THE MANOMETERS. 



49 



When any operation is to be performed with a manometer which 

 might endanger the calibrated portion, or contaminate the mercury 

 in it, or foul the walls, the instrument is cut into two parts, the point 

 of severance being usually between the bulbs, when that is practicable. 

 If necessary, another piece of suitable form is then attached to the 

 manometer, e. g., as when the instrument is to be placed in the steel 

 block. Afterwards the detached portion is restored to its place. 





Fig. 27.— "Brass block." 



Construction like that of "steel block" (see Figure 22), except the manometer attachments, which 



are like those used with the cells. 



In Figure 28 is seen an instrument much used in the manipulation 

 of the manometers. The manner of its use will be best illustrated by 

 describing a few of the operations in which it is most frequently employed. 



1. Suppose the whole instrument (Figure 18 or 19), except the space 

 occupied by the nitrogen, is filled with mercury, and it is necessary 

 to cut the tube between the bulbs 1 and 2, either for the purpose of 

 replacing the detached piece by another of similar form, or by a simpler 

 piece of glass tubing. The rubber-covered cone which is usually upon 

 the open end of the manometer, or a sharply sloping stopper through 

 which the end has been passed, is placed in the cup 1. The air is then 

 exhausted through a rubber tube attached to the stem at 2. When a 

 sufficient quantity of mercury has been drawn into the cup, the whole 

 arrangement is tipped backwards until the end of the manometer is 

 exposed. Air is then cautiously readmitted to fill the space in the 

 instrument which was previously occupied by the mercury removed. 



