the late Dr. Joule's Thermometers. 489 



would have taken place, and owing to the sticking of the 

 thread the first few observations would have been uncertain. 



Table IV. 

 y. , Fall in divisions per cm. 



a e * of mercury pressure. 



April 28, 1892 '0564 



» » '0567 



Feb. 22, 1894 -0563 



•0559 

 •0548 



Mean -0560 



Mean excluding last number . *0563 



The observation which is quoted in full above is the one 

 which shows greater irregularities in the rise than the others, 

 and it gave the result (*0548) which differs most from the 

 mean. Part of the discrepancies between the different ob- 

 servations is no doubt due to the irregularities in the 

 graduation of this thermometer, but the result is sufficiently 

 accurate for the purpose for which it is intended. 



Reduced to degrees, the final results are as follows : — 



In degrees per cm. 

 of mercury. 



Coefficient of external pressure .... 0*002400 



„ internal „ .... 0*002554 



Knowing the distance of any scale-division from the centre 

 of the reservoir, we may calculate the corresponding pressure 

 correction. A table was calculated once for all, giving in 

 this way the differences between the readings of the ther- 

 mometer in the horizontal and vertical positions for every 50 

 divisions. The numbers are given in the last column of 

 Table I. It will be noticed that an error of 1 per cent, in 

 the pressure correction would cause a difference of less than 

 0°*002 on a range of over 30°, which difference of course 

 would be quite inappreciable. 



Some experiments were made to find how much the 

 thermometer A lagged behind when placed in water the 

 temperature of which was uniformly rising. They were 

 carried out according to the manner described by Thiesen* 

 and gave sufficiently consistent results showing the time 

 constant to be 12 s . 



* Guillaume, ThermomUrie, p, 187* 



