, ACTION AT A DISTANCE. 



so that the number of the lines wliich pass through an area, say of one square 

 inch, indicates the intensity of the force acting through the area. Tims tin- 

 lines of force become definite in number. The strength of a magnetic pole i- 

 mosimrH by the number of lines which proceed from it; the electro-tonic state 

 of a circuit is measured by the number of lines which pass through it. 



In the second place, each individual line has a continuous existence in 

 space and time. When a piece of steel becomes a magnet, or when an electric 

 current begins to flow, the lines of force do not start into existence each in 

 its own place, but as the strength increases new lines are developed within 

 the magnet or current, and gradually grow outwards, so that the whole system 

 expands from within, like Newton's rings in our former experiment. Thus every 

 line of force preserves its identity during the whole course of its existence, 

 though its shape and size may be altered to any extent. 



I have no time to describe the methods by which every question relating 

 to the forces acting on magnets or on currents, or to the induction of currents 

 in conducting circuits, may be solved by the consideration of Faraday's lines of 

 force. In this place they can never be forgotten. By means of this ii- 

 symbolism, Faraday defined with mathematical precision the whole theory of 

 electro-magnetism, in language free from mathematical technicalities, and appli- 

 cable to the most complicated as well as the simplest cases. But Faraday did 

 not stop here. He went on from the conception of geometrical lines of force 

 to that of physical lines of force. He observed that the motion which the 

 magnetic or electric force tends to produce is invariably such as to shorten 

 the lines of force and to allow them to spread out laterally from each other. 

 He thus perceived in the medium a state of stress, consisting of a tension, 

 like that of a rope, in the direction of the lines of force, combined with a 

 pressure in all directions at right angles to them. 



This is quite a new conception of action at a distance, reducing it to a 

 phenomenon of the same kind as that action at a distance which is exerted 

 by means of the tension of ropes and the pressure of rods. When the muscles 

 of our bodies are excited by that stimulus which we are able in some unknown 

 way to apply to them, the fibres tend to shorten themselves and at the same 

 time to expand laterally. A state of stress is produced in the muscle, and the 

 limb moves. This explanation of muscular action is by no means complete. 

 It gives no account of the cause of the excitement of the state of stress, nor 

 does it even investigate those forces of cohesion which enable the muscles to 



