This experiment should interest you very much, because it is going to
lead up to the subject of weather instruments, and is absolutely
essential that you understand the fundamental principles in order to
intelligently interpret these instruments. This experiment will explain
one of the principles of the barometer.
Take a glass tube thirty-two inches long and one-quarter or one-eighth
inch in diameter, and fill it with mercury, care being used to get rid
of all the air bubbles. The mercury should be poured in with an eye
dropper, one end of the tube being sealed, until filled, and then the
finger is placed over the open end. (See Fig. 7A). The tube is inverted
and immersed in a reservoir of mercury and clamped to an upright stand.
Immediately the mercury falls to about thirty inches. (See Fig. 7B). Ask
yourself what held the mercury up in the tube. Again the answer is that
the pressure of the air on the mercury in the reservoir causes it to
rise and fall in the tube, as the pressure of the air changes. You will
soon learn what causes these changes in the pressure of the air.
[Illustration:
Fig. 8
]