~s 
THE 
LONDON, EDINBURGH anv DUBLIN 
PHILOSOPHICAL MAGAZINE 
AND 
JOURNAL OF SCIENCE. 
[FOURTH SERIES.] 
JANUARY 1860. 
I. On the Construction of certain new forms of Thermo-Barome- 
ters. By.T. Tarr, Esq.* 
HESE instruments are highly useful on account of being 
more sensitive than the mercurial barometers. The 
thermo-barometer now commonly used is called a Sympieso- 
meter by the instrument makers ; it consists of an upright tube 
having a globe at the top bent downwards, and a cup at the 
lower extremity bent upwards; the globe is filled with hydrogen 
gas; and a liquid, usually strong sulphuric acid, stands in the 
tube and cup. The variations of the pressure of the atmosphere 
are indicated by the elevation of the liquid in the tube, a cor- 
rection being made for the change of temperature. For this 
purpose there are two scales, one moveable, called the barome- 
trical scale, the other fixed, called the thermometrical scale ; 
the latter is graduated into degrees of temperature, and the 
former into equal divisions representing inches and tenths of a 
mercurial column. Now the construction of this instrument 
is not correct in principle; for it is constructed on the assump- 
tion that the variations of atmospheric pressure are in pro- 
portion to the changes in the height of the column of liquid. 
The globe is bent downwards to facilitate the graduation of the 
thermometrical scale, which is used for giving the correction 
for temperature: the globe being plunged into a water-bath, 
brought to different degrees of heat, corresponding marks are 
then made on the scale on a level with the liquid in the tube. 
Now it will be observed that, in this process, the gas occupying 
the tube is not brought to the temperature of the water-bath ; 
whereas to have a correct scale of temperature, the whole volume 
of gas, as well as the column of liquid, should be equally heated ; 
* Communicated by the Author. 
Phil. Mag. 8. 4. Vol. 19. No. 124. Jan. 1860. B 
