AIR 



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. 



85. The Danger of Compression. Air under ordinary atmos- 

 pheric 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 correspond- 

 ingly greater. If too much air is blown into a toy balloon, 

 the balloon bursts because it cannot support the great pres- 

 sure exerted by the compressed air within. What is true of 

 air is true of all gases. Dangerous boiler explosions some- 

 times occur because the walls were not strong enough to with- 

 stand the pressure of the steam (which is water in the form 

 of gas). The pressure within the boilers 

 of engines is frequently as much as 240 

 pounds to the square inch, and such a 

 high pressure needs a boiler of strong 

 construction. 



86. How Pressure is Measured in 

 Buildings. It was made clear in the pre- 

 ceding Section that undue pressure of 

 a gas may cause explosion. It is im- 

 portant, 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. 



Pressure in a gas pipe is measured in 

 the following way by a very simple instrument called the 

 pressure gauge : The gauge consists of a bent glass tube 



FIG. 52. A pressure 

 gauge. 



