102 



ELEMENTS OF LABORATORY WORK 



which was used in the last experiment, maintained a longer 

 column of mercury in the other limb. The height of the 

 column supported may be noticed to vary slightly at different 

 times, provided the tube be long enough. In order to accurately 

 measure this variation in the distance between the level of the 

 mercury in the cistern and that in the tube, certain details of 

 construction are required. The bottom of the cistern A is 

 formed of leather, which can be raised or 

 lowered by a screw B working against it. 

 By this means the surface of the mercury 

 in the cistern may be always made to touch 

 an ivory pointer c, placed immoveably with 

 regard to the scale. The distance is then 

 always measured from the bottom of this 

 pointer to the top of the column of mercury 

 within the tube, and since the scale is 

 rigidly fixed to the tube (for it is marked 

 on the brass tube partly encasing it), a 

 band with a horizontal lower edge, moving 

 over the tube and graduated so as to form 

 a vernier, enables the exact position of the 

 column with regard to the scale to be read. 

 When the centre of the mercury surface and 

 the front and back of the band all seem to 

 be at the same level, their vertical heights 

 coincide. It will be noticed that the sur- 

 face of the mercury is somewhat convex. 

 We measure from the centre of the surface, 

 which will be the highest point, provided 

 the tube be vertical. This is ensured by 

 allowing the tube with its brass case to 

 hang freely. 



The accurate comparison of the pressure 

 of the atmosphere at different times requires 

 **; more than the reading of the height of the 



column of mercury it supports. The height 

 of the column of liquid supported under these conditions has 

 been shown to be inversely proportional to the density. The 



