ON THE MECHANICAL EQUIVALENT OF HEAT 



365 



The interval between the and 100 points, as Welsh found it, was 

 180 -12, usinff barometer at 30 inches, or 180 -05 as corrected to 

 760 mm. of mercury. 8 At the present time it is 179 -68,* showing a 

 change of 1 part in 486 in twenty-five years. This fact shows that 

 the ordinary method of correcting for change of zero is not correct, and 

 that the coefficient of expansion of glass changes with time. 10 



I have not been able to find any reference to the kind of glass used 

 in this thermometer. But in a report by Mr. Welsh we find a com- 



TABLE VI. COMPARISON BY WELSH, 1852. 



parison, made on March 19, 1852, of some of his thermometers with 

 two other thermometers, one by Fastre, examined and approved by 

 Eegnault, and the other by Troughton and Simms. The thermometer 

 which I used was made a little more than a year after this; and it is 



8 Boiling point, "Welsh, Aug. 17, 1853, 212 -17; barometer 30 in. 

 Freezing point, " " " 32 -05. 



Boiling point, Rowland, June 22, 1878, 212 -46; barometer 760 mm. 

 Freezing point, " " 32-78. 



The freezing point was taken before the boiling point in either case. 

 9 179 -70, as determined again in January, 1879. 



10 The increase shown here is 1 in 80 nearly ! It is evidently connected with the 

 change of zero ; for when glass has been heated to 100, the mean coefficient of ex- 

 pansion between and 100 often changes as much as 1 in 50. Hence it is not 

 strange that it should change 1 in 80 in twenty-five years. I believe this fact has 

 been noticed in the case of standards of length. 



