wf 
SSS 
i Temperattire of Mercury in Siphon Barometer. 251 
e “ ” : 
©) posite to each other, about four tenths of an inch wide, parallel 
to the axis, and extending 9 or 10 inches from the extremities to- 
wards the centre. The design of this is to expose to view the 
tremities of the column, and tojallow the requisite movement 
to the vernier. ‘The remaining central portion, which is about 
15 inches in length, and has no perforation. through its surface, 
embraces the attached thermometer. This arrangement evident- 
ly exposes the bulb and the tube to different influences. The 
bulb is comparatively small, and its Connedpn ‘with the mount- 
ing is more intimate than it is with the tu e, and ee 
it tends to indicate the temperature of the former more truly than 
that of the latter; and more especially so asa like connection 
between the tube and the mounting is unattainable with a due 
tegard to the safety of the instrument. And besides, allowing 
the attached thermometer to mark truly the temperature of that 
part of the column in its vicinity, and which, like it, is protected 
by the mounting, it might materially err in respect to the remain- 
ing portions which are exposed to the direct and variable influen- 
ces of the atmosphere., These ‘remarks, suggested by the con- 
struction of the instrument, I verified experimentally in the fol- 
lowing manner. I filled with mercury a tube 14 inches long and 
of a diameter not much larger than that of the barometric tube, 
and inserted into the open end a tube of less diameter ; joining 
the two firmly with sealing wax. This was introduced into the 
central part of the brass mounting. I took a series of observa- 
tions with this instrument in a cellar where the temperature was 
low and uniform, and another series in a room where the temper- 
ature was high and also uniform. ~In both these series of obser- 
vations the temperature indicated by the attached thermometer 
Must have been very nearly the same as that of the mercury in 
the tube. Denoting the means of these temperatures by (¢,) 
(t”) respectively, and the mean altitudes of the upper surface of 
the mercury by (a,) (a”,) we evidently have ¢”=¢+-A(a”—a); 
being a constant depending upon the volume of ‘mercury and 
the diameter of the lesser tube. For the same reason, if (t’) is 
the temperature of the mercury at any other time, and (a’) the 
teading of the upper surface, we have =t+A(a’—a). Elimi- 
hating the constant A between these two equations, and resolving 
for (t,) there results ¢="” —!(q/_a)+4t (1). I then made the 
—a 
fi 
following observations. 
