ON TIIK SI'KCII-IC 1JKS1STANCK OK MKRCURY. 357 



If we pnt. = 4l'45, aa found above, then 



8/ = -41! X a, 



and this is the value used for the tubes VI., VIII., and II. in the Tables which follow. 

 In the case of the wider tubes, it was not at first so clear what the amount of this 

 correction ought to be. If ACB, fig. 2, again represent the meniscus, it was evident 

 that B was not vertically below A, and, further, that the extreme point C did not lie 

 on the axis of the tube. 



Let AE be vertical through *A, and FC'CG a vertical touching the end of the 

 meniscus, and let DC' be the axis of the tube. 



Then, in the case of one tube of 1'9 mm. in diameter, we found that 



AF = -46 mm., 

 CC'= -11 inm., 

 BG = '34 mm., 



approximately. Observations on other tubes gave somewhat similar values, though 

 the difference between BG and AF was not always so great as in the above. The 

 exact calculation of the volume between the plane through FG and the meniscus 

 ACB is not possible. We have calculated the correction on the assumption that it is 

 the same as that for a spherical surface through AC'E ; the effect of gravity has been 

 to draw this surface down into the position ACB, and it is assumed that the volume 

 ACBE is approximately the same as AC'E. 



Taking the values given above, we find on this assumption that 



8/ = '46 X a. 



It seemed desirable, however, to verify this result by direct experiment, and this 

 we did, following the method adopted by Lord RAYLEIOH. Ebonite plugs were 

 turned which exactly fitted the ends of the tubes, and these plugs were inserted and 

 pressed up against the ends of the mercury columns so as to flatten them, and the 



