 

 needles, mounted and unmounted. These are capable 

 of possessing a great magnetic power in proportion 

 to their weight, and they can be procured at small 

 expense. 



6. Mounting of magnets for experiments. 



In experiments where the position taken by the 

 magnet, or its vibration, or its displacement by the 

 action of an external magnetic substance, is to be ob- 

 served, it is desirable that the magnet (and, if suspended, 

 its suspending apparatus) should be inclosed in a glass 

 case. For many ordinary experiments, the support of 

 the magnet upon a fine point, as in the common com- 

 pass, is sufficiently delicate ; especially if the point be 

 made of the hard iridium-ore, now universally employ- 

 ed for the compasses of the Royal Navy. But for 



