38 1 >r. < '. ' 



where / and j>/ f are the temperatures, on the gas and platinum - 

 of the sulphur boiling point at standard pressure. 



II ML-C if (10) be correct, we have, pi answering to the temperature of 

 sulphur vapour when the pressure is p mm., 



pi, = pi - 0-082 (;> - 760) [1 - 100--S(2/, - 100) 



- 100-28 x 0-082(^-760)] (12). 



Supposing, for instance, 8 = 1-5, and t g = 444'53, then 



2>f t = pi - 0-082 (/) - 760) [0-88164 - 0-000012 (p - 760)] . 



For any ordinary barometric range the term containing p - 760 inside 

 the square bracket is negligible, and so we may take 



pt, = pt- 0-0723 Q-760) (13), 



where the last significant figure is, of course, uncertain. 



In actual application pt is the observed value of 100(R - Ro)/(Ri - RO) 

 in a sulphur point experiment taken when the reduced barometer read- 

 ing is p, while pt g is the value calculated from this experiment for the 

 platinum temperature of the boiling point of sulphur at standard pres- 

 sure, on the assumption that (10) is correct. 



36. In calculating pt# I adopted two methods : First I took for 

 RI - RO in each group of experiments with a given thermometer (a >jrnnp 

 including all the experiments made during one year, or during the time 

 one particular calibration was in vogue) the mean of the observed 

 values of the fundamental interval during the epoch in question. Next 

 I used for each sulphur experiment the individual value of RI - RO 

 found from the corresponding ice and steam point observations. These 

 observations were usually taken on the same day as the sulphur point 

 observation, or on the previous day. I thus obtained two series of 

 values of pl g . In the first, one principal source of uncertainty was 

 largely eliminated ; while, in the second, another principal source of 

 uncertainty was largely reduced. 



The following table shows the results for the longest epoch con- 

 sidered, viz., from September, 1897, to January, 1899, inclusive, cover- 

 ing the time when Calibration III was in vogue. During a set of 

 sulphur experiments with the different thermometers the barometric 

 height of course varied somewhat, thus the pressures given in the first 

 column of the table are only approximate. The break in the table 

 separates the cases where the reduced barometric height was al>ove 

 760 mm. from those in which it was below. The results in the last 

 two lines are based on the exact barometric readings observed during 

 the experiments on the individual thermometers : 



