416 THIRD REPORT — 1833. 



A varnished paper is pasted on the front of the tube, marked 

 27, 28, 29, 30, &c., to denote the height of the mercurial co- 

 lumn in inches ; these measures being taken with care from the 

 surface of mercury in the bulb to that in the tube. The index 

 is set as nearly as possible when the altitude corresponds to 

 fixed divisions of the scale or measure- 

 There is a thermometer close to the mercurial column, the 

 bulb of which is placed in a small cistern of mercury, to indi- 

 cate the temperature, and a hygrometer to measure the change 

 which may be supposed to happen in the silk line, to which 

 the cone resting on the mercury is attached ; but the author 

 has found that by employing fine unspun silk the changes are 

 quite unimportant. 



The quantity of mercury in the instrument is about 15 lbs. 

 In order to fill the tube clear of air, the following process was 

 adopted as a substitute for boiling. A small iron cap, polished, 

 was first cemented air-tight upon the end of the tube, and into 

 this was screwed an iron stopcock : a long glass tube was then 

 cemented to the stopcock, furnished with iron caps, &c., so 

 that by reverting the instrument and steadying the tubes by 

 cross-bars of wood, tied with silk-ribbon band, the whole may 

 be screwed into the plate of a good air-pump. 



The air being withdrawn as completely as a good air-pump 

 will effect, the cock is closed, the whole is detached from the 

 pump, the long tube removed, and the barometer tube trans- 

 ferred to a cistern of mercury, under the surface of which the 

 stopcock is fairly immersed, whilst the tube is inclined as much 

 as possible. The operator, being placed in a convenient position, 

 supports the ball of the tube in his hand, and turns the cock 

 gradually, so as to allow the mercury to be pressed up in an 

 extremely small stream into the tube, and to flow down without 

 violence into the ball. During this process the ball is gently 

 moved about with an easy circular motion, which allows of the 

 more speedy union of the mercury and displacement of the air. 

 An assistant should be ready to close the cock occasionally, for 

 the purpose of examining the state of the mercurial mass 

 within the tube. 



In this way the barometer tube may be filled with great 

 nicety, so as to show a most resplendent surface, equal in ap- 

 pearance to that produced by boiling even under a powerful 

 magnifying glass. 



When the tube is complete to the point required, the stop- 

 cock is again closed, the whole is reverted, and the tube is 

 placed in its intended place ; the cock is then opened by de- 

 grees, and the mercury will gradually descend to the level of 



