322 



NATURE 



\Feb. 21, 1873 



of which we still regard the motive tendency as similar to that 

 of other mechanical forces, becauSe it can maintain equilibrium 

 with them. 



If the natural office of force as a bond between space and 

 matter is to ward off contact between material points, and to 

 endow them with impenetrability (for this marked feature as far 

 as it has been explored appears to be inherent in all matter) by 

 absorbing at the proper places the energy of motion, and by curbing 

 and accelerating it elsewhere according to these dictates, it must be 

 implanted in material points in such permanent pairs as have 

 just now been described ; for the third law in Newton's master- 

 summary is often held (with how much correctness, perhaps, 

 may be questioned), to assert that all the forces of nature consist 

 of an action and a reaction in equal and opposite pairs, and be 

 omnipresent with a particle to protect it, its permanent impulse, 

 or rate of doing work, being at the same time referrible to the 

 space or distance between the mutually impinging, or colliding 

 pair of atoms. A mechanical system so constituted, as we have 

 seen, if not disturbed by the forces of foreign bodies outside of it 

 will have the sum of its actual and potential energies constant. 

 If we include those foreign bodies (endowing their forces at the 

 same time with persistency), and if we find that the whole 

 material universe as far as we can explore it admits of being 

 comprehended in one system of this kind, a mechanical explana- 

 tion of all known physical agencies might thus, apparently, 

 be arrived at. In every particular of the motion of any group 

 of bodies in it, however, except the single one of its total 

 energy, we would be obliged to abandon (as Newton does) 

 the local centres of reckoning of the several force pairs, and 

 betake ourselves to the mass-centre of the group as our 

 origin of reference for noting all the forces, and tracing all 

 the motions of a body completely in its wanderings through the 

 system. If this obligation, and the end to which it leads us of 

 referring force at last to perfectly abstract realms of space and 

 matter terrifies and affronts the scientific sense, it is sufficient 

 consolation to observe that if force were not ultimately so referred, 

 and if its impulse was exerted in those local spaces only in which 

 we find it acting apparently as a simple action and reaction, 

 there would be as many distinct kinds of energy of motion and 

 of configuration (which we would still retain as expressing the 

 Iccal laws of force) as there are local spaces, with countless com- 

 plex rules for mutual equivalence of these several energies con- 

 nected with the common path of the body in several of them 

 together, which would effectually defy even a modern physicist 

 to disentangle and employ ! We may rest thankfully contented 

 with the laws that Newton has traced out. But does the simple 

 mechanical system which we have just imagined really represent 

 that of nature? Are nature's force pairs really all permanent? 

 (We will suppose that they are all dual and reciprocal, for, as will 

 presently be noticed, a special and peculiar explanation only can 

 be given of forces which ?re absolutely external or solitary 

 without any physical qualification) ; and is the sum of their 

 mechanical energies a constant quantity, as we know that it 

 becomes when all the other kinds of energy in material nature 

 are added to it ? The answer is very obvious, but it betrays at 

 the same time a complete ignorance of the extent and depth 

 of the question that we put. The readily-preferred response is 

 *' No ; the sum of the mechanical energies, as far as they can be 

 recounted, is not constant, for numberless forces, and mechanical 

 energies proceeding from them are being constantly produced by 

 heat, radiation, chemical action, and other physical agencies, or 

 are being employed to renovate those agents with energy in 

 equivalent stores, but whose special kinds are not mechanical." 

 At the same time the progress of physical inquiry reveals to us 

 in the operations of these agents microscopical, or rather hyper- 

 microscopical, actions of force, and invisible charges of actual 

 energy, with which our earlier knowledge of these agencies was 

 entirely unacquainted, and a little step of inductive generalisation 

 only is needed (I believe that this view was unfolded by Helm- 

 holtz many years ago, but I have not been able to rediscover his 

 remarks and demonstrations^) to suppose that all physically- 

 generated forces form but visible and changing resultant-links in 

 an invisible chain of persistent forces, of which we hold some of 

 the most prominent loops in our hands, and thoughtfully wonder 

 what they are. 



In his Glasgow lecture on "Force" (in Nature, vol. xiv. p. 

 463), an important hint was offered by Prof. Tait regarding an 

 apparent character of potential energy, founded on the mathe- 

 matical condition that quantities whose units of measurement are 



' They are, I find, contained in a paper of some length in vol. \'i. of 

 " Taylor's Scientific Memoirs," 1853, pp. 114 — 162. 



of the same dimensions in space, mass, and time, are of the sama 

 kind. It follows from this that the potential energy of a force, 

 or the energy received from and transferred to it is of the same 

 kind as actual energy of material motion, because they are both 

 measured by the same combination of the units of space, time, 

 and mass. 



Let us first observe that it cannot be matter in motion which 

 constitutes the potential energy, unless this matter can traverse 

 itself and other matter freely (because, itself the cause of force, it 

 cannot itself experience any), and therefore that it is something 

 not matter, but both resident in and proportional to matter, and 

 also free ; and that its energy of motion as an occupant of matter 

 is actual, and in a state of freedom is potential energy. Imagine 

 a perfect reflection of the material universe to be formed by a 

 plane in space ; then the changes of energy of motion of the 

 reflected image of any mass particle taken negatively are equal 

 and opposite to the similar changes of energy of the panicle 

 itself, and would measure in a kinetic form the amount of the 

 work of force upon it ; but the tendency of energy of this form and 

 of the actual form to pass into each other would not be accounted 

 for. In our present knowledge of its transformations it scarcely 

 seems possible that a simpler picture of potential energy as a 

 form of energy of motion than this downright imitation of the 

 actually-existing motions could be reasonably offered. The pro- 

 position that force must be a process of transformation of a neio 

 energy of motion, so astonishingly complex, I confess staggered me, 

 and even led me to doubt il the simple laws of force and motion laid 

 down in the ' ' Principia " could be really so perfect and comiDlete 

 as they appear, amid the pell-mell of motions which the thought 

 suggests ! I began this letter shortly before going to Plymouth, 

 intending to recommend much more careful experiments than 

 even Mr. Crookes has carried out with the radiometer, and with 

 his recent, most effective form of the instrument, the otheoscope, 

 in order to test and examine the question of the laws of for^e 

 (especially with the idea of possibly isolating a single force) 

 seriously ; and though much induced to do so by the warm and 

 timely words of commendation passed on Mr. Crookes' labours 

 in his opening address to Section A at Plymouth (Nature, 

 vol. xvi. p. 314) by Prof. G. C. Foster, I have been unable 

 from other occupations to finish it until now. But I have 

 entirely abandoned my original intention, in great part, by reason 

 of a new light on the exceedingly abstruse and puzzling question 

 which, the able remarks by " X " in Nature (vol. xvi. pp. 438, 

 457) have afforded me about the real characters of force and of 

 potential energy. 

 Newcastle-on-Tyne A. S. IIekscuel 



( To be continued.) 



Cumulative Temperatures 



Under the above heading, among the "Meteorological 

 Notes " in your issue of last week, I notice the announcement 

 that "To simplify the difficulty of obtaining sums of tempera- 

 ture . . . M. von Sterneck has recently proposed to obtain these 

 indirectly by observation of the sums of actions produced by the 

 temperature." And that M. von Sterneck's proposal is to 

 employ for this purpose a pendulum clock in which the variation 

 of rate due to the raising or lowering of the centre of gravity of 

 the pendulum under variations of temperature is, through its 

 influence upon the daily error of time shown [on the dial, em- 

 ployed for determining the mean temperature of the air through- 

 out the twenty-four hours. And the notice concludes by saying 

 that M. von Sterneck has also proposed to apply the same 

 principle to determine the variations in atmospheric pressure 

 and in the intensity of magnetism. 



In reference to this subject it is only right to point out to the 

 readers of Nature (a term synonymous with the general body 

 of lovers of science all over the world) that the merit of these 

 suggestions is, by priority, due to one of our own countrymen, 

 Mr. W. F. Stanley, who, at the soiree given by the President of 

 the Royal Society as far back as April, 1876, exhibited two 

 instruments in which the chronometrical method of determining 

 thermometric and barometric averages was carried out with very 

 marked success. 



One of these to which the name chrono-thermometer was 

 applied, consisted of a clock, the pendulum of which was a 

 mercurial thermometer, its centre of gravity being raised or 

 lowered by the expansion or contraction of a column of mercury 

 under variations of temperature. In the other instrument, or 

 chrono-barometer, the pendulum consisted of a glass lube con. 



