(115 ) 
ature of the room; this height co-incided with the zero of the 
cathetometer scale. 
The preceding table gives the mercury heights to be summed for 
the measurements on Aug. 25% 1900. (Comp. p. 000). 
From the corrected mercury position in the open manometer 
reduced at the time of the observation augmented by the height of 
the barometer at the level of mercury and the corrected position of 
the level glass, the corrected height in the piezometer tube must 
be subtracted and to this we must apply the correction for the 
compression of the mercury; the pressure then found is that of the 
hydrogen at the temperature measured; in order to reduce this 
pressure to 20° C. I have assumed 0.003663 for the co-efficient of 
expansion of hydrogen, while the influence of the pressure on that 
co-efficient could be left out of consideration owing to the small 
deviation of the temperatures measured from 20° C. 
The pressure now measured is the one immediately above the 
mercury and hence the mean pressure of the hydrogen is lower. 
The greatest difference (when the mercury is at the lower end of the 
stem) however remains when p is the mean pressure in atm. below 
0.000004 p atm., which therefore may be neglected in comparison 
with p atm. 
And hence we obtain for the pressure the following calculation: 
August 25th TABLE II. 
| Reading Corr. | Temp. |Corr. for Corrected | Corr. Pressure reste 
Time. of for Pressure.| _ of rat 
piezom. | depr. | of H. | temp. | reading. | compr. H at 20° 45° Ni. 
2.40 | 
2.48 | 60.907 | 0.115); 19.78° |—0.193 | 60.829 || 0.00 | 2685.55 | 2687.52] 35.387 
3.09 | 60.897 | 0.127 | 19.78° | —0.193| 60.831 || 0.00 | 2684.15 | 2686.12 | 35.368 
3.28 | 60.881 |0.117| 19.78° |—0.193| 60.805 || 0.00 | 2682.63 | 2684.60 | 35.348 
3 
3.48 | 60.868 [0.123 | 19.785 | —0.193| 60.798 || 0.00 | 2681 .06 | 2683.03 | 35.327 
i 
4.07 | 60.850 | 0.125 | 19.78° | —0,193| 60.782 || 0.00 | 2679.55 | 2681 .52 35.307 | 
4.26°| 60.825 |0.126| 19.782 | —0.193| 60.758 || 0.00 | 2677.96 | 2679.93! 35.286" 
