202 ON MAGNETISM. 



If (as with the other instruments) a concave mirror 

 be attached to the magnet and throw the image of a 

 fixed light to a photographic barrel (whose axis is 

 vertical) at the distance p inches : then the direction of 

 the reflected beam will be changed through the angle 

 2i/r, which will cause the light-spot to move through 



2p . i|r inches. For 1 inch of motion, ty will = ^- , and 



BF = JT^_ 

 H ~ 2p . T 2 ' 



87. Results obtained from the continuous registers 

 of small changes in terrestrial magnetism. 



When the sheets of photographic paper are detached 

 from their barrels, and a large number of these sheets 

 (extending for instance through a year or through 

 several years) are examined, they present the most 

 capriciously discordant appearance that can be imagined. 

 Thus, Figure 60 represents the curve given by the Hori- 

 zontal-Force-Magnetometer on a quiet day (1869, Octo- 

 ber 17) : Figure 61 represents that given by the same 

 instrument on a day of disturbed magnetism (1869, 

 March 10). It appears from such records that the 

 terrestrial forces are at every moment in a state 

 of change, though in very different degrees on 

 different days. The laws of change extend without 

 sensible alteration over considerable geographical dis- 

 tances: the writer of this treatise has compared many 

 photographic records made at the Royal Observatory 

 of Greenwich with those made at the same time 



