130 PROCEEDINGS OP THE AMERICAN ACADEMY. 



as to be negligible. According to the report of the National Bureau of 



Standards, a millimeter in this part of the scale has the average value 



of 1.0004; hence a single dial division has the value in millimeters of 



9 f)008 



= 0.012325 ±0.000005. This value was used throughout the 



162.34 



work. 



Before proceeding to the measurement of the pressures exerted by the 

 gas during the experiment, that constant correction due to the difference 

 in the distance apart of the' platinum points in the bulb and in the side 

 tube was evaluated. As in the preliminary experiments, the bulb was 

 placed at the arbitrary standard distance from the telescope and the dif- 

 ference in level of the upper pair of points was observed by successive 

 settings of the cross-hair of the telescope upon each point. The tele- 

 scope was then lowered and a similar set of observations of the lower 

 points was made. Then the position of the upper points was again 

 determined. This first series showed the points in the bulb to be 

 higher than those in the side tube — for the upper pair by 0.749 mm., 

 0.750 mm., 0.752 mm. ; 0.751 mm., average 0.751 ; and for the lower 

 pair by 0.1 G7 mm., 0.169 mm., average 0.1 68 mm. 



These showed as the average the distance apart of the points in the 

 bulb to be 0.583 mm. greater than the corresponding distance in the 

 side tube. 



The second series of measurements was made after the bulb had been 

 readjusted in respect to its distance from the telescope. The further 

 precaution was also taken of viewing the points from the opposite side 

 of the bulb in order to eliminate any error due to possible irregular re- 

 fraction of the glass. 



In the second series, the heights of the bulb points exceeded those of 

 the side points, in the case of the upper pair by 0.753 mm., 0.754 mm., 

 0.752 mm., average 0.753 ; and in the case of the lower pair by 

 0.167 mm., 0.169 mm., 0.170 mm., average 0.169. 



These gave the average greater distance apart of the bulb points as 

 0.584 mm. (agreeing as nearly as is reasonable with the proceeding), 

 and so for the total average of all determinations 0.5835. These 

 measurements were made at 20°, while the tube was used at 0° and 32.4°, 

 but the effect of the corresponding dilation is negligible. The difference 

 of mercury level at 32.4° must nevertheless be reduced to 20° (at which 

 the main barometer was read), before it is added to the reading of the 

 main barometer. The value then becomes 0.582, which is used as K in 

 the calculation on page 135. 





