TERRESTRIAL MAGNETISM 



313 



magnet increases by M<5H, and this is indicated by its angular 

 displacement. 



The vertical force magnet is provided with a knife edge at 

 its centre, resting on a plane, and is so adjusted that with 

 the average vertical force it is 

 horizontal. It is practically a 

 balance arm. If the vertical com- 

 ponent changes, either through 

 change in direction or magnitude 

 of the total intensity, the magnet 

 tilts. Special methods are 

 adopted to calibrate the records 

 on the revolving drum (Ency. FIG. 232. 



Brit., I.e.). 



Results. Observation shows that the magnetic elements are 

 subject to continual "disturbances." On quiet days, in this 

 country, the disturbances in the declination are not usually more 

 than a few minutes. There are days on which the disturbances are 

 greater, and sometimes are so large that they are described as due 

 to magnetic storms. But it is very rare in this country for the 

 range of declination to exceed 5. When the value of an element 

 for each hour of the day is averaged for a large number of days, it 



FIG. 233. 

 Continuous line, summer. Dotted line, winter. 



is found that there is a diurnal periodicity or inequality differing 

 at different parts of the earth, and differing again in summer and 

 winter. Thus the declination at Kew shows a diurnal inequality 



