62 



cury from the end of the barometer tube. The small portion of mer- 

 cury that is apt to remain should be poured out afterwards, removing 

 first the tube or barrel K, then the clamp ring A, holding the glass 

 cistern in position by the hand until the last portion of mercury is 

 emptied. 



To remove the barometer tube from the sheath, the cap of the 

 air vent must be first removed. 



It is always best not to disturb any more than necessary the various 

 leather washers and fittings. 



144. In replacing the parts he sure the cap of the air vent is 

 screwed up before introducing the mercury. 



After inserting the glass cistern and washers screw down upon, it 

 the clamp ring h. Then filter in as much mercury as practicable, 

 observing the precautions mentioned in paragraph 140. Next screw 

 down snugly the tube or barrel K, and add the remaining mercury. 

 Finally, replace the piston and cap q. 



If the piston does not fit the barrel snugly enough, the washers 

 should be tightened by turning up the screw _/, using the special 

 wrench. The parts of the piston should not be taken apart nor the 

 washers disturbed, except possibly to tighten the screw mentioned 

 above. 



During the operation of replacing the piston it should not close 

 down upon the mercury, which would be subjected to severe pressure 

 thereby. At last, tighten the screw Z. 



Before the instrument is placed erect the piston should be screwed 

 up close to the mercury, still leaving a small space, as shown by the 

 air bubble, visible in the cistern when the barometer is held nearly 

 horizontal. 



145. Suggestions about moving and packing 'barometers. — Pre- 

 paratory to moving invert the barometer as described in paragraph 

 111. 



The most approved methods of packing barometers for transporta- 

 tion are to be learned by carefully observing the manner followed in 

 packing instruments sent out by this office. The instrument should 

 be shipped in a horizontal position. The air-bubble space in the 

 cistern should be small, but still sufficient to admit of expansion 

 with temperature changes. 



When carried about by hand, the cistern should be uppermost. 



The barometer should be first wrapped in soft paper, then with a 

 thick layer of 'cotton. sheeting and an outer wrapping of heavy paper. 

 Thus prepared, it is then placed in the middle of a strong wooden box 

 and completely and closely surrounded with good excelsior or cotton 

 or similar elastic packing material. The lid of the box must be 

 screwed down, not nailed, and a strong handle attached to the middle, 

 so that the box may be carried by one hand in a horizontal position. 



146. Leather cai'rying cases. — In using the leather carrying case, 

 supplied when barometers are to be transported by hand, secure the 

 barometer in the hinged wooden sheath, being careful to observe that 

 the latter closes tightly without straining either the milled head 

 for regulating the vernier or the attached thermometer. The wood 

 should be neatly cut away, if necessary, but only sufficiently to receive 

 these projecting parts. Insert the barometer, cistern uppermost., into 

 the leather case. 



