$14 Description of a new portable Barometer,  [(Aprit, 
- The principle upon which this instrument depends is the regular 
expansion or contraction of a permanently elastic fluid on a dimi- 
nution or increase of the pressure under which it exists. 
The objection to an instrument of this kind, and it appears at 
first to be insurmountable, is the difficulty of measuring the ex- 
pansion of the included air, which must vary according to its quan- 
tity; but this objection is entirely overcome by having the elastic 
fluid so placed that, in proportion as the weight of the atmosphere 
decreases, it continues to throw upon itself a column of mercury 
until its height balances the diminished pressure without. The 
quantity of expansion is not, therefore, the measure required ; it is 
the height of the mercurial column, as in the common barometer, 
without reference to its diameter. 
This instrument is composed of a cistern of iron turned truly 
cylindrical, and is one inch in its internal diameter, and 12 inch in, 
depth. The top and bottom of this cistern are also made of iron, 
and adapted to it by screws. 
Through the centre of the top a glass tube about 10 inches long 
is made to project into the cistern exactly to half its depth, and_is: 
there firmly cemented. The bore of this tube is about + of an 
inch in diameter, and must be chosen as perfectly cylindrical as 
possible. 
At the side of this tube a delicate thermometer is likewise intro- 
duced through the top of the cistern, so that its bulb shall be a. 
short distance within it. It must also be cemented in, as well as 
the top and bottom of the cistern, so that the whole shall be per- 
fectly air tight. 
The cistern is now filled with mercury to such a height that the 
end of the tube inserted in it is immersed under a greater quantity 
of that fluid than the whole length of its bore could contain, which 
prevents it from ever being uncovered in any possible position of the 
instrument so that the portion of included air could escape.. This 
is most conveniently done by drilling a small hole through the side 
of the cistern at the calculated height, and pouring in mercury by 
the glass tube until it flows out of this aperture, which is then 
secured by a screw and cement. The temperature of the cistern is 
brought to 75° Fahr. at the time of sealing. A small quantity of 
mercury is afterwards added until it stands in the tube at a conve- 
nient height above the cistern. 
We have now a close vessel containing a portion of air confined 
by mercury, and exposed to the variations of weight in the atmos- 
phere. The obvious consequence of any diminution of its pressure 
will be a proportional expansion of the included air, and an eleva- 
tion of mercury in the tube, until its re-action forms a counterpoise 
to the expansive force of the confined elastic fluid. 
The height of this column will be in proportion to the dimi- 
nished pressure without; but an inch of the common barometer 
will be expressed by less than an inch on this instrument, from the 
mercury sinking in the cistern as it rises in the tube, and because a 
certain force is absorbed in the increased space which the included 
