The Rev. J. M. Heath on the Principles of Thermodynamics. 429 



as the imperfections arising from chromatic aberration are alto- 

 gether absent, owing to the homogeneous quality of the light of 

 the protuberances ; and hence, as I have convinced myself by 

 repeated experiment, non-achromatic lenses, when suitably se- 

 lected, may be employed without hesitation for arrangements of 

 the nature here contemplated. 



This extremely compact form of instruments suitable for ob- 

 serving the solar protuberances admits of a delicate motion 

 being given to them with facility by clockwork, and holds out 

 the prospect of our seeing realized by these simple means very 

 shortly the idea already broached by me in my former paper, of 

 obtaining an artificial solar eclipse, of any duration desired, for 

 the simultaneous observation of all the protuberances situated on 

 the edge of the sun. 



Leipzic, August 26, 1869. 



LII. On the Principles of Thermodynamics. 

 By the Rev. J. M. Heath*. 



I HAVE to apologize to Mr. Rankine for attributing to him 

 an admission which it appears he never intended to make. 

 I understood him to mean that the period in which the best and 

 original writers on thermodynamics had been careful observers 

 of the true principles of the science was to be dated from the 

 time when the revived theory of molecular vibration in gases 

 had superseded the older one of centres of repulsive force ; and 

 I believed that this event did not happen until long after the 

 first speculations in thermodynamics. But since Mr. Rankine 

 disclaims having made any such admission at all in any shape, 

 I of course acknowledge my mistake and beg to withdraw the 

 assertion. But this point is not very material to the main pur- 

 pose of the argument of this discussion. The new position from 

 which we now start is this. Mr. Rankine contends that all good 

 writers on thermodynamics, up to the very earliest of them, have 

 been reasoning correctly and from properly assumed premisses ; 

 whereas my complaint is that all those same writers, down to 

 the very latest of them, have been and are reasoning from pre- 

 misses improperly assumed at the beginning. To be of any use, 

 therefore, this discussion must now be turned to the considera- 

 tion of what these assumptions are, whether or not they have 

 been justifiable, and, if not, what others ought to have been as- 

 sumed in place of them. 



Mr. Rankine has stated it to be the business of the therm o- 



* Communicated by the Author. 



