53 



111. Never remove a good barometer from its supports while the 

 mercurial column is at or near its normal height. Always screw up 

 the cistern until the top of the column is just visible at the tof of the 

 opening m the brass case. Do not subject the barometer to quick 

 movements or sudden changes in its position ; always move it about 

 slowly and regularly and change its position gradually. Do not 

 handle or carry the barometer in an upright position. Handle it 

 horizontally, or upside down as far as possible, preferably the latter. 

 The proper procedure to invert a barometer is as follows : 



Examine the cistern to see if there is any special air vent as at d, 

 Figure 9, in the Tuch cistern. Screw up the mercury until it reaches 

 the top of the cistern. Then close tightly the air verity and continue 

 screwing up the cistern until the top of the mercurial column reaches 

 the summit of the opening in the metal tube. Always avoid screw- 

 ing up the cistern until the tube is entirely filled with mercury. It is 

 impossible to tell exactly when the tube is full, and a turn too much 

 of the screw is almost sure to force the mercury through the joints 

 of the cistern or even the pores of the leather bag and lead to very 

 serious injury of the barometer. Do not strain the screw if it goes 

 hard. Mercury may have leaked from the cistern, and what remains 

 be insufficient to fill the tube. A barometer can be safely inverted 

 even if there is quite a deficient supply of mercury in the cistern. 



When the mercury is near the top of the tube, remove the barome- 

 ter from its supports and incline slowly, listening, meanwhile, for 

 any slight sound or " click " that may be emitted from the top of 

 the barometer. When the tube is nearly horizontal, watch for the 

 appearance of an air bubble at the cistern end showing there is 

 still a small free space within. From the horizontal position the 

 instrument may be turned cistern end up without any special pre- 

 cautions and may then be handled and carried with ease and safety. 

 .It is even advisable now to loosen the cistern screw a turn or so, 

 that there may be plenty of free space in the cistern. 



112. Special instructions for handling marine barometers. — It will 

 be readily understood from the description of the marine barometer 

 and its ciste'rn that special care must be employed in inverting such 

 a barometer. In this case the cistern is not and can not be filled 

 with mercury. Moreover, the constriction of the tube prevents the 

 vacuum from filling quickly, so that to invert a barometer of this 

 kind the tube must be inclined 30° or 40° from the vertical and 

 held in such a position, and farther inclined, if necessary, until the 

 vacuum chamber is completely filled with mercury, whereupon the 

 instrument may be fully inverted and handled without danger, 

 cistern uppermost. 



Since the cistern is only partly fidl ordinarily, the marine baro- 

 meter is more liable to injury in shipment than other barometers 

 and, in fact, is best transported by hand. 



113. The " metallic c/icA;."— The so-called " metallic click " is best 

 produced while the barometer is inclined at about 45°, or possibly 

 still more nearly horizontal at high-level stations. The cistern must 

 not be screwed up too much. The " click " occurs just as the mer- 

 cury moving up the tube reaches the top and completely fills it. If 

 the barometer is quickly inclined, the violent shock of the mercury 

 against the top of the tube is sometimes sufficient to crack the tube 



