49 



of the mercury to the ivory point accurately requires care and sharp 

 scrutiny. 



Adjust?}ie/it of cistern., first method. — One way is to sight between 

 the point and the mercury and watch for the slightest thread of light 

 that can be detected. The screw should be turned very carefully 

 until this thread of light just disappears. This method is believed to 

 be best and is uniformly practiced at the central office. It is equally 

 applicable to new barometers with bright mercurial surfaces and to 

 older ones the mercury of whicli is more or less oxidized. The light 

 should be strong from behind the barometer and the front of the 

 cistern should be in shadow. 



jSeeand method. — The adjustment of the mercury to the ivory point 

 may also be determined by watching the formation and disappearance 

 of the small, dimplelike depression made in tlie mercury when the 

 ivory point is pressed into tlie mercury a little and again withdrawn. 

 When the dimple just disappears tlie surface may be supposed in con- 

 tact with the ivory point. The mercury often clings to the ivory 

 point, especially when the ivory is newly cut. This method, however, 

 is not so reliable and accurate, and, in general, can be followed only 

 with clean mercury. Moreover, it is not good practice to lower the 

 mercury any slight amount after it is once raised to the ivory point. 

 The effect of this generally is to change simply the convexity of the 

 meniscus at the top of the column, and this gives rise to a new and 

 unknown correction for capillarity. The most uniform results are 

 obtained by gradually raising the mercury until precise contact is 

 secured. If it is imagined the mercury has been raised too much, 

 lower it until entirely free from the point and adjust again. 



Third method. — Another method that is often given is to watch 

 closely until the reflected image of the ivory point coincides with the 

 point itself. This also requires clean, bright mercury and is there- 

 fore not a general method. Great precision in the adjustment of the 

 contact of the ivory point with the mercury may be attained with a 

 little care and practice, and observers may scarcely be conscious of 

 precisely the manner of making the adjustment. 



98. AdjustTrhent of the vernier. — The level of the mercury being 

 adjusted to the ivory .point, the vernier must next be brought to the 

 top of the column. Greater uniformity and accuracy are insured if 

 the fingers be now tapped smartly against the side of the metal 

 barometer tube. This aids the mercury in detaching itself from the 

 glass and forming into a normal meniscus. The proper setting of the 

 vernier is made when the light is just cut off from across the extreme 

 summit of the meniscus. The Figures on page 11 indicate how the 

 vernier should be set to the mercurial meniscus. The lower edge of 

 the vernier must be brought just to the level of the extreme siim/niit of 

 the meniscus. The eye must be held so that both front and hack edges 

 of the vernier are in the line of vision. 



99. It is needless to say that throughout the setting of the barome- 

 ter, as described above, the column must be maintained vertical, 

 either by means of the fixed supports or by the skillful handling of 

 the freely suspended barometer, so that at the critical moments when 

 contacts are judged to be made the instrument is truly vertical. 



100. After the cistern and vernier are adjusted in the manner 

 described above it remains only to read the scale and vernier in 

 accordance with the instructions in paragraphs 13, 14, and 15. 



