s; at that height
the barometer stands at 7 inches instead of at 30 inches, and the
internal pressure in cells and tissues is not balanced by an equal
external pressure. The unequalized internal pressure forces the blood
to the surface of the body and causes rupture of blood vessels and
other physical difficulties.

81. Use of the Barometer. Changes in air pressure are very closely
connected with changes in the weather. The barometer does not directly
foretell the weather, but a low or falling pressure, accompanied by a
simultaneous fall of the mercury, usually precedes foul weather, while
a rising pressure, accompanied by a simultaneous rise in the mercury,
usually precedes fair weather. The barometer is not an infallible
prophet, but it is of great assistance in predicting the general trend
of the weather. There are certain changes in the barometer which
follow no known laws, and which allow of no safe predictions, but on
the other hand, general future conditions for a few days ahead can be
fairly accurately determined. Figure 48 shows a barograph or
self-registering barometer which automatically registers air pressure.

[Illustration: FIG. 48.--Barograph.]

Seaport towns in particular, but all cities, large or small, and
villages too, are on request notified by the United States Weather
Bureau ten hours or more in advance, of probable weather conditions,
and in this way precautions are taken which annually save millions of
dollars and hundreds of lives.

I recollect a summer spent on a New Hampshire farm, and know that an
old farmer started his farm hands haying by moonlight at two o'clock
in the morning, because the Special Farmer's Weather Forecast of the
preceding evening had predicted rain for the following day. His
reliance on the weather report was not misplaced, since the storm came
with full force at noon. Sailing vessels, yachts, and fishing dories
remain within reach of port if the barometer foretells storms.

[Illustration: FIG. 49.--Isotherms.]

82. Isobaric and Isothermal Lines. If a line were drawn through all
points on the surface of the earth having an equal barometric pressure
at the same time, such a line would be called an isobar. For example,
if the height of barometers in different localities is observed at
exactly the same time, and if all the cities and towns which have the
same pressure are connected by a line, the curved lines will be called
isobars. By the aid of these lines the barom