889 



skeleton is highly developed, the verfcebrfje appear to have the double 

 concave articulation common amongst fish and enaliosaurs. 



The author, in conclusion, acknowledges his obligations to Sir 

 Philip M. de Grey Egerton, M.P., Dr. Mantell, Mr. Binney, Mr. 

 J. E. Gray and Mr. Searles Wood, for their valuable co-operation 

 in supplying many important specimens for examination. 



4. " On the Mechanical Equivalent of Heat." By J. P. Joule, 

 Cor. Associate R. Acad. Sciences, Turin, &c. Communicated by 

 M. Faraday, D.C.L., F.R.S., Foreign Memb. Acad, of Sciences, 

 Paris, &c. 



After passing in review the experimental researches of Rumford, 

 Davy, Dulong, Faraday, and others who have successively discovered 

 facts tending to prove that heat is not a substance, but a mode of 

 force, the author mentions the papers he has already communicated 

 to the Royal Society, and published in the Philosophical Magazine, 

 in which he has endeavoured to show that in the production of heat 

 by the expenditure of force, and vice versa, in the production of 

 force by the expenditure of heat, a constant relation always subsists 

 between the two. This relation he denominates the " Mechanical 

 Equivalent of Heat," and the object of the present paper is to advance 

 fresh proofs of its existence, and to give to it the numerical accuracy 

 requisite to fit it as a starting-point for further inquiries. 



In carrying out the above design, the author has determined the 

 relation of work done to heat produced in the cases of the friction, — 

 1st, of water; 2nd, of mercury ; and 3rd, of cast iron. 



In the experiments on the friction of the fluids, the liquid was 

 contained in a covered cylindrical vessel of copper or iron, and the 

 agitation was effected by vanes of brass or iron, fixed to a vertical 

 axis revolving in the centre of the vessel, whilst fixed vanes pre- 

 vented the liquid being whirled in the direction of rotation. In the 

 experiments on the friction of solids, a disc of cast iron was rotated 

 against another disc of cast iron pressed against it ; the whole being 

 immersed in a cast-iron vessel filled with mercury. 



The force expended was measured by the descent of the weights 

 employed in rotating the apparatus ; and great care was taken to 

 correct it for the friction of the axes of the puUies employed, &c. 



The heat evolved by the friction was measured by exact thermo- 

 meters, and very laborious precautions were taken in order to elimi- 

 nate the effects of radiation or conduction of heat to and from the 

 surrounding atmosphere. The corrected thermometric effect was 

 then reduced to a known capacity for heat, by means of extensive 

 series of experiments made in order to ascertain the specific heat of 

 the materials in which the thermometric effect was observed. 



In this way the number of units of work, estimated in pounds one 

 foot high, required to .be done in order to develope one degree Fahr. 

 in one pound of water taken at about 50°, was found to be as 

 follows : — 



772'692 from friction of water, a mean of 40 experiments. 

 774"083 from friction of mercury, a mean of 50 experiments. 

 774*987 from friction of cast iron, a mean of 20 experiments. 



