92 ELEMENTARY LESSONS ON [CHAP. n. 



52 and 53. In Fig. 52 the poles are of opposite kinds, 

 and the lines of force curve across out of one pole into 

 the other; while in Fig. 53, which represents the action 



of two similar poles, the lines of force curve away as if 

 repelling one another, and turn aside* at right angles. 

 Musschenbroek first pointed out the essential difference 

 between these two figures. 



Fig. 52. Fig. 53. 



111. Magnetic Writing. Another kind of magnetic 

 figures was discovered by De Haldat, who wrote with the 

 pole of a magnet upon a thin steel plate (such as a saw- 

 blade), and then sprinkled filings over it. The writing, 

 which is quite invisible in itself, comes out in the lines 

 of filings that stick to the magnetised parts ; this magic 

 writing will continue in a steel plate many months. The 

 author of these Lessons has produced similar figures in 



