( 111 ) 



within 0.01 mm. ; it is therefore possible to measure mercury pressures 

 of 10 cm. with certainty to 1 in 10000. 



In conclusion it may bo mentioned that the heights of the edges 

 of the menisci were read at tlie near side. As comparatively little 

 light could be introduced into the interior of the volumenometer jacket, 

 readings at the sides of the volumenometer tube were matters of 

 considerable difficulty, but by making (hem in front sharp readings 

 could be obtained, for, with suitable illumination from an electric 

 hand lamp the meniscus edge could always be seen as a sharp line 

 at which a number of images came to an end. As the near edge 

 of the meniscus was about 8 mm. nearer the objective of the telescope 

 than the top the same artifice was employed to eliminate parallax 

 errors in this method of reading as was indicated above in the deter- 

 mination of the corrections for the manometer. Glasses were placed 

 in front of the telescope objective and were firmly attached to the 

 telescope tube as is shown in PI. II. fig. 2. The optical shortening 

 of the path of the light rays was about 8 mm. By turning the 

 glasses round their axis and thus causing them to approach or recede 

 from each other the adjustments could be made perfectly free from 

 parallax. The top of the meniscus was read through the glasses and 

 the edge through the uncovered portion of the objective. Optical 

 errors arising from the glasses used in this apparatus and also from 

 those already mentioned as used for the manometer tube were accu- 

 rately determined beforehand. 



TABLE III. 

 Comparison of the different measurements. 



1) Method II could not be applied to this screen. For this method to be 

 applied, the screens must be very clean and quite dry. In this instance it was 

 impossible to fulfill the latter condition, for when the water was being drawn off 

 from the jacket this lowest screen was kept moist by the water that remained 

 behind since the level of the tap was but very little below the mark on the screen. 



