252 Prof. J. G. MacGregor on the 



incompatibility of action at a distance (1) with the law of 

 the conservation of energy and the third law of motion, and 

 (2) with the former law alone, and to be able to prove its 

 incompatibility with the third law alone. In my paper 

 (p. 139) I showed that the incompatibility w r ith the law of 

 conservation and the third law had not been proved, and that 

 the incompatibility with his law and the third law was a 

 necessary consequence of his definitions. I showed also that 

 the argument by which he had sought to prove the incom- 

 patibility with the law of conservation alone (expressed in the 

 form of the impossibility of the perpetual motion) was not 

 sound. I also called attention to the fact that no attempt to 

 substantiate the third of the above claims, a very important 

 one, had been made. In the present paper he makes no 

 reference to all this, but he makes a dogmatic statement 

 which appears at first sight to be a reiteration of the second 

 claim mentioned above, viz., u Energy is only really con- 

 served under conditions of universal contact-action " (p. 16). 

 Prof. Lodge is obviously under the impression that in making 

 this statement he is taking up a position which is opposed to 

 that of all his " prehistoric " dynamical brethren. In reality, 

 however, any one who holds to the ordinary law of conser- 

 vation and is able to regard action at constant distance as 

 u practically "" contact-action, must admit it ; for, subject to 

 the last proviso, it is an obvious deduction from the ordinary 

 law. We may express that law as follows : — In any isolated 

 system of bodies, the sum of their kinetic working-power, 

 the working-power they may possess because of their being 

 individually in a state of strain, and the working-power of 

 the system due to actions between the bodies at variable 

 distance, must be constant. Now the first two kinds of 

 working-power constitute what Prof. Lodge calls energy, 

 and the third must vary with the configuration of the system. 

 Hence Prof. Lodge's energy can be conserved only subject to 

 the condition of action at constant distance. If, therefore, 

 action at constant distance be admitted to be u practically " 

 contact-action, Prof. Lodge's statement is seen to be an 

 immediate deduction from, and therefore merely a particular 

 case of, the ordinary law of conservation. 



(6) Relative generality and precision of Prof. Lodge's Law 

 of Conservation and the ordinary law; localization of 

 Potential Energy ; " Identity " of Energy. 



Prof. Lodge makes no reference to the argument by which 

 I sought to prove that his law of the conservation of energy 



