422 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Measurements. 



The first measurement to be undertaken was the determination of the 

 required volumes. The bulb A and side tube C, before being placed in 

 the bath, but while held in an upright position, were filled with mercury. 

 The end of the side tube was then closed firmly with a pinch-cock at d, 

 and the mercury filling the space between the stop-cock at the top of the 

 bulb and the point a, and that between the point a and the point b were 

 in turn run out and separately weighed. Allowance was made for the 

 weight of mercury held by the stop-cock. The temperature was noted, 

 and the volume calculated from the tables of the specific gravity of 

 mercury given by Laudolt and Bornstein. In correcting for the expan- 

 sion of the glass, the value 0.000025 for the coefficient of expansion of 

 glass was assumed as accurate enough for preliminary experiments. In 

 conclusive work this value should of course be determined. 



Temperature Weight of Weight of Volume of large Volume of expansion 



Centigrade. mercury in mercury in bulb at O o C . bulb at 32 5° 0. 



° large bulb. expansion bulb. 



grams. grams. c.c. c c. 



20.0° 3709.7 441.90 273.72 32.631 



20.0° 3710.5 442.02 273.77 32.640 



20.0° 3709.5 441.93 273.70 32.633 



Average . . . 273.73 ± 0.01 32.635 ± 0.002 



The " probable errors," obtained by the well known method of least 

 squares, show that the average values are sufficiently accurate. 



The large bulb must have been 0.222 c.c. larger at 32.4° than at 0°. 

 Thus, since the volume of the large bulb at 0° is 273.73 c.c, that of the 

 total, as far as the point b, at 32°. 4, is 306.59 c.c. 



In an ideally perfect apparatus of this type the distances between the 

 platinum points a and b on the one side and the points a' and b' on the 

 other should be exactly the same. Since, however, it is not possible to 

 seal these platinum points into the glass tubes with the requisite accuracy 

 of position, it is necessary actually to measure the distances between the 

 two sets of points and apply the corrections thus obtained to the succes- 

 sive observed readings of the pressures in the reservoir R. 



Iu measuring the difference between the distances a to b and a' to b\ 

 the bulb, before being mounted in its final position, was inclined very 

 slightly, at such an angle that the points b and b' were at exactly the 

 same level, as shown by their simultaneous electrical contact on slowly 

 raising the mercury in the tubes. The difference in level between the 



