172 ON MAGNETISM. 



steel magnet in all respects but this, that, while 

 perhaps no change can be remarked in hours or days, 

 it infallibly diminishes in a long time. To express 

 this partially permanent character, the term 'Sub- 

 permanent Magnetism ' has been adopted. 



In single bars, the subpermanent magnetism di- 

 minishes sensibly in a few hours, and is lost in a few 

 days. In some large iron ships, a portion of it has 

 remained unaltered for many years. It would seem 

 that where the operation of magnetizing by hammer- 

 blows has been rapid, the magnetism is not very 

 firmly fixed: but where the violence has been long 

 continued, the magnetism is so firmly established as 

 to become an immovable quality of the iron. 



