BETWEEN ONE ATMOSPHERE AND HALF AN ATMOSPHERE OF PRESSURE. 359 



The value of B calculated without allowance for this correction would be too large, 

 so that a gas which really obeyed BOYLE'S law exactly would appear to be too 

 condensable, like CO 2 . From a value of B so calculated we are to subtract '000021. 



Comparison of Manometers. 



This comparison is effected by combining the manometers in parallel so that the 

 mercury at the lower levels is subject to the pressure of one continuous quantity of 

 gas, while the mercury at the upper levels is in vacuo, or at any rate under the 

 pressure of the same very rare gas. Any difference that may manifest itself may be 

 estimated by finding what change of gas- volume is required in order to pass from the 

 pressure appropriate to one manometer to that appropriate to the other. 



The first matter requiring attention is the vertically of the measuring rods, or 

 rather of the lines joining the point* actually applied to the mercury. The points 

 were visually projected upon a plumb-line, hung a few centimetres away, and were 

 observed through a hole of 2 millims. or 3 millims. diameter perforated in a black 

 card. If the adjustment is perfect the same position of the card allows accurate 

 projection upon the thread of the upper and lower points ; if not, the necessary 

 motion of the card, perpendicular to the line of sight, gives data for estimating the 

 amount of the error. If x and y be the linear horizontal deviations from true 

 adjustment thus determined in any two perpendicular planes, I the length of the rod, 

 the angular error is \/(3?-\-y*)-l, and the proportional error of height for the 



a -2_i_ v 3 

 present purpose is "L" . When the manometers were compared, no value of x or y 



21 



exceeded millim., so that with / = 380 millims. the error of vertically could be 

 neglected. 



In effecting the comparison of the manometers, both mercury levels must be set 

 below (in order to make the gas volume definite), while the settings above are made 

 alternately. It was at once apparent, when the right-hand manometer was set, 

 that the rod on the left was a little too long, a perceptible interval being manifest 

 between the upper point and the mercury. In these experiments the total gas 

 volume was about 2845 cub. centims., the principal part being the volume of a large 

 bottle protected from rapid changes of temperature by a packing of sawdust. The 

 necessary changes were produced by causing mercury to rise and fall in a vertical 

 tul>e of small bore, the position of the meniscus being noted at the moment when a 

 setting was judged to be good. The settings of the two manometers must be made 

 alternately in order to eliminate temperature changes ; and the result of each set of 

 observations was derived from the means of four settings of one manometer, and of 

 five of the other. The lowering of the mercury in the auxiliary tube required to pass 

 from a setting of the left-hand manometer to a setting of the right-hand manometer, 

 was found on three separate days to be 50'3 millims., 51 '3 millims., 49'9 millims., or, 



