the Lines of Magnetic Force. 403 



as far as we can test it by the resultant phenomena. It cannot 

 be deflected, or even affected, in its course. Neither is the action 

 in one line at all influenced, either in direction or amount, by a 

 like action in another line ; i. e. one particle gravitating toward 

 another particle has exactly the same amount of force in" the 

 same direction, whether it gravitates to that one alone or towards 

 myriads of other like particles, exerting in the latter case upon 

 each one of them a force equal to that which it can exert upon 

 the single one when alone : the results of course can combine, 

 but the direction and amount of force between any two given 

 particles remain unchanged. So gravity presents us with the 

 simplest case of attraction ; and appearing to have no relation to 

 any physical process by which the power of the particles is carried 

 on between them, seems to be a pure case of attraction or action 

 at a distance, and offers therefore the simplest type of other cases 

 which may be like it in that respect. My object is to consider 

 how far magnetism is such an action at a distance ; or how far 

 it may partake of the nature of other powers, the lines of which 

 depend, for the communication of force, upon intermediate phy- 

 sical agencies (3075.). 



3246. There is one question in relation to gravity, which, if 

 we coidd ascertain or touch it, would greatly enlighten us. It 

 is, whether gravitation requires time. If it did, it would show 

 undeniably that a physical agency existed in the course of the 

 line of force. It seems equally impossible to prove or disprove 

 this point; since there is no capability of suspending, changing, 

 or annihilating the power (gravity), or annihilating the matter 

 in which the power resides. 



3247. When we turn to radiation phsenomena, then we obtain 

 the highest proof, that though nothing ponderable passes, yet 

 the lines of force have a physical existence independent, in a 

 manner, of the body radiating, or of the body receiving the rays. 

 They may be turned aside in their course, and then deviate from 

 a straight into a bent or a curved line. They may be affected 

 in their nature so as to be turned on their axis, or else to have 

 different properties impressed on different sides. Their sum of 

 power is limited ; so that if the force, as it issues from its source, 

 is directed on to or determined upon a given set of particles, or 

 in a given direction, it cannot be in any degree directed upon 

 other particles, or into another direction, without being propor- 

 tionately removed from the hist. The lines have no dependence 

 upon a second or reacting body, as in gravitation; and they 

 require time lor their propagation. In all these things they are 

 in marked contrast with the lines of gravitating force. 



3248. When we turn to the electric force, we are presented 

 with a very remarkable general condition intermediate between 



2D2 



