; 258 Pemperaticgs of Mercury in a Siphon Barometer. ae 
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 PQR repre- Fig. Aes 
sent the tube of a siphon barometer; |? 
EAF’, eaf the.forms of the terminal a 
segments of the column, when the ,l/_le Wy 
upper and lower readings are respec- Vea 
b 
: 
tively (a,) (6;) HBI, Abi the forms 
itions of these segments, 
when the readingsare (a’,)(b’). 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 me 
this meniscus should assume the 2 
form of that of EAF’, without dis- _ 
