172 



GENERAL SCIENCE 



215 cubic inches. The tidal air, or the air we inhale and exhale, 

 amounts to about 20 to 30 cubic inches. 



FIG. 135. 



FIG. 136. 



Amount of Air Breathed. An adult uses about 30 cubic inches 



of air at each breath. The aver- 

 age person breathes about twenty 

 times per minute. 



Fill a large bottle or cylindrical jar 

 with water and invert it in a large pan 

 of water. Place a rubber tube under 

 the mouth of the jar, as shown in Fig. 

 137. Take the deepest breath possible. 

 Place one end of the tube in the 

 mouth and force as much air as possible 

 out of the lungs into the bottle. De- 

 termine from this the number of cubic 

 inches of air forced into the bottle. 

 Such an instrument may be called a 

 spirometer, an instrument for measuring 

 lung capacity. 



By placing a strip of paper on the 

 side of the bottle, a scale may be drawn 

 so as to show the number of cubic 



