152 JAMES CLERK MAXWELL 



charged with po.iitive electricity, and placed in any given 

 losition, will experience a force urging it in a certain direction. 

 If the small body be now negatively electrified, it will be urged 

 by an equal force in a direction exactly opposite. 



"The same relations hold between a magnetic body and the 

 north or south pole* of a small magnet. If the north pole is 

 urged in one direction, the south pole is urged in the opposite 

 direction. 



"In this way we might find a line passing through any 

 point of space, such that it represents the direction of the force 

 acting on a positively electrified particle, or on an elementary 

 north j>ole, and the reverse direction of the force on a negatively 

 electrified particle or an elementary south pole. Since at every 

 point of space such a direction may be found, if we commence 

 at any point and draw a line so that, as we go along it, its 

 direction at any point shall always coincide with that of the 

 resultant force at that point, thU curve will ' indicate the 

 direction of that force for every point through which it passes, 

 and might be called on that account a line of furcf. We might 

 in the same way draw other linos of force, till wo had filled all 

 space with curves indicating by their direction that of the force 

 at any assigned i>oint 



44 We should thus obtain a geometrical model of the physical 

 phenomena, which would tell us the directim of the force, but 

 we should still require some method of indicating the intensity 

 of the force at any point. If we consider these curves not as 

 mere lines, but a* fine tul>es of variable section carrying an 

 incompressible fluid, then, since the velocity of the fluid is 

 inversely as the section of the tube, we may make the velocity 

 vary according to any given law, by regulating the section of 

 the tube, and in this way we might represent the intensity of 

 the force as well as its direction by the motion of the fluid in 

 these tubes. This method of representing the intensity of a 

 force by the velo.-ity of an imaginary fluid in a tube i? 

 applicable to any conceivable system of forces, but it is 

 capable of great simplification in the case in which the forces 

 are such as can be explained by the hyjMjthesis of attractions 

 varying inversely a* tho squire of the distance, such as those 



