TRANSACTIONS OP THE SECTIONS. 45 



On the Temperature-equilibriwm of an Enclosure in which there is a Body in 

 Visible Motion. By Prof. Balfour Stewaet, F.B.S. 

 It is now several years since Professor Tait and the author of this paper came 

 jointly to entertain the belief that there is some transmutation of energy, the 

 exact nature of which is unknown, when large bodies approach or recede from one 

 another. It is desirable to vindicate an idea of this nature, both from the theo- 

 retical and the practical point of view — that is to say, we ought, if possible, to 

 exhibit it as a probable deduction from those laws of nature with which we are 

 ali'eady acquainted ; and, on the other hand, it ought to be supported by observa- 

 tions and experiments of a new kind. In om* case the experiments and observations 

 have been of a difficult nature, and are yet in progress ; it is therefore premature to 

 bring them before the notice of the Association. A theoretical vindication of the 

 idea has been obtained by Professor Tait, and more recently one has occurred to 

 the author of these remarks, which he now ventures to bring forward. Men 

 of science are now sufficiently well acquainted with Provost's theory of exchanges, 

 and its recent extension. We know that in an enclosure, the walls of which are 

 kept at a constant temperature, every substance will ultimately attain the very- 

 same temperature as these walls, and we know also that this temperature-equili- 

 brium can only be brought about by the absorption of every particle being exactly 

 equal to its radiation, an equality which must separately hold for every individual 

 kind of heat which the enclosure radiates. This theoretical conclusion is sup- 

 ported by numerous experiments, and one of its most important applications has 

 been the analysis of the heavenly bodies by means of the spectroscope. Let us 

 now suppose that in such an enclosure we have a body in visible motion, its tem- 

 perature, however, being precisely the same as that of the walls of the enclosure. 

 Had the body been at rest, we know from the theory of exchanges that there 

 would have been a perfect equilibrium of temperature between the enclosure and 

 the body ; but there is reason to believe that this state of temperature-equilibrium 

 is broken by the motion of the body. For we know both from theory and expe- 

 riment that if a body, such for instance as a star, be either rapidly approacliing the 

 eye of an observer or receding from it, the rays from the body which strike the eye 

 will no longer be precisely the same as would have struck it had the body been at 

 the same temperature and at rest — ^just as the whistle of a railway engine rapidly 

 approaching an observer will have to him a different note from that which it 

 would have had if the engine had been at rest. The body at motion in the 

 enclosure is not therefore giving the enclosure those precise rays which it would 

 have given it had it been at the same temperature and at rest ; on the other hand, 

 the rays which are leaving the enclosure are unaltered. The enclosure is there- 

 fore receiving one set of rays and giving out another, the consequence of which 

 will be a want of temperature-equilibrium in the enclosure, in other words, all 

 the various particles of the enclosure will not be of the same temperature. Now, 

 what is the consequence of this ? The consequence will be that we can use these 

 particles of different temperatm-e so as to transmute part of their heat into the 

 energy of visible motion, just as we do in a steam-engine ; and if it is allowable to 

 suppose that during this process the moving body has retained all its energy of 

 motion, the result will be an increase of the amount of visible energy within the 

 enclosure, all the particles of which were originally of the same temperature. But 

 Sir W. Thomson has shown us that this is impossible ; in other words, we cannot 

 imagine an increase of the visible energy of such an enclosure imless we acknow- 

 ledge the possibility of a perjoetual motion. It is not, therefore, allowable to sup- 

 pose that in such an enclosure the moving body continues to retain all its energy 

 of motion, and consequently such a body will have its energy of motion gi-adually 

 stopped. Evidently in this argument the use of the enclosure has been to enable 

 us to deduce our proof from the known laws of heat and energy, and we may alter 

 the shape of the body without affecting the result ; in other words, we should 

 expect some loss of visible energy in the case of cosmical bodies approaching or 

 receding from one another. 



On a new Steam-gauge. By Prof. Ch. V. Zengtie. 

 This gauge is intended to avoid the defects of common air-gatiges, which have 



