[From the Philosophical Magazine, Vol. xxi.] 



XXIII. On Physical Lines of Force. 



PAET I. 

 THE THEORY OP MOLECULAR VORTICES APPLIED TO MAGNETIC PHENOMENA. 



IN all phenomena involving attractions or repulsions, or any forces depend- 

 ing on the relative position of bodies, we have to determine the magnitude and 

 direction of the force which would act on a given body, if placed in a given 

 position. 



In the case of a body acted on by the gravitation of a sphere, this force 

 is inversely as the square of the distance, and in a straight line to the centre 

 of the sphere. In the case of two attracting spheres, or of a body not spherical, 

 the magnitude and direction of the force vary according to more complicated 

 laws. In electric and magnetic phenomena, the magnitude and direction of the 

 resultant force at any point is the main subject of investigation. Suppose that 

 the direction of the force at any point is known, then, if we draw a line so 

 that in every part of its course it coincides in direction with the force at that 

 point, this line may be called a line of force, since it indicates the direction 

 of the force in every part of its course. 



By drawing a sufficient number of lines of force, we may indicate the 

 direction of the force in every part of the space in which it acts. 



Thus if we strew iron filings on paper near a magnet, each filing will be 

 magnetized by induction, and the consecutive filings will unite by their opposite 

 poles, so as to form fibres, and these fibres will indicate the direction of the lines 

 of force. The beautiful illustration of the presence of magnetic force afforded 

 by this experiment, naturally tends to make us think of the lines of force as 

 something real, and as indicating something more than the mere resultant of 

 two forces, whose seat of action is at a distance, and which do not exist there 



572 



