56' 



position for cleaning a barometer is to be seated in front of a desk 

 with a drawer at the top and side partly opened. This affords 

 convenient corners in which the barometer can be rested in upright 

 positions during the process. 



126. Tlie barometer will be removed from its box or support and 

 inverted, as described in paragraph 111, 



Unscrew with one hand the portion of the cistern marked " S," 

 Figure 3, grasping witli the other hand only the narrow -flange R.^ 



127. Next separate the two wooden portions of the cistern marked 

 i and j by loosening the four screws uniting the split-ring clamp 

 marked " I " and " il/," in Figure 3. It is important that each screw 

 be loosened a little in turn, otherwise an uneven strain may be thrown 

 upon some portion of the fragile wooden flange and chip out a piece. 

 After loosening each of the screws one may be taken out entirely, 

 and the whole system of split rings still interlocked by the screws 

 will generally unfold from around the cistern. Sometimes another 

 screw must be taken out. 



If the rings are separated, they should afterwards be united again 

 precisely in the original relation. When removing the wooden piece 

 j, to which the leather bag is attached, lift it cautiously directly up 

 from the part i so as not to spill the mercury, which is thereby 

 exposed and should just about fill i. Hold a clean, dry vessel close 

 under the flange of i and pour out steadily from the cistern all or 

 nearly all the mercury it contains. The mercury will not leave the 

 open end of the barometer tube so long as the latter is not raised 

 much above a horizontal position, and generally not then unless the 

 opening is large and the tube shaken or jerked a little. Care must 

 be taken to prevent the mercury from passing out of the tube. The 

 barometer is then returned to its inverted position and the remaining 

 parts of the cistern removed by loosening the screws P and P ; here 

 again each screw must be loosened a little in turn to avoid clipping or 

 cracking the glass cylinder. If a small globule of mercury remains 

 in the glass cistern, allow the latter to rest in its position, while the 

 boxwood piece ^, the metal flange R, and the screws P, are removed. 

 Then holding the glass cylinder in position with the fingers, empty 

 what remains of the mercury in the cis'iern. In handling the little 

 leather washers taken from the parts of the cistern, avoid wrinkling 

 or creasing them or otherwise changing their form, as any injury of 

 this kind will probably result in leaks that can not be prevented 

 except by new washers. 



128. The barometer tube and attached wooden piece G, Figure 3, 

 may be next withdrawn from the metal sheath and all the parts thor- 

 oughly cleaned. Before removing the tube notice exactly the position 

 of the ivory point in reference to the outside sheath so that it may be 

 returned to this position, otherwise a change may be introduced in 

 the correction for instrumental error. In all probability small quan- 



« If mercury has leaked out of the cistern this will generally be Indicated by the 

 presence of minute globules of mercury adhering to the threads of the screw O. In such 

 a case the cap at the extreme bottom of the cistern should be unscrewed, instead of the 

 portion marked " S," thereby preventing the escape of the mercury which has leaked out. 

 As this mercury, by reason of its contact with the metal parts of the cistern, is impure, 

 it must be emptied separately, and under no circumstances afterwards used in the 

 barometer or mixed with good mercury, as the whole will be rendered impure. 



To empty this impure mercury from the cistern the finger must be used to force the 

 kid-skin bag up into the cistern while the barometer is inclined and the impure mercury 

 poured out. 



