6
ON SOUND.
temperature; (II) the variation produced in that law
by a permanent change of the temperature of the air;
(III) the variation produced in that law by a sudden
change of the compression of the air.
6. Construction of the Barometer.
In order to explain the experimental investigations
upon which law (I) is established, it is necessary to de¬
scribe in its essential points the common barometer.
Take a straight tube of glass, not less than 32 inches
long, open at one end and closed at the other, hold it
for a short time with the closed end downwards, and
pour quicksilver (mercury) into it till it is quite full :
then carefully stop the open end, either by pressing
it with the finger or by inserting any tight plug which
can be easily withdrawn ; then invert the tube so that
its open end is downwards, dip that open end (before
the plug is withdrawn) into a cistern of mercury, and,
when the end is securely lodged below the surface of
the mercury, withdraw the finger or plug. (Figure 1
represents the apparatus thus arranged, in a shape con¬
venient for experiment : the glass tube being carried
by a tripod stand which prevents the open end of the
glass tube from touching the bottom of the cistern.)
Immediately the surface of the mercury in the tube will
fall, till its height above the surface of the mercury in
the cistern is a quantity not absolutely constant but
(when the place of experiment is near the level of the
sea) seldom less than 28 English inches, and seldom