316 Description of a new portable Barometer, [APRIiL, 
the hand, or under the arm, for ashort time before it is to be used ; 
and the cooling of the cistern is so slow, from its being incased 
with tubes preserving a small space between each other, as to allow 
of sufficient time for making the observation without risk of error. 
The temperature of 75° Fahr. is chosen, from its being one 
which can easily be obtained by the heat of the body in almost 
every climate. 
In order to prevent the escape of mercury when the instrument 
is inverted, a small piece of porous wood is inserted in the extre- 
mity of the bore of the tube, which allows of the free passage of 
air, but prevents the transmission of a denser fluid. The tube is 
afterwards terminated by a stop-cock cemented on so as to be air- 
tight. 
The instrument thus prepared is inclosed in a brass tube having a 
slit in front about three quarters of an inch wide, which reaches 
from the top of the cistern to the extremity of the scale. This front 
slit shows the fixed thermometer and barometer tube and scale, 
which is divided in the usual manner, and may be read off to the 
thousandth of an inch by means of a vernier moveable by a screw, 
Above the fixed thermometer a detachable one is placed for taking 
the temperature of the air. A narrow slit behind gives light to 
enable the observer to make a tangent of the lower edges of the 
vernier with the convex surface of the mercury, as in the best kind 
of barometer. Another thin brass tube, slit in the same way, is 
afterwards fixed on, which, by turning half round, covers the front 
aperture in the usual manner; and the top is terminated by a small 
ferrule, or cap, which unscrews, and shows a milled head for 
moving the vernier, and a steel ring by which the instrument may 
be suspended. 
The barometer is now completed. In order to employ it in the 
mensuration of heights, the temperature of the air is taken in the 
usual manner, by means of the detached thermometer. During 
the time occupied in doing this, the cistern of the barometer is 
held in the hand, under the arm, or other warm part of the body, 
until the temperature is raised a few degrees above 75° Fahr., the 
point at which it is graduated. This will be accurately ascertained 
by the included thermometer. When that degree of heat is indi- 
cated, the stop-cock is opened, and the barometer suspended by the 
iron ring, or rested on the knee in the usual manner. A few gentle 
taps on the tube will make the mercury oscillate freely, and take its 
proper level. As the temperature approaches the standard, the 
vernier is moved by the milled head, so that its lower part shall be 
a tangent to the convex surface of the mercury, when that degree is 
indicated by the fixed thermometer, ‘The change of temperature, 
as before observed, is sufficiently slow to admit of perfect accuracy. 
The height being then read off and registered, the observation is 
completed; and the same means may be employed for deducing its 
metrical value as with the common barometer.* 
* Mr. Thomas Jones, of Charing Cross, has just published a set of very useful 
tables on this subject. He has undergone the labour of calculating by the loga- 
