380 Messrs. W. Makower and H. II. Xobk, [June 2ft, 



This was repeated a second time, care being taken to admit just the 

 quantity required to fill the apparatus to the pressure desired for 

 taking observations. The instrument was then ready for use. 



4. Correction of Pressure Measurements for Parallax. The pressures 

 exerted by the oxygen in the thermometer bulb were determined by 

 measuring the height of a mercury column on a glass scale placed in 

 front of it. In these circumstances errors due to parallax were apt to 

 occur in the observations, unless great care was taken to bring the axis 

 of the telescope into line with the top of the meniscus, or the glass 

 point. This means that all readings were to be made at the intersection 

 of the cross wires in the telescope. It was obviously impossible to 

 read both point and meniscus at the centre simultaneously. A series 

 of observations was therefore taken to determine in what way the 

 apparent distance between a fixed graduation on the scale and the 

 glass point (which was, of course, also fixed) varied with the position 

 of the axis of the telescope. Care was always taken to adjust the 

 telescope so as to make the parallax correction small and in most cases 

 negligible ; but when necessary the appropriate correction was applied. 



5. Calculation of Results. The pressure of the oxygen contained in 

 the thermometer bulb was measured at the freezing point and at the 

 boiling point of water. 



The pressure coefficient (a) was deduced from the well-known 

 definition 



P*- Po 

 T76T' 

 where P0 is the pressure at the boiling point (6) of steam on the 



Centigrade scale 



and P that at the freezing point at a constant volume unity; 

 P0 and P were calculated respectively from the formula! 



where P is the observed pressure at the ice point 

 and P' is that at the steam point, 



f s and tg are the temperatures Centigrade at the ice and steam 



point observations respectively, 

 / and t' are the temperatures of the dead space at the ice and 



steam point observations, 



Vb is the volume of the bulb at the ice point 94-2096 c.c., 

 V'o is the volume of the bulb at the steam point 94-2096 



(1 + 0-00002850), 



Vg is the volume of the stem (-1031 c.c.), 

 v is the volume of the dead space. 



