TERRESTRIAL MAGNETIC ELEMENTS. 2QI 



From this point of view we must attribute to the fluids, to the 

 magnets, and to magnetic substances, a certain number of properties 

 by which all the experiments may be explained. 



We assume, then, the existence of two imponderable magnetic 

 fluids, consisting, like the electrical fluids, of molecules which act by 

 repulsion on the molecules of the same fluid, and by attraction on 

 molecules of a different kind, these reciprocal actions being inversely 

 as the square of the distance. The combination of these two fluids 

 in equal quantities has no action on external bodies, and constitutes 

 what may be called the neutral fluid. 



In virtue of the phenomena of induced magnetisation, we must 

 assume that the neutral fluid exists in almost unlimited quantity in 

 magnetic bodies, and is divided into two distinct fluids under the 

 influence of the magnet. 



Since permanent or temporary magnets are always complete, 

 whatever may be their dimensions, we must assume also that the 

 fluids present in an element of volume never quit it to pass to an 

 adjacent element, so that the separation of these fluids is confined to 

 the extent of each molecule. 



Finally, no internal force opposed to the directive actions of the 

 magnetic fluids, hinders their separation or their reunion in soft iron. 

 In cast iron and in steel, on the contrary, there is a special resistance 

 (a kind of friction called coercive force), which restricts the magnetisa- 

 tion by induction, and then hinders the recombination of the fluids 

 when the external force has disappeared. 



It is not surprising that the theory of fluids, with all the acces- 

 sories which are only arbitrarily connected with it, can explain the 

 phenomena ; in these conditions the agreement of experiment with 

 theory affords no argument in favour of the exactitude of the 

 hypothesis ; and we shall make no further use of it. 



305. DEFINITION OF TERRESTRIAL MAGNETIC ELEMENTS. 

 The magnetic field which surrounds the earth, and which may be 

 called the terrestrial field, is sensibly uniform throughout a space of 

 small dimensions as compared with that of the terrestrial radius ; but 

 the direction and the intensity of the force vary from one point to 

 another. In fact the force in any one place changes in magnitude 

 and direction in the course of time ; we shall disregard for the 

 moment these variations which are feeble, and shall suppose we are 

 considering the magnetic state of the globe at a definite time. 



The magnetic axis of a magnet suspended freely by its centre of 

 gravity, and withdrawn from any other action than that of the 

 terrestrial magnetic field, would, when in equilibrium, assume the 



u 2 



