482 Mr. J. P. Joule and Prof. Thomson on the Thermal Effects 



tested either experimentally or theoretically. The experiments 

 described in the present commimication were commenced by the 

 authors jointly in Manchester last May. The results which have 

 been already obtained, although they appear to establish beyond 

 doubt a very considerable discrepancy from IMayer's hypothesis 

 for temperatures from 40^ to 170° Fahr., arc far from satisfac- 

 tory ; but as the authors are convinced that, without apparatus 

 on a much larger scale, and a much more ample source of me- 

 chanical work than has hitherto been available to them, they 

 could not get as complete and accurate results as are to be 

 desired, they think it right at present to publish an account 

 of the progress they have made in the inquiry. 



The following brief statement of the proposed method, and the 

 principles on which it is founded, is drawn from §§ 77, 78 of 

 Part IV. of the series of articles on the Dynamical Theory of 

 Heat republished in this Magazine from the Transactions of the 

 Royal Society of Edinburgh in 1851* (vol. xx. part 3. pp. 296, 

 297). 



Let air be forced continuously and as uniformly as possible, 

 by means of a forcing-pump, through a long tube, open to the 

 atmosphere at the far end, and nearly stopped in one place so as 

 to leave, for a short space, only an extremely narrow passage, on 

 each side of which, and in every other part of the tube, the passage 

 is comparatively very wide ; and let us suppose, first, that the air 

 in rushing through the narrow passage is not allowed to gain any 

 heat from, nor (if it had any tendency to do so) to part with any to, 

 the surrounding matter. Then, if Mayer's hypothesis were true, 

 the air after leaving the narrow passage would have exactly the 

 same temperature as it had before reaching it. If, on the contrary, 

 the air experiences either a coohng or a heating effect in the cir- 

 cumstances, we may infer that the heat produced by the fluid fric- 

 tion in the rapids, or, which is the same, the thermal equivalent 

 of the work done by the air in expanding from its state of high 

 pressure on one side of the narrow passage to the state of atmo- 

 spheric pressm-e which it has after passing the rapids, is in one 

 case less, and in the other more, than sufficient to compensate 

 the cold due to the expansion ; and the hypothesis in question 

 would be disproved. 



The apparatus consisted principally of a forcing-pump of 

 lOA inches stroke and If internal diameter, worked by a 

 hand-lever, and adapted to pump air, through a strong copper 

 vessel t of 136 cubic inches capacity, (used for the purpose of 



* See also Dynamical Theory of Heat, part 5. Trans. Roy. Soc. Edinb. 

 1852. 



t This and the forcing-pump are parts of the apparatus used by Mr. 

 Joule in his original experiments on air. See Phil. Mag. S. 3. vol. xxvi. 

 p. 370 (1845). 



