10 PEOF. HUGH L. CALLENDAR AND MR. HERBERT MOSS 



vark* with temperature. The compressibility of mercury, however, is so small that 

 it would require a variation of 50 per cent, to affect the results appreciably even at 

 300* C. It is most improbable that the variation of compressibility with temperature 

 is as great as this. It would have been necessary to apply a pressure of 2 or 3 

 atmospheres to the gauge to test this point satisfactorily, and it was not considered 

 advisable to do this, since any accidental failure of any of the joints or taps under 

 pressure at high temperatures might have involved dismounting and filling the whole 

 apparatus afresh, and would have seriously interfered with the continuity of the 

 observations. 



4. Method of Filling the Apparatus. 



The adoption of the multiple manometer method, which was rendered necessary in 

 order to avoid excessive length and pressure, entailed some difficulty in filling the 

 apparatus. After adjusting the tubes in position, as shown in fig. 1, the small bore 

 steel tubes at the top and bottom of the hot and cold columns were connected by 

 horizontal glass tubes, as shown in fig. 5a, with taps attached at right angles. The 

 glass tubes nearly fitted the steel tubes, and the joints were made tight by running 

 in a mixture of beeswax and resin. The taps were suitably supported, and projected 

 fanwise, upwards at the top, and downwards at the bottom, as shown in the side 

 view in fig. 1. After the taps had been connected and the whole tested for leaks, 

 the apparatus was heated to about 350 C., and evacuated, and dried by passing 

 filtered air through it. When cool, the apparatus was evacuated as completely as 

 possible, and mercury was admitted by connecting a reservoir to each of the lower 

 taps in turn, until the level of the mercury rose nearly to the upper cross tubes. 

 The apparatus was again evacuated by connecting the pump to each of the upper 



Fi - to. Fig. 54. 



taps in turn in order to remove any air displaced by the mercury. The filling was 

 iiifii completed and the gauge tubes conneotefl TJ V> c 



oJ e th?l h u at> ^1 the leVd Q , 7 "^ WCre d ^ W ^ ^Jm^btoToS 



ttjffifcST InTJchaT * "' aUd mei " CUry re " rV irS e> f 



the other side of the gauge. If the fevd ^Ld ^> ut ! ^y 



