in the air, and they fall to the ground as rain.

Part of the rain soaks into the ground. Some of it gradually seeps
down through the ground to an underground stream. This has its outlet
in a spring or well, or in an open lake or the ocean. But the rain
does not all soak in. After the storm, some of the water again
evaporates from the top of the ground and mixes with the warm air, and
it goes through the same round. Other raindrops join on the ground
to form rivulets that trickle along until they meet and join other
rivulets; and all go on together as a brook. The brook joins others
until the brooks form a river; and the river flows into a lake or into
the ocean.

Then again the sun warms the surface of the ocean or lake; the water
evaporates and mixes with the air, which rises, expands, and cools;
the droplets form and make clouds; the droplets join, forming big
drops, and they fall once more as rain. The rain soaks into the
ground or runs off in rivulets, and sooner or later it is once more
evaporated. And so the cycle is repeated again and again.

And all this is accounted for by the simple fact that when water
evaporates its vapor mingles with the air; and when this vapor is
sufficiently cooled it condenses and forms droplets of water.

THE BAROMETER. In predicting the weather a great deal of use is made
of an instrument called the _barometer_. The barometer shows how
hard the air around it is pressing. If the air is pressing hard, the
mercury in the barometer rises. If the air is not pressing hard the
mercury sinks. Just before a storm, the air usually does not press so
hard on things as at other times; so usually, just before a storm,
the mercury in the barometer is lower than in clear weather. You
will understand the barometer better after you make one. Here are the
directions for making a barometer:

EXPERIMENT 87. _To be done by the class with the aid of the
teacher._ Use a piece of glass tubing not less than 32 inches
long, sealed at one end. Fill this tube to the brim with
mercury (quicksilver), by pouring the mercury into it through
a paper funnel. Have the sealed end of the tube in a cup, to
catch any mercury that spills.[7] When the tube is full, pour
mercury into the cup until there is at least half an inch of
it at the bottom. Now put your forefinger very tightly over
the open end of the tube, take hold of the sealed end with
your other hand, and turn the tu