ACTION AT A DISTANCE. 323 



And these lines must not be regarded as mere mathematical absti-actions. 

 They are the directions in which the medium is exerting a tension like that 

 of a rope, or rather, like that of our own muscles. The tension of the medium 

 in the direction of the earth's magnetic force is in this country one grain 

 weight on eight square feet. In some of Dr Joule's experiments, the medium 

 has exerted a tension of 200 Ibs. weight per square inch. 



But the medium, in virtue of the very same elasticity by which it is able 

 to transmit the undulations of light, is also able to act as a spring. When 

 properly wound up, it exerts a tension, different from the magnetic tension, by 

 which it draws oppositely electrified bodies together, produces effects through 

 the length of telegraph wires, and when of sufficient intensity, leads to the 

 rupture and explosion called lightning. 



These are some of the already discovered properties of that which has 

 often been called vacuum, or nothing at all. They enable us to resolve several 

 kinds of action at a distance into actions between contiguous parts of a con- 

 tinuous substance. Whether this resolution is of the nature of explication or 

 complication, I must leave to the metaphysicians. 



412 



