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XXX. On the Mechanical Equivalent of Heat^ as determined 

 by the Heat evolved by the Friction of Fluids. By J. P. 

 Joule, Secretary to the Literary and Philosophical Society 

 of Manchester *, 



IN the Philosophical Magazine for September 1845 I gave 

 a concise account of some experiments brought before the 

 Cambridge Meeting of the British Association, by which I 

 had proved that heat was generated by the friction of water 

 produced by the motion of a horizontal paddle-wheel. These 

 experiments, though abundantly sufficient to establish the 

 equivalency of heat to mechanical power, were not adapted to 

 determine the equivalent with very great numerical accuracy, 

 owing to the apparatus having been situated in the open air, 

 and having been in consequence liable to great cooling or 

 heating effects from the atmosphere. I have now repeated 

 the experiments under more favourable circumstances, and 

 with a more exact apparatus, and have moreover employed 

 sperm oil as well as water with equal success. 



The brass paddle-wheel employed had, as described in my 

 former paper, a brass framework attached, which presented 

 sufficient resistance to the liquid to prevent the latter being 

 whirled round. In this way the resistance presented by the 

 liquid to the paddle was rendered very considerable, although 

 no splashing was occasioned. The can employed was of cop- 

 per, surrounded by a very thin casing of tin. It was covered 

 with a tin lid, having a capacious hole in its centre for the axle 

 of the paddle, and another for the insertion of a delicate ther- 

 mometer. Motion was communicated to the paddle by means 

 of a drum fitting to the axle, upon which a quantity of twine 

 had been wound, so as by the intervention of delicate pulleys 

 to raise two weights, each of 29 lbs., to the height of about 5^ 

 feet. When the weights in moving the paddle had descended 

 through that space, the drum was removed, the weights wound 

 up again, and the operation repeated. After this had been 

 done twenty times, the increase of the temperature of liquid 

 was ascertained. In the second column of the following 

 table the whole distance through which the weights descended 

 during the several experiments is given in inches. I may 

 observe also that both the experiments on the friction of water, 

 and the interpolations made in order to ascertain the effect of 

 the surrounding atmosphere, were conducted under similar 

 circumstances, each occupying forty minutes. 



* Read before the Mathematical and Physical Section of the British 

 Association at Oxford, and communicated by the Author. 



