138 



Lord Rayleigh. 



[Mar. 23, 



is continued by squeezing the tube at P between a pair of hinged 

 boards, gradually approximated by a screw. This plan, though appa- 

 rently rough, worked perfectly, leaving nothing to be desired. 



It remains to explain the object of the vessel shown at Q. In the 

 early trials, when the rubber tube was connected directly to R, the 

 gradual fouling of the mercury surface, which it seems impossible to 

 avoid, threatened to interfere with the setting at B. By means of 

 Q, the mercury can be discharged from the measuring chambers, and 

 a fresh surface constituted at B as well as at C. 



The manometer above described was constructed by my assistant, 

 Mr. Gordon, at a nominal cost for materials ; and it is thought that 

 the same principle may be applied with advantage in other investiga- 

 tions. In cases where a certain latitude in respect of pressure is 

 necessary, the measuring rod might be constructed in two portions, 

 sliding upon one another. Probably a range of a few millimetres 

 could be obtained without interfering with the india-rubber con- 

 nexions. 



The length of the iron rod was obtained by comparison under 

 microscopes with a standard bar R divided into millimetres. In 

 terms of R the length at 15° C. is 762*248 mm. It remains to 

 reduce to standard millimetres. Mr. Chaney has been good enough 

 to make a comparison between R and the iridio-platinum standard 

 metre, 1890, of the Board of Trade. Prom this it appears that the 

 metre bar R is at 15° C. 0'3454 mm. too long ; so that the true dis- 

 tance between the measuring points of the iron rod is at 15° C. 



762-248 x 1-0003454 = 762 511 mm. 



Connexions with Pump and Manometer. 



Some of the details of the process of filling the globe with gas 

 under standard conditions will be best described later under the head 

 of the particular gas ; but the general arrangement and the con- 

 nexions with the pump and the manometer are common to all. They 

 are sketched in fig. 2, in which S represents the globe, T the 

 inverted bell-glass employed to contain the enveloping ice. The con- 

 nexion with the rest of the apparatus is by a short tube U of thick 

 rubber, carefully wired on. The tightness of these joints was always 

 tested with the aid of the Toppler X, the tap Y leading to the gas- 

 generating apparatus being closed. The side tube at D leads to the 

 vacuum chamber of the manometer, while that at E leads to the 

 pressure chamber B. The wash-out of the tubes, and in some cases 

 of the generator, was aided by the Toppler. When this operation 

 was judged to be complete, Y was again closed, and a good vacuum 

 made in the parts still connected to the pump. W would then be 

 closed, and the actual filling commenced by opening Y, and finally 



