176 CORRELATION OF PHYSICAL FORCES. 



twisting or tearing asunder matter previously held together by 

 cohesive attraction, or in overcoming gravitation or inertia, 

 the same amount of heat or electricity would not apparently 

 be evolved as if such obstacle were non-existent, and the 

 initial force were wholly employed in producing, not in 

 opposing. There is great difficulty in devising experiments 

 in which some portion of the force is not so employed. 



The initial force, however, that has been employed for such 

 disruption is not lost, as at the moment of disruption the 

 bodies producing it fly off, and carry with them their force. 

 Thus, let two weights be attached to a cord placed across a 

 bar.; when their force is sufficient to break the cord or the 

 bar, the weights fall and strike the earth, making it vibrate, 

 and so conveying away or continuing the force which the 

 bar or cord had been acting on. If, instead of breaking a 

 cord, the weights be employed to bend a bar, their gravitating 

 force, instead of making the earth vibrate, produces heat in 

 the bar, and so with whatever other force may be employed 

 to produce effects of disruption, torsion, &c., so that, though 

 difficult in practice, the numerical problem of the equivalent 

 of the force is not theoretically irresolvable. 



The voltaic battery affords us the best means of ascertaining 

 the dynamic equivalents of different forces, and it is probable 

 that by its aid the best theoretical and practical results will 

 be ultimately attained. 



In investigating the relation of the different forces, I have 

 in turn taken each one as the initial force or starting-point, 

 and endeavoured to show how the force thus arbitrarily 

 selected could mediately or immediately produce and be 

 merged into the others ; but it will be obvious to those who 

 have attentively considered the subject, and brought their 

 minds into a general accordance with the views I have sub- 

 mitted to them, that no force can, strictly speaking, be initial, 

 as there must be some anterior force which produced it : we 

 cannot create force or motion any more than we can create 

 matter. 



Thus, to take an example previously noticed, and trace it 

 backwards : the spark of light is produced by electricity, 

 electricity by motion, and motion is produced by something 



