72 GAUSS AND WEBER ON TERRESTRIAL, MAGNETISM. 



needle is thereby removed. The measurement is still dependent 

 on the remaining peculiarity of the needle, namely, its own 

 magnetism ; and as soon as we can reduce this to an absolute 

 measure, the force of terrestrial magnetism itself may also be 

 reduced to an absolute measure; for we have only to divide 

 the number which expresses the moment of rotation by the 

 number which measures the magnetism of the needle. In fact, 

 the earth's magnetism is measured by a force equal to itself, 

 whose action on the unit of magnetism of the needle consists in 

 a moment of rotation, measured by the force which the unit of 

 weight exerts on a lever of the unit of length." 



If, therefore, T signifies the terrestrial magnetism, and M the 

 magnetism of the needle, or of the vibrating bar, 



T= ^ (I.) 



" We might be inclined to suppose that the weight which a 

 magnetic needle can carry would afford a standard by m hich the 

 force of magnetism developed in the needle might be estimated ; 

 but a closer examination will show that this method is quite un- 

 availing for our purpose. The determination itself is incapable 

 of much precision ; for repeated experiments give very different 

 results ; but there is a still more important objection : the capa- 

 bility of sustaining weight has no necessary connexion with the 

 magnitude of the development of magnetism in the needle, in 

 the sense in which it must here be understood. The moment of 

 rotation is due to the magnetism of all the parts of the needle, 

 upon which the terrestrial magnetism acts equally, and in pa- 

 rallel directions. The sustaining power, on the contrary, is 

 chiefly due to the magnetism situated in the ends nearest to the 

 Aveight, which, moreover, is modified every moment by the reci- 

 procal action of the magnet-bar and the suspended iron. 



" A magnetic needle, at a given place, acts on, every point of 

 space, in an amount and direction determined by its distance and 

 position. In the immediate neighbourhood its action is strong, 

 but very unequal on different pai'ts ; at great distances the action 

 is weak, but almost uniform in strength and direction within a 

 moderate space. The greater the distance, the nearer the law of 

 the force approaches to a rule, which is very simple, and is com- 

 pletely given by theory : we may limit ourselves here to the con- 

 sideration of a single case, which is sufficient for our purpose. 



