nd eyelids of explorers. The walls of the little blood-vessels
are broken by the expansion of the air that is inside. At the sea-level
air presses at the rate of fifteen pounds per square foot in all
directions. As one ascends to higher levels, the air pressure becomes
less and less.

The barometer is the instrument by which the pressure of air is
measured. A glass tube, closed at one end, and filled with mercury, the
liquid metal often called quicksilver, is inverted in a cup of the same
metal, and so supported that the metal is free to flow between the two
vessels. The pressure of air on the surface of the mercury in the cup is
sufficient at the sea-level to sustain a column of mercury thirty
inches high in the tube. As the instrument is carried up the side of a
mountain the mercury falls in the tube. This is because the air pressure
decreases the higher up we go. If we should descend into the shaft of
the deepest mine that reaches below the sea level, the column of air
supported by the mercury in the cup would be a mile higher, and for this
reason its weight would be correspondingly greater. The mercury would
thus be forced higher in the tube than the thirty-inch mark, which
indicates sea-level.

Another form of barometer often seen is a tube, the lower and open end
of which forms a U-shaped curve. In this open end the downward pressure
of the air rests upon the mercury and holds it up in the closed end,
forcing it higher as the instrument is carried to loftier altitudes. At
sea level a change of 900 feet in altitude makes a change of an inch in
the height of the mercury in the column. The glass tube is marked with
the fractions of inches, or of the metre if the metric system of
measurements is used.

It is a peculiarity of air to become heated when it is compressed, and
cooled when it is allowed to expand again. It is also true that when the
sun rises, the atmosphere is warmed by its rays. This is why the hottest
part of the day is near noon when the sun's rays fall vertically. The
earth absorbs a great deal of the sun's heat in the daytime and through
the summer season. When it cools this heat is given off, thus warming
the surrounding atmosphere. In the polar regions, north and south, the
air is far below freezing point the year round. In the region of the
Equator it rarely falls below 90 degrees, a temperature which we find
very uncomfortable, especially when there is a good deal of moisture in
the air.

If