360 



ox TIIK C<)MPI;KSSH:IIJTY <>K CASKS 



as a mean, 50 '5 millims. As regards the section of the auxiliary tube, it was found 

 that a mercury thread occupying 85 millims. of it weighed 5 '3 35 grammes. The 

 proportional difference of volume is thus 



50-5X5-335 

 85X13-54X2845 



= -000082 ; 



and the same fraction represents the proportion by which the left-hand measuring 

 rod exceeds in length its fellow on the right. It would seem that by this procedure 

 the lengths of the rods are compared to about a millionth part. 

 In the notation employed in the calculations 



whence 



[ a = 1 -000082 H 2 , 



= 1-000041. 



2H, 



It may be observed that an error in the comparison of H t and H 3 enters to only half 

 its amount into the final result. 



The, Observations. 



The manipulation necessary to imprison the right quantity of gas was described in 

 the former paper. When this has once been secured the observations are straight- 

 forward. On each occasion 6 readings were taken, extending over about an hour, 

 during which time the temperature always rose, and the means were combined into 

 what was treated as one observation. 



A complete set usually included eight observations at the high pressure, in four of 

 which one large bulb was in use, and in the second four the other bulb. Interpolated 

 in the middle of these were the observations (usually six in number) of the low pressure 

 where both large bulbs were occupied by gas. Further, each set included eight obser- 

 vations relating to the side apparatus, in which the large bulbs stood charged with 

 mercury. In this way each set contained within itself complete material for the 

 elimination of V 3 , which might possibly vary from time to time with the character of 

 the contact between mercury and glass in the lower chamber of the right-hand 

 manometer. Finally the means were taken of all the corresponding observations, no 

 further distinction being maintained between the two large bulbs. 



The following table shows in the notation employed the correspondence of volumes 

 and temperatures : 



