: Temperature of Mercury ina Siphon Barometer. 257 ~ 
The imperfections of a barometer may chiefly be classed under 
the following heads. fa . 
1, An erroneous scale. 
_- 2. An imperfect vacuum. 
3. Variableness of the friction of the mercury on the interior 
surface of the tube. 
4. The want of equality and uniformity of those parts of the 
two branches within the range of the mercurial surfaces. 
The first of these must evidently occasion uncertain results in 
the calculated temperature, as well as in every other object of re- 
search, and the above formula furnishes no means of detecting or 
appreciating the error of the scale. An imperfect vacuum, al- 
though it influences the length of the column, and on this account 
injures the instrument, has no effect upon the calculated tempera- 
ture. For, in this respect, it is evidently immaterial whether the 
inferior, surface falls, and the superior one rises by atmospheric 
Pressure alone, or whether it is modified by the elasticity of the 
enclosed air. . Hake gc: 
The third imperfection, although little attention has been be- 
stowed upon it hitherto, is one to which barometers of all con- 
structions are exposed, and is probably among the most difficnlt to 
tectify. The effects of this are to sustain the column at a height 
different from that due to the atmospheric pressure, and also to 
change the forms of its convex terminations. Both these circum- 
stances occasion error as it respects the height of the column, while 
the latter, or change of form only, can affect the correctness of the 
calculated temperature. 
The altitudes of the segments which terminate the column are 
found to vary more or less at different times in the same barome- 
ter. In some, the variation is quite inconsiderable, if necessary 
care is taken in observing; while in others, one or both the ter- 
minal surfaces may assume all forms from a plane to an apparent 
hemisphere. When the atmospheric pressure is increasing, the 
Inferior meniscus tends to become less convex, and the superior 
*he more so. For, as the column lengthens, and the mercury 
Consequently descends in the shorter branch of the siphon, the 
cylindric surface of mercury which is in immediate contact with 
€ glass tube is retarded by its friction against the glass, and re- 
lards in its turn, though in a less degree, the next concentric sur- 
face of mercury ; and this the succeeding one, and so on; the 
central filament being least retarded of all.. This would cause 
