250 Mr. W. J. M. Rankine on the Conservation of Energy. 



racy, still it represents the essential distribution of beat in the 

 diffi'action-spectrum. Draper's assertion, that the maximuni 

 heat falls in the yellow, is certainly thereby supported ; but it 

 by no means follows thence, that "the distribution of heat corre- 

 sponds to the distribution of light" as Draper asserts. Draper, 

 indeed, entirely ignores the dark portion of the thermal spec- 

 trum, appearing to imagine that the intensity of heat, like that 

 of light, diminishes equally towards the violet and red bounda- 

 ries of the visible spectrum. 



Because, in the refraction-spectrum, the cm-ve of luminous 

 intensity differs so entirely from the course of the thermal 

 curve, the two curves cannot coincide in the diffraction-spec- 

 trum, although in the latter the luminous and thermal maxima 

 approach one another incomparably more closely than in the 

 refraction-spectrum. 



We see from fig. 4, p. 249, that the dark thermal rays in 

 the diffraction-spectrum occupy a space which is about 3^ times 

 as broad as the whole visible spectrum. In the diffraction image 

 formed by a grid*, the dark part of the first thermal spectrum 

 extends accordingly on both sides as far as the violet of the 

 third light spectrum, that is, from R to V. Hence almost the 

 whole of the second light spectrum is covered by the dark 

 thermal rays of the first spectrum, — a circumstance which, put- 

 ting aside other difficulties, renders it impossible to trace directly 

 the curve of intensity of a single diffraction-spectrum. 



XXXVII. On the Consei-vation of Energy. 

 By W. J. Macquorn Rankine, C.E., LL.D., F.R.S. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



THE extract from a recent work of Dr. Paraday, which you 

 have reprinted in your Magazine for March, has suggested 

 to me the following remarks : — 



1. It is certain that no law of "conservation" is applicable 

 to the tendency of a body to change its place, nor to any mere 

 tendency whatsoever. 



2. The quantity whose amount is '^ conserved" during all the 

 mutual actions amongst a system of bodies, is always the product 

 of two factors ; and when one of those factors is the magnitude 

 of a tendency towards change of a particular kind, the other factor 

 is the magnitude of the change throughout which that tendency is 

 capable of continuing to act. 



When the magnitude of the tendency is variable, it is of 



* See my ' Physics,' ed. 6, vol. i. p. 622, fig. 699. 



