Sept. 27, 1877] 



NA TURE 



459 



live force, or tension, or stress, tliey are driven to thrust it into 

 the body, notwithstanding tlie perplexities and contradictions 

 caused by so doing, and notwithstanding the painful necessity 

 incurred thereby of hoodwinking poor P. in the above manner, 

 and endeavi!uring to hoodwink the metsphysiciar.s. But there 

 is really no reason why force with the saving reservation should 

 not be introduced as freely into the discu-sion of E. as into 

 other questions of dynamics ; and the physicists often do intro- 

 duce it thereinto ; but then, when frightened at what they have 

 done, they will silently withdraw it again. All the inconsistencies 

 of the doctors, and their capriciously varying moods of freedom 

 and shyness respecting " force," and their stepping up and down 

 from one platform of thought to another, perplex poor P. beyond 

 measure. He knows nothing but what they tell him ; and he 

 dares not attribute his difficulties to anyihing but either the 

 abstruseness of the subject or his own stupidity. 



But probably 'here was another motive, also, for this melan- 

 choly idea of putting the E. of position into the body, viz , the 

 desire for simplicity of arrangement Since E. is E. .and the 

 kinetic E. is undeniably in the body, it would seem to be an 

 orderly proceeding to put the other there too. But this would 

 be as if a methodical housekeeper should keep her coals and her 

 blankets in the same **h()le'' because they are both warming 

 apparatus, though in very different ways. And besides, we 

 shall find that whatever may be the gain in this respect in putting 

 both the types of E. into the body, it is outweighed by a certain 

 loss of true correspondence and clear analogy, which will be 

 mentioned farther on. 



We now come to the more direct consideration of the merits 

 of this procedure of putting the E. of position into the body. 

 Let us begin with an interesting little illustration of its character. 

 It is the ordinary and legitimate mode of expression to say that 

 when a stone is projected vertically upwards, the gravitational 

 attraction between the earth and stone draws the stone down again 

 and gives it the kinetic E. with which it strikes the earth. And the 

 gravitation attraction is usually and conveniently conceived and 

 spoken of as being all the earth's : and the stone is usually 

 regarded as being simply attracti-i/. Every doctor will frequently 

 speak thus ; and nevertheless he will also, and sometimes in the 

 same breath,' tell us that it is the stone, say at the highest point 

 of its ascent, that has E. of position due to its height from the 

 ground. So then the connecting attractive force, which is to do 

 the work of drawing the stone down again, and which is therefore 

 one factor of the E. present, is regarded as being in the earth, 

 but the E. a.s being in the stone ! This is one way no doubt of 

 teaching poor P. the difference between force and E ! Take 

 another illustration. Some of our foremost doctors " tell us that 

 when a bow is drawn and about to discharge the arrow or the 

 bolt it is the arrow or the bolt that has E. of position ; in this 

 they have at least the merit of consistency. Poor P. generally 

 feels that this conveys no distinct idea at all to his mind ; of 

 course he dares not thing it wrong. Then he finds other doctors ' 

 who tell him (though in so doing they are inconsistent '' with 

 themselves) that in this case the E. is in the bow. What is to 

 be done now? Is this distracting E. of position "like a bird so 

 that it can be both here and there at the same time " ? Or are the 

 doctors on one side — how shall we write it — wrong ? At any 

 rate, since the doctors differ, poor P. must needs choose for 

 himself, and in order to escape the above perplexities and also 

 for the following reasons, he elects to conceive of the E. of 

 position as net in the body but in the force or forces concerned 

 w'liich are at least virtually there ; it being an ulterior and quite 

 another question, what is force ? 



The discussion is of course now, as it has been all along, only 

 as to modes of conception or of expression, and not as to the 

 science of our doctors. All agree that if you spend E. against 

 the resistance of the inertia of a mass in giving it velocity or 

 acceleration, you have bestowed your E. on the inertia of that 

 body, you have transferred your E. to that inertia. So, in exact 

 correspondence and analogy, if you spend E. against the resist- 

 ance of the gravitation attraction, for instance, in raising a stone 

 to a certain height you have bestowed your E. on that 

 ' Clerk Maxwell's "Heat," p. 281 bottom; seeWillson's "Dynamics," 



= E.g. Balfour Stewart, " Cons, of E.," p. 25 (but see his " Elem. Phys.," 

 p. 106). 



3 Tait, "Recent Advances," p. 18; Willson, "Dynamics, p. 278. 



< The inconsistency is startlingly exh bited in a single sentence or which 

 two doctr rs are responsible, " Uns. Univ.." p iii, "the potential E of a 

 raised weight or bent sprii g." If the potential E. is in either one of these it 

 cannot be in the other. W e have the same in a single sentence in Thomson 

 and Tait, p. 178 (two doctors, again, responsible) ; also in Tait's Glasgow 

 lecture. 



attraction, you have transferred your E. to gravity. That 

 attraction was beforehand pulling at the stone as hard as it 

 could ; but it had no power of doing work, according to the 

 definition of work, i.e., it had no energy according to the defini- 

 tion of E. You have given it E., or the power of performing 

 work by affording it the condition necessary for its doing work, 

 viz., space to work through. Why will not the doctors say this 

 in so many words, when they do say it virtually in various forms? 

 From Newton down they tell us this, that the work done by a 

 force is/.t [s being the space through which the force / acts) • 

 but the work done is the measure of the preceding E. or power 

 which of course the force had of doing that work ; why then 

 will they scarcely ever say that the E. of a force is fs {s being 

 now the space through which he force will have opportunity of 

 acting) ? When they do say in substance what we want them to 

 say, they avoid most carefully the direct clear statement of it in 

 so many words.i "This kind of E. [potential] depends upon 

 the work which the forces of the system would do if the parts of 

 the system were to yield to the action of those forces." That, 

 of course, ra^m\s precisely the same as the following, which, how- 

 ever, expresses the thing more directly. This kind of E. 

 (potential) is the E. which the forces of the system possess in 

 consequence of the possible displacements of the parts of the 

 system under the action of those forces. Tait himself, both in his 

 Glasgow lecture and in his " Recent .Advances," tells us that 

 a wound up spring or bent bow has potential E. Clerk Maxwell 

 tells us the same. If so we have a right to speak of the energy 

 of the gravitation attraction. In a certain respect the cases are 

 difTerent, but not so as to affect the present point. 



This, our putting of the E. of position into the forces, instead 

 of into the body or bodies, does not, of course, explain the action 

 any more than the other does, but it gives a conception (pro- 

 visional, if you like) which is much clearer and in better 

 analogy, and, as we have said, free from all the above-recounted 

 confusions. Moreover, the expression "E. of a force " has the 

 great advantage of keeping before the mind of poor P. the fact 

 that force and energy are not the same, a distinction which he is 

 slow to apprehend, and which it is of the utmost importance to 

 him that he should get proper hold of. 



And now that we have got our E. of position into its most 

 convenient seat, what shall we call it, and how shall we speak 

 of its action? We cannot be dreadfully wrong if we call it by 

 a name suggested by an expression of Helmholtz ; let it be 

 " Emrgy o; Tension." Does it not seem more logical to desig- 

 nate it by its essential characteristic than by what is only a 

 condition though an indispensable one ; for this latter we do when 

 we call it E. of position or configuration. And as to its action let 

 us say that when E. is being, as it is usually expressed, transformed 

 frdm potential to actual E., ox -nee versA, it is transferred from 

 the forces to the bodies of the system, or vice versA. If these 

 expressions are unsuitable and erroneous, then let every one abstain 

 from language which is precisely tantamount to them. But our 

 doctors do not do this ; and it fortifies us greatly in the belief 

 that we are right to know that our doctors, when they are quite 

 themselves, say the very same in substance, though not in so 

 many words. On the other hand, if these expressions recom- 

 mend themselves to us, let us use them boldly and consistently 

 without mincing matters. Deschanel seems to have been on 

 the point of using them in one place.' However, the fear of his 

 conjrires suddenly rose before bis eyes, and having written (or 

 his translator for him) the word " transferred," he stops short 

 without telling us frsm what and to what the transference is 

 made ; he leaves us to complete for ourselves the sense of the 

 passage, which clearly is that the transference is from the forces 

 to the bodies, and vice versS. 



Poor Publius and myself have smeral other complaints to 

 make ; but probably we have said enough to excite the sympathy 

 of all considerate persons. 



Dublin X- 



New Electric Lights 



Under the above title Mr. Munro describes, in Nature, 

 vol. xvi. p. 422, M. Lodighin's device for an electric light. 

 This is 110 novelty but a simple repetition of an invention made 



■ The only exception tbat I remember to have seen is afforded curiously 

 enouBh, by Rnnkine himself, the inventor of E m posse. In Ph.. A/a^-., 

 Februa^, .853. he say.,, " E. of gravitation ; and in Encycl. Brit., 

 vol. xiv., " Mechanics," he speaks ol the E. of an etfort. 



= '• Nat. Phil ," p. -9 



