Notes on Barometer Construction, 51 



through it, while the tube itself is supported over a warm 

 stove or other suitable source of heat. The caoutchouc 

 connector leading to the aspirator is attached to the tube 

 over A; on the open end of the long arm a chloride of 

 calcium tube is also attached by an in diarubber joint. 



" The mode of filling is the following : — First the tube, 

 very carefully dried, is fastened to a narrow wooden board 

 in the manner shown in the engraving. This board ends 

 below in a screw, which is screwed into a base also of 

 wood, and which is supported by three wooden levelling 

 screws. The board has at its upper end a ring for the 

 purpose of hanging up the instrument. 



" Thus mounted, with the stopcock A open and the stop- 

 cock B closed, well cleaned dry mercury heated to about 

 100° C. is poured into the tube through a small funnel with 

 capillary termination, which holds back all dust. The mer- 

 cury drives before it slowly and gradually the air in arm I., 

 causing it to escape through stopcock A. Finally mercury 

 also passes through the stopcock A and the tube above it. 

 Now A is shut and B opened; the mercury now conse- 

 quently falls out of the arm II. until its surface in this limb 

 has descended to the point of junction of the branch tube, 

 while in arm I. a column approximating the true barometric 

 column remains suspended. The space thus existing above 

 the mercurial column is not quite air free, although in a 

 highly attenuated condition. The instrument may be made 

 to act as a mercurial air-pump upon the air which adheres 

 to the inner surface of the glass tube and on that drawn in 

 by the warm mercury. For a few minutes, however, the 

 instrument is allowed to remain at rest in the condition just 

 described. 



" In the next place the stopcock B is closed, the stopcock 

 A also remaining closed ; heated mercury is again poured 

 into the open tube II., fiUing it completely; the small quan- 

 tity of air contained in the vacuum chamber is compressed 

 into a very small bubble close under the stopcock ; A is then 

 opened, allowing this bubble to escape, and afterwards mer- 

 cury ; after this mercury is again fed in again at C, when a 

 stream of air-free mercury flows through A, sweeping with 

 it mechanically all air attached to the glass inner surfaces ; 

 after several grammes have thus flowed out A is closed, B is 

 opened, allowing once more the efflux of the mercury from 

 t;he latter. The chamber above the mercury column is now 



