June i, 1893J 



L , .,..,„. 



^^Ke all beside the point, for they all dealt with contacts between 



^^Wbodies," not between a body and an "ether" in which it was 



iP^oving. We must therefore begin de novo, and we must start 



this time with some definition or explanation of what he means 



respectively l>y " the bodies " and "the ether " which surrounds 



them. 



But the point I particularly wished to discuss was his view 

 of the " identity " of energy. I do not think any such identity 

 can be recognised, at any rate if we grant I'rof. Lodge's own 

 hypothesis that energy on being transferred from one body to 

 another is always transformed from Kinetic lo Potential energy, 

 or vice -jersn, for I maintain that potential energy, as such, be- 

 longs to a system of bodies not to any particular one of them, 

 and so has no local habitation even though it has a name. 



The law of the conservation of energy is usually expressed by 

 the formula — 



Kinetic + Potential Energy = Constant. 



But if this is to be a physical law, and not a mere truism, its terms 

 must be defined in such a way that it is not a mere formal conse- 

 quence of their definitions. As to Kinetic energy, everybody is 

 practically agreed in defining it as 'X\mv-, or, which is the same 



thing, as 2 / nmdv. But if we define potential energy, as 



Prof Lodge would apparently have us do, as 2 I —Ft/s, the 



formula does not assert a physical fact — at least no new one — 

 but is merely an identity. The equation of energy in this form 

 would, indeed, be quite useless, for we should have to know 



the previous path of each particle in order to evaluate / Fds. 



NATURE 



loi 



=/■ 



And so we find that in the equation of energy, as used by 

 mathematicians, ihe "Potential Energy" has nothing to do 

 with any paths the particles may have described, but is a mere 

 function of their present co-ordinates. 



The truth is that the physical fact implied in the law of con- 

 servation is not that the energy in genei'al is conserved through- 

 out all changes in the system, but merely that the kinetic 

 energy is always the same whenever the system returns lo the 

 same configuration ; that term being held, if necessary, to .in- 

 clude, not only geometical form, but such conditions as tem- 

 perature, chemical or electrical state, &c. 



The law of energy is then better stated thus : " In any inde- 

 pendent system of bodies — 



Kinetic energy -J- A function of the configuration of the 

 system = Constant." 



And we may, if we like, call this function " Potential 

 energy,' since it diminishes as the kinetic energy increases; 

 but we have no right to assume a priori that it is the same 

 sort of thing as kinetic ener-y. It is true that in some 

 cases what used to be called potential energy is now regarded 

 as in great part kinetic, but this can only be done if at the same 

 time we change our conception of the " configuration " of the 

 system. If we regard the energy stored in a reservoir of com- 

 pressed air as kinetic instead of potential, we must include the 

 average positions of its particles in our statement of the configur- 

 ation of the system. 



But the important conclusion to be drawn from this is that 

 potential energy (qua potential) does not belong to a single 

 panicle but to the system as a whole, or at least it can only be 

 allocated to such portions of the system as may by themselves 

 be regarded a< independent systems. If ever all energy were 

 explained to be kinetic energy, and if we could then explain 

 how it comes to be transferred from one body to another, we 

 might be able to trace the biography of apiece of energy as we 

 might that ofan atom of matter. But even if "potential energy" 

 may thus be regarded as only a name used to veil our present 

 ignorance of what has happened to the kinetic energy, it is still 

 illogical to talk of the " identity" of energy till this veil has been 

 removed. And I cannot see that anything Prof. Lodge has 

 said helps us in the smallest degree to remove it. 



Edward T. Dixo.n. 



Trinity College, Cambridge, May 27. 



On the Velocity of Propagation of Gravitation Effects. 



If, according to the accepted kinetic theory of gases, the 



velocities of molecules "vary between zero and infinity" 



NO. I 23 I, VOL. 48] 



(Maxwell): it must certainly result that frequently enormous 

 velocities are accidentally attained by even gross molecules, and 

 this produces no perceptible disturbance measured by us. It 

 would be admittedly almost puerile to ask how high a velocity 

 might normally be possessed by a large number of particles of 

 matter (as an (/ priori question, that is), provided the particles 

 be perfectly elastic, so that there is no jar at their encounters, 

 but the movement goes on with perfect smoothness, so that its 

 existence may escape detection by the senses. Moreover there 

 is no resistance in space to free motion of material particles. 



In regard to the effects of gravity then, the practical question 

 for us (in regard to their elucidation) becomes, What is the 

 velocity demanded for the transmission of gravity ? This 

 velocity, whatever it be (if very great, but finite), may then reason- 

 ably be considered to exist in matter in some form, or to be 

 posse>sed by it. To assert a priori that the existence (say among 

 particles of matter) of a velocity even many times that of light, 

 IS unlikely, or to view this with incredulity as an abstract fact 

 apart from its possible utility — would seem to partake somewhat 

 of the nature of a prejudice, due possibly to absence of adequate 

 reflection. 



A high normal velocity has the undoubted mechanical advantage 

 of being able to produce a given dynamical effect by means of 

 very small panicles, i.e., without demanding for such efl'ect any 

 large collective mass or the employment of a great quantity of 

 material. Smallness in size moreover allows the particles to 

 possess a very long mean path: and they have the advantage of 

 occupying, in toto, very little room (although they may be 

 relatively numerous). 



Without going into the question of the modus operandi of 

 such effects as explosions of gases, dynamite, &c., it at least 

 appears manifest that by the rejection of "action at a distance," 

 a store of motion of a very high intensity in the matter of space 

 would be consistent with, or would be demanded in order to 

 give some rational account of sudden developments or 

 transferences of motion. It may appear questionable whether 

 a normal velocity of matter in space only equal to that of light, 

 would be sufficient to account for the explosive violence of somt 

 transferences of motion. The rate of travel of light when viewed 

 in relation to the intervening distances of the chief bodies of 

 the universe — may appear even slow. More than three years, 

 lor instance, are occupied in the transmission of a wave from the 

 nearest star to our system. 



It may be reasonable then to assume that the possibilities for 

 the existence of a higher rate of intercommunication than this 

 (that of luminous efiecls) may exist in nature, and that the bodily 

 mass movements of the units of the universe may influence each 

 other more quickly than their molecular movements ; since 

 gravitational disturbances or their measures appear to demand 

 this. It is so far certain that in addition to the luminiferous 

 ether there may be plenty of room for finer and therefore more 

 mobile material : or no one, as far as I am aware, has urged a 

 diflSculty on this head, provided its presence were subservient to 

 some great mechanical purpose. 



Hamburg, May 16. S. Tolver Preston. 



Singular Swarms of Flies. 



It may interest some of your readers if I describe a sight 

 which I saw this forenoon, which was quite new to me and 

 apparently to all who witnessed it. Alter a brisk N.N.E. 

 breeze in the morning, at about u a.m. it fell flat calm, the sky 

 becoming inky black, with every sign of a heavy thunderstorm 

 impending. Soon after, looking out of my office window at a 

 belt of trees some hundred yards away, my eye was caught by 

 a most singular and to me (at first) uncanny sight. 



Above the trees, apparentlyoneon each principal prominence of 

 their outline, there appeared a numberof slim clouds, like straight 

 wreaths of thin smoke, slanting upwards into the sky. Though 

 they maintained their positions, they seemed alive and moving, 

 in a manner partly suggestive of the twisting motion of a water- 

 spout. A field-glass showed the clouds to be swarms of small 

 flies ; and looking around, similar swarms were seen above 

 all the trees everywhere. They were perfectly visible to 

 the n.iked eye a quarter of a mile off, and the glass showed 

 them on the furthest trees in sight, these being nearly a mile 

 away. All seemed to have much the same peculiar slant, 

 pointing more or less towards the (then invisible) sun. Some 

 of the swarms looked to be fifteen or twenty feet long. 



Over a few of the low bushes on a bank of rough ground 



