6 Mr. T. Tate on the Construction of 
and therefore free from those errors which necessarily arise from 
an empirical principle of construction, such as that adopted in 
the construction of the foregoing instrument. I have used this 
new instrument for some time, and find that its indications 
closely agree with those of the common barometer, excepting 
when the atmosphere is in an unsettled state, and then the want 
of agreement is clearly due to the resistances or sluggishness of 
the mercurial column. 
This instrument consists of a glass globe A and tube ASP, 
containing a portion of strong 
sulphuric acid, bent at S to 
an angle of about 45°; MN 
a stout scantlingof hard wood 
fixed ina level position, having 
a slit in it extending nearly 
from end to end, to allow the 
lower portion of the tube to 
slide through it, and having 
a circular groove extending 
from F to N, in which the 
globe A slides; TV a thin 
board placed at the back of 
MN; Oa round pin on a 
level with the centre A of the 
globe (this pin may be placed 
higher if necessary) upon 
which the tube SP slides; e7 a scale of temperature attached 
to the tube SP; EGK a sliding square, the stock EG sliding 
in a groove formed in the scantling from M to F; mn the ther- 
mometrical scale, which is transferred from the scale ev in a way 
hereafter described ; DC the barometrical scale, sliding on the 
blade GK, and graduated into equal parts so as to read off the 
height of the mercury column balancing the pressure of the 
atmosphere ; Q a piece of thin india-rubber tied over the top of 
the tube to keep the external air from coming into contact with 
the sulphuric acid. 
The observations of atmospheric pressure are made in the fol- 
lowing manner :—The temperature, as indicated by a delicate 
thermometer, being first noted, the globe A is shifted along the 
groove F N until the liquid in the tube stands at this tempera- 
ture, as indicated on the scale er; the barometrical scale D C is 
‘then shifted until its middle point p coincides with the same 
point of temperature indicated upon the scale mn, and then the 
point on the scale D C coincident with the level of the liquid in 
the tube will give the pressure of the atmosphere as measured 
by a column of mercury. 
