HOW LIGHT EXERTS PRESSURE I? 



may be the actual modification of the medium 

 between the two electrifications we may symbolize 

 its effect by saying that " lines of force," or " tubes 

 of force," as in fig. 4, start from the negative on 

 the wax to the positive on the conductor, and that 

 these tubes of force are trying to contract and pull 

 the bodies together. The action is like that which 

 would occur if every tube of force were a stretched 

 india-rubber cord with its ends fastened on the two 

 surfaces. Imagine, then, a bundle of such stretched 

 rubber cords. As they shorten lengthways they 

 tend to bulge out sideways and press against each 

 other. In somewhat similar manner the tubes of 

 force in the modified medium, while exerting end 

 pulls are also exerting against each other a side 

 pressure as if they too were trying to bulge side- 

 ways. It is this side pressure of the tubes against 

 each other or against any body on which they 

 impinge sideways which is their essential property 

 for our present purpose. 



We may symbolize magnetic attraction in a 

 similar way. If, for example, the north-seeking 

 pole of a magnet is drawing towards itself a piece 

 of iron it makes a south-seeking pole in the nearest 

 part of the iron ; and we may think of tubes of 

 force as stretching from one pole to the other 

 through the intervening medium. These tubes of 

 force tend to shorten, and so to draw the magnet 

 and iron together. We have much reason to sup- 



B 



