85.  Bicycle Tires.  We know very well that we cannot put more than a certain amount of water in a tube, but we know equally well that the amount of air which can be pumped into a bicycle or automobile tire depends largely upon our muscular energy.  A gallon of water remains a gallon of water and requires a perfectly definite amount of space, but air can be compressed and compressed, and made to occupy less and less space.  While it is true that air is easily compressed, it is also true that air is elastic and capable of very rapid and easy expansion.  If a puncture occurs in a tire, the compressed air escapes very quickly; that is, the compressed air within the tube has taken the first opportunity offered for expansion.

[Illustration:  FIG. 51.—­By squeezing the bulb, air is forced out of the nozzle.]

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.

[Illustration:  FIG. 52.—­A pressure gauge.]