54 



Hence, sudden movements of this sort are always attended with 

 danger to the barometer. 



Many think they can judge of the excellence of the vacuum in a 

 barometer by the character of the " metallic click." It is exceedingly 

 deceptive, however, and even experts are able to draw only approxi- 

 mately correct conclusions from its character. The greatest caution 

 should be exercised in producing the click, as, if the vacuum is first 

 class, it tends to injure the barometer. A good plan is to incline the 

 barometer, as described above, until the mercury almost reaches the 

 top of the tube ; then, holding it in this position, move it somewhat 

 quickly, but very slightly and regularly, back and forth three or four 

 times exactly in the direction of its length, and, if necessary, changing 

 the angle of inclination and increasing, very cautiously, the intensity 

 of the shaking motion until two or three gentle clicks may be heard. 

 Too great care can not be exercised in this respect, and only the most 

 gentle clicks should be produced. Even then, with very perfect 

 vacua, the internal stress is very great, and barometer tubes that 

 have been subjected to boiling in the process of filling and are not 

 thoroughly annealed are sometimes in such a state of internal stress 

 as to be very easily cracked and injured. 



114. Handling harometers at elevated stations. — ^In the case of sta- 

 tions from 3,000 to 10,000 feet or more above sea level the top of the 

 mercurial column, in extreme cases, is a long distance from the top 

 of the tube. It is not advisable, therefore, when it is desired to invert 

 such a barometer, to screw up the cistern immediately until the col- 

 umn reaches nearly the top of the tube. A better plan is to raise the 

 column only 2 or 3 inches, then, while gradually inclining the instru- 

 ment, continue to screw up the cistern until the column is about to 

 disappear from view at the top. The object of this is to avoid sub- 

 jecting the cistern to the considerable hydrostatic pressure that occurs 

 if the column is raised several inches above that which the air pres- 

 sure itself is capable of supporting. 



At an elevated station the barometer must be in a much more nearly 

 horizontal position to produce the " metallic click " than at sea level. 



115. The best possible care a barometer can receive is to be pro- 

 tected from accumulations of dust, etc., and left quite alone.- When 

 readings must be taken, and the barometer is suspended from a hoop 

 upon which it is drawn out to a position convenient for reading, the 

 rough sliding of the barometer along the hook, together with the 

 springing movement up and down, and finally the knocks the cistern 

 is apt to receive when the instrument is returned to the box, are all 

 very injurious to the condition of the barometer and are to be avoided 

 by gentle and careful handling. 



116. The results of comparative barometer readings conclusively 

 show that in spite of every care a difference of several thousands of 

 an inch in the indications of two or more instruments can not ordi- 

 narily be avoided. Any change of instruments at a station therefore 

 is apt to make a more or less objectionable break in the strict con- 

 tinuity of the pressure reports fron that station and obviously such 

 changes should be made only when absolutely necessary. 



117. After continued use the mercury in the cistern of a barometer 

 loses its brilliant surface and becomes coated with a slight film of 

 oxide. This does not impair the barometer to any serious extent, 



