258 Temperature of Mei^ciiry ifi a Siphon Barometer. 



the inferior meniscus to become less convex when the column is 

 lengthening, and more convex when it is contracting. In like 

 manner, the superior meniscus would have its convexity increas- 

 ing while that of the inferior one is diminishing, and vice versa. 

 This tendency, which in good barometers, I apprehend is small, 

 may be chiefly, and perhaps sufficiently counteracted by causing 

 the barometer to vibrate once or twice far enough to produce an 

 oscillation of the column in respect to the axis of the tube, and 

 then, after having firmly secured the instrument in a perpendicular 

 position, by giving it a few gentle taps with the fingers. These 

 irregularities are greatly increased in some barometers by the va- 

 riable adhesiveness of the mercury to the sides of the tubes, at dif- 

 ferent sections of them, arising partly perhaps from the impurity 

 of the mercury, but more probably from imperfections of the in- 

 terior surfaces of the tubes. If these latter imperfections exist to 

 any considerable extent, which may be ascertained by measuring 

 the altitudes of the terminal convex segments of the column, the 

 barometer is unfit for delicate purposes ; not merely because the 

 temperature is thereby rendered uncertain, but more particularly 

 from their influence upon the height of the column. When the 

 altitudes of these terminal segments are not so variable as to prove 

 fatal to the instrument, it is desirable to apply a suitable correc- 

 tion to the formula for temperature. 



For this purpose let PCm repre- Fig. 2. 



sent the tube of a siphon barometer ; 

 EAF, eaf the forms of the terminal 

 segments of the column, when the 

 upper and lower readings are respec- 

 tively (a,) {b;) HBI, hbi the forms 

 and positions of these segments, 

 when the readings are («',)(&'). If the 

 two meniscuses of the longer branch 

 are similar, and also the two men- 

 iscuses of the shorter one, it is evi- 

 dent that {a') and {b') would require 

 no correction ; since the correction 

 for {a',) for example, is obviously 

 AB, the height to which the vertex 

 B of the meniscus HBI must rise if 

 this meniscus should assume the 

 form of that of EAF, without dis- 



H 



E 



1? 



