QUALITY OF THE EARTH'S MAGNETISM, 



15 



purposes the Earth may be represented by such a 

 magnet. 



The importance of this inference for theories of 

 magnetism cannot be over-estimated. It shews, not 

 only that the Earth's red and blue poles must be 

 considered to be on the south and north sides, but also 

 that the quality of the Earth's magnetism is the same 

 as that of a steel magnet. 



Advantage may be taken of this principle, in ex- 

 periments, for removing terrestrial influence. If, as 

 in Figure 5, a large magnet be placed at a proper 

 distance below a table, magnetic experiments may be 

 performed upon that table without disturbance by 

 terrestrial magnetism. 



Fig. 5. 



There is however another way of neutralizing the 

 Earth's action, by use of the " astatic needle." This 

 instrument, represented in Figure 6, consists of two 

 needles of equal magnetic power, attached to the same 

 central axis, with their poles in opposite positions. In 

 this state, the action of the Earth on one needle is 



