PRODUCTION OF SUBPERMANENT MAGNETISM. 169 



and one surface containing the direction of local dip ; 

 and to fix it to the floor. Lay a piece of iron plate or 

 iron bar, in contact with the dip plane, and with its 

 length approximately in the direction of dip : and 

 strike it repeatedly with an iron hammer. On re- 

 moving it, it will be found to be a true magnet, the 

 end which was lowest being charged with red mag- 

 netism: and this magnetism is not transient like the 

 induced magnetism of soft iron, changing its place in 

 the bar with every change in the position of the bar 

 (see Article 60) ; but is constant like that of a steel 

 magnet, retaining the same magnetism whatever be 

 the position of the bar. An iron bar which has not 

 been struck, if applied in the horizontal position end- 

 on towards the center of a compass, does not disturb 

 the needle at all; but the same iron bar when it has 

 been struck in the manner described, if applied end-on 

 to the center of a compass, disturbs it powerfully : one 

 end deflecting the needle in one direction, and the 

 other end deflecting the needle in the opposite direc- 

 tion, exactly as a steel magnet would do it: and in 

 all respects comporting itself as a steel magnet. 



76. Variations of the experiment. All lead to the 

 supposition that iron, in a state of tremor or jar, is 

 peculiarly able to receive induced magnetism and to 

 retain it firmly. 



The circumstances of the last experiment, in which 

 the receipt of magnetism depends evidently on the 



