and the Conservation of Energy. 135 



of motion and contact-action, and (/;) to deduce, not a law of 

 the conservation of energy, but of its conservation during 

 transference merely. 



(a) The unacknowledged assumption is made in the " de- 

 finition " of energy :■ — " Whenever work is done upon a body, 

 an effect is produced in it which is found to increase the 

 working-power of that body (by an amount not greater than 

 the work done) ; hence this effect is called energy, and it is 

 measured by the quantity of work done in producing it. 

 Whenever work is done by a body, i. e. anti-work done on it, 

 its working-power is found to be diminished (to at least the 

 extent of the work done), and it is said to have lost energy — 

 the energy lost being measured, as before, by the anti-work 

 done in destroying it." The words "is found " indicate an 

 appeal to experience. We may readily recognize what it is, 

 if we note that the increment or decrement of working-power 

 which is produced in a body on or by which work has been 

 done, may be kinetic or potential, and that, as Prof. Lodge 

 says"*, "for a body to possess kinetic energy you must have 

 not merely motion, you must have a guarantee of persistence 

 of motion, the body must possess inertia," and '" for a body to 

 possess potential energy we must have two things — the exer- 

 tion of a force, together with a guarantee that that force shall 

 be exerted over a certain distance ; i. e. a continuance of the 

 force even after motion is permitted." If, then, these two 

 guarantees be expressed quantitatively, so as to ensure the 

 equality of the change of working-power to the work done, 

 they will form a statement of the experience to which 

 appeal is made. When so expressed, the former is seen to be 

 Newton's second law of motion, and the latter the axiom that 

 the work done by the mutual forces between the parts of a 

 material system during any change of its configuration de- 

 pends only on the initial and final configurations. If these 

 guarantees cannot be deduced from the third law and universal 

 contact-action, they are thus unacknowledged assumptions 

 in the argument under consideration. 



We may assume that Prof. Lodge will not hold it to be 

 possible to make this deduction in the case of the latter of 

 the two assumptions mentioned. He does hold, however, 

 that Newton's second and third laws of motion are different 

 aspects of one law f , and he may therefore regard the first of 

 the two guarantees mentioned above as not forming an addi- 

 tional assumption in his argument. I have not access to k The 

 Engineer' of 1885, in which he says his argument in support 



* Phil, Mag. [5] vol. xix. (1885) p. 485. 

 t Ibid. vol. xix. (1885) p. 483. 



