CHAP, ii.] ELECTRICITY AND MAGNETISM. 121 



Kew and other magnetic observatories the daily and 

 hourly variations of the magnet are recorded on a 

 continuous register. The means employed consists in 

 throwing a beam of light from a lamp on to a light mirror 

 attached to the magnet whose motion is to be observed. 

 A spot of light is thus reflected upon a ribbon of photo- 

 graphic paper prepared so as to be sensitive to light. 

 The paper is moved continuously forward by a clock- 

 work train ; and if the magnet be at rest the dark trace 

 on the paper will be simply a straight line. If, however, 

 the magnet moves aside, the spot of light reflected from 

 the mirror will be displaced, and the photographed line 

 will be curved or crooked. Comparison of such records, 

 or " magneto graphs" from stations widely apart on the 

 earth's surface, promises to afford much light upon the 

 cause of the earth's magnetism and of its changes, of 

 which hitherto no reliable origin has been with certainty 

 assigned 



