The fact that air is elastic has added materially to the comfort of the world. Transportation by bicycles and automobiles has been greatly facilitated by the use of air tires. In many hospitals, air mattresses are used in place of hair, feather, or cotton mattresses, and in this way the bed is kept fresher and cleaner, and can be moved with less danger of discomfort to the patient. Every time we squeeze the bulb of an atomizer, we force compressed or condensed air through the atomizer, and the condensed air pushes the liquid out of the nozzle (Fig. 51). Thus we see that in the necessities and conveniences of life compressed air plays an important part.
86. The Danger of Compression. Air under ordinary atmospheric conditions exerts a pressure of 15 pounds to the square inch. If, now, large quantities of air are compressed into a small space, the pressure exerted becomes correspondingly greater. If too much air is blown into a toy balloon, the balloon bursts because it cannot support the great pressure exerted by the compressed air within. What is true of air is true of all gases. Dangerous boiler explosions have occurred because the boiler walls were not strong enough to withstand the pressure of the steam (which is water in the form of gas). The pressure within the boilers of engines is frequently several hundred pounds to the square inch, and such a pressure needs a strong boiler.
87. How Pressure is Measured in Buildings. In the preceding Section we saw that undue pressure of a gas may cause explosion. It is important, therefore, that authorities keep strict watch on gases confined within pipes and reservoirs, never allowing the pressure to exceed that which the walls of the reservoir will safely bear.
FIG. 52.—A pressure gauge.
Pressure in a gas pipe may be measured by a simple instrument called the pressure gauge: The gauge consists of a bent glass tube containing mercury, and so made that one end can be fitted to a gas jet (Fig. 52). When the gas cock is closed, the mercury stands at the same level in both arms, but when the cock is opened, the gas whose pressure is being measured forces the mercury up the opposite arm. If the pressure of the gas is small, the mercury changes its level but very little. It is clear that the height of a column of mercury is a measure of the gas pressure. Now it is known that one cubic inch of mercury weighs about half a pound. Hence a column of mercury one inch high indicates a pressure of about one half pound to the square inch; a column two inches high indicates a pressure of about one pound to the square inch, and so on.
This is a very convenient way to measure the pressure of the illuminating gas in our homes and offices. The gauge is attached to the gas burner and the pressure is read by means of a scale attached to the gauge. (See Laboratory Manual.)
In order to have satisfactory illumination, the pressure must be strong enough to give a steady, broad flame. If the flame from any gas jet is flickering and weak, it is usually an indication of insufficient pressure and the gas company should investigate conditions and see to it that the consumer receives his proper value.
87. The Gas Meter. Most householders are deeply interested in the actual amount of gas which they consume (gas is charged for according to the number of cubic feet used), and therefore they should be able to read the gas meter which indicates their consumption of gas. Such gas meters are furnished by the companies, and can be read easily.
