Osr THE MECHANICAL EQUIVALENT or HEAT 



357 



has made the determination for common glass between and 100. 

 I have found approximately the constants for my formula in this case, 

 and have calculated the values in the fourth column of the following 

 table. 



TABLE IV. RECKNAGEL'S RESULTS COMPARED WITH THE FORMULA. 



J=290, a = .000 000 33, 



It will be seen that the values of the constants are not very different 

 from those which satisfy Eegnault's experiments. 



There seems to be no doubt, from all the experiments we have now 

 discussed, that the point of maximum difference is not at 50, but at 

 some less temperature, as 40 to 45, and this agrees with my own 

 experiments, and a recent statement by Ellis in the Philosophical 

 Magazine. And I think the discussion has proved beyond doubt that 

 the formula is sufficiently accurate to express the difference of the 

 mercurial and air thermometers throughout at least a range of 200, 

 and hence is probably very accurate for the range of only 100 between 

 and 100. 



Hence it is only necessary to find the constants for my thermometers. 

 But before doing this it will be well to see how exact the comparison 

 must be. As the thermometers are to be used in a calorimetric research 

 in which differences of temperature enter, the error of the mercurial 

 compared with the air thermometer will be 



= a \U' 2 (J + 



