ON THE MECHANICAL EQUIVALENT OF HEAT 445 



about seven revolutions of the chronograph, and consequently seven 

 readings of the torsion circle, each one of which was the average for a 

 little time as estimated by the eye. 



I have obtained more than thirty series of results, but have thus far 

 reduced only fourteen, five of which are preliminary, or were made with 

 the simple jacket instead of the water jacket, the radiation to which 

 was much greater, as there was a hole at the bottom which allowed more 

 circulation of the air. The mean of the preliminary results agrees so 

 closely with the mean of the final results, that I have in the end given 

 them equal weight. 



On March 24th, the same thermometer was used for a second experi- 

 ment directly after the first, seeing that the chronograph failed to work 

 in the first experiment until 8 was reached. The error from this cause 

 was small, as the first experiment only reached to 26 C., and hence 

 there could have been no change of zero, as this is very nearly the tem- 

 perature at which the thermometer was generally kept. 



Having thus calculated the work in conjunction with the tempera- 

 ture, I have next interpolated so as to obtain the work at the even de- 

 grees. The tables so formed I have combined in two ways : first, I have 

 added to the column of work in each table an arbitrary number, such as 

 to make the work at 20 about 10,000, and have then combined them as 

 seen in Table LI, and, secondly, I have subtracted each number from 

 the one 10 farther down the table, and divided the numbers so found 

 by 10, thus obtaining the mechanical equivalent of heat. 



In these tables four thermometers have been used, and yet they were 

 so accurate that little difference can be observed in the experiments 

 which can be traced to an error of the thermometer, although the Kew 

 standard has some local irregularities. The greatest difference between 

 any column of Table LI and the general mean is only 10 kilogramme- 

 metres, or 0-023 degree, and this includes all errors of calibration of 

 thermometers, radiation, &c. This seems to me to be a very remarkable 

 result, and demonstrates the surpassing accuracy of the method. In- 

 deed, the limit of accuracy in thermometry is the only limit which we 

 can at present give to this method of experiment. Hence the large 

 proportional time spent on that subject. 



The accuracy of the radiation is demonstrated, to some extent, by 

 the agreement of the results obtained even with different temperatures 

 of the jacket. But on close observation it seems apparent that the 

 coefficients of radiation should be further increased as there is a ten- 

 dency of the end figures in each series to become too high. This is 



