164 



GENERAL SCIENCE 



STEAM GAUGE 



other objects. Since they are so numerous and strike 

 against the wall of the vessel containing them, they 

 produce what is called pressure. It is like a continuous 

 pushing against a wall. If a stream of water from a 

 hose is directed against a wall, it exerts a continuous 

 force due to the water molecules 

 striking the wall. The air exerts a 

 pressure of 15 pounds per square 

 inch against the walls of a room at 

 sea level. Within the tube of an 

 automobile tire the molecules of air 

 may produce a pressure of 100 

 pounds or more per square inch, 

 and in a steam engine the pressure 

 of the steam may be 200 pounds per 

 square inch. The steam made from 

 one cubic centimeter of water will 

 occupy 1600 cubic centimeters of 

 space when under about 15 pounds 

 pressure. So in steam the mole- 

 cules are 1600 times as far apart as 

 they are in water and are moving 

 much faster than they do in water. 

 Steam engines have a steam gauge 

 to measure the molecular pressure of the steam. There is 

 a pressure gauge on the carbon dioxide tank of the chemi- 

 cal wagon of fire companies. These gauges in principle 

 are very much like an aneroid barometer. Mercury 

 gauges are also sometimes used; in these the gas lifts a 

 column of mercury. Every two inches of the mercury 

 column is approximately equal to one pound of pressure 

 per square inch. The object in measuring the pressure 

 of gases is to prevent explosions. 



INTERIOR OF STEAM 

 GAUGE 



