be over. Lower the open end,
with your finger over it, into the cup. When the mercury in
the cup completely covers your finger and the end of the tube,
remove your finger carefully so that no air can get up into
the tube of mercury. Let the open end of the tube rest gently
on the bottom of the cup, and hold the tube upright with your
hand or by clamping it to a ring stand. Hold a yardstick or
meter stick beside the tube, remembering to keep the tube
straight up and down. Measure accurately the height of the
mercury column from the _surface_ of the mercury in the cup.
Then go to the regular barometer hanging on the wall, and read
it.

[Footnote 7: If mercury spills on the floor or table during
this experiment, gather it all into a piece of paper by
brushing even the tiny droplets together with a soft brush;
squeeze it through a towel into a cup to clean it. It is
expensive; so try not to lose any of it.]

The reason your barometer may not read exactly the same as the
expensive laboratory instrument is that a little air and
water vapor stick to the inside of the tube and rise into the
"vacuum" above the mercury; also, the tube may not be quite
straight up and down. Otherwise the readings would be the
same.

[Illustration: FIG. 152. Filling the barometer tube with mercury.]

Of course you understand what holds the mercury up in the tube. If you
could put the cup of mercury into a vacuum, the mercury in the tube
would sink down into the cup. But the pressure of the air on the
surface of the mercury in the cup keeps the mercury from flowing out
of the tube and so leaving a vacuum in there. If the air pushes down
hard on the mercury in the cup, the mercury will stand high in the
tube. This is called _high pressure_. If the air does not press hard
on the mercury in the cup, the mercury stands low in the tube. This is
called _low pressure_.

[Illustration: FIG. 153. Inverting the filled tube in the cup of
mercury.]

HOW WEATHER IS FORECAST. Weather forecasters make a great deal of use
of the barometer, for storms are usually accompanied by low pressure,
and clear weather nearly always goes with high pressure.

The reason storms are usually accompanied by low pressure is this: A
storm is almost always due to the rising of air, for the rising
air expands and cools, and if there is much water vapor in it, this
condenses when it cools