We are practically never conscious of this really enormous pressure of the atmosphere, which is exerted over every inch of our bodies, because the pressure is exerted equally over the outside and the inside of our bodies; the cells and tissues of our bodies containing gases under atmospheric pressure.  If, however, the finger is placed over the open end of a tube and the air is sucked out of the tube by the mouth, the flesh of the finger bulges into the tube because the pressure within the finger is no longer equalized by the usual atmospheric pressure (Fig. 47).

[Illustration:  FIG. 47.—­The flesh bulges out.]

Aeronauts have never ascended much higher than 7 miles; 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.]