1817.] and a new Hygrometer. $15 
air must occupy from the rising of the mercury. The height of 
the mercurial column will, therefore, be a mean proportional be- 
tween the action and re-action of these different forces, and it is a 
fixed quantity subject to no disturbing cause except temperature. 
In order to graduate this instrument, the end of its tube is 
cemented into a glass vessel, connected with an air-pump and 2 
condensing apparatus, to which an accurate barometer gauge is 
attached. The new instrument is placed in an upright position, 
with its cistern resting in a vessel of water at the temperature of 
75° Fahr. The air in the receiver is now condensed until the 
standard barometer indicates 32 inches, when the height of the 
mercury in the new instrument is to be accurately marked. The 
receiver is then exhausted until the barometer falls to 20 inches, 
and the position of the mercury is again carefully ascertained in the 
new instrument. It will be found that, as the exhaustion goes on, 
for every inch the mercury falls in the standard barometer, it will 
rise in the proportion of about *75 of an inch in the new instru- 
ment, and the same ratio will be preserved throughout all the 
space. 
The range thus obtained is divided into twelve equal parts, which 
represent barometrical inches. 
The temperature of the new barometer must be preserved the 
same during the operation, which the included thermometer indi- 
cates with much precision. It is also necessary to ascertain the 
temperature of the barometer gauge ; and if it varies from 60° Fahr. 
or any other fixed point which it may be found convenient to assume, 
the well-known correction for diminution or increase of gravity in 
the mercury must be applied to the new barometer, by raising or 
lowering the fixed points, in order to form a proper compensation. 
As the temperature of the external air acts alike on both instru- 
ments, it is not necessary to observe it. By this means the new 
barometer at the temperature of 75° Fahr. will always correspond 
in its indications with the common barometer when the mercurial 
column of the latter is at 60° Fahr., whatever may be the heat of 
the atmosphere. But a variation in the temperature of either in- 
strument will occasion a disagreement in their indications; on this 
account a correction for heat is necessary. 
This correction is very simple, from the circumstance of gases 
undergoing precisely equal augmentations of bulk from equal in- 
crements of temperature, and as mercury is governed at a low heat 
by the same law. Its amount is ascertained Ly immersing the 
cistern in water of different temperatures until the rate of expansion 
is obtained. It must also be observed that no change takes place 
during the operation in the state of the barometer. ‘The value of a 
iven number of thermometrical degrees in thousandths of an inch 
Is engraved on the barometer scale, and the manner of applying 
it as a correction will be afterwards explained. But entirely to 
ge the necessity of any correction, it is found convenient to 
ting the instrument, during each observation, to the temperature 
at which it was graduated, ‘This is readily effected by holding it in 
