SOIL PHYSICS 



463 



takes place in the early morning and the forenoon. Thus we 

 see that the earth takes one long, slow breath each day. 



543. Effect of Pressure upon the Volume of Air. But the 

 change in the soil temperature is not the only cause of the earth's 

 breathing.. Changes in the pressure of the atmosphere, 

 ATMOSPHERIC PRESSURE (Chap. IV, Sec. II) , also affects the 

 soil-air. 



It is easily proved in every high school laboratory that in- 

 creasing the pressure upon a certain quantity of air decreases 

 the volume of the air. In fact, it was shown by an English 

 philosopher, Robert Boyle, 

 about 1650, that, if the 

 pressure was doubled, the 

 volume of the gas was re- 

 duced one-half; if the pres- 

 sure was made four times as 

 great, the volume became 

 one-quarter its former 



Fl0 ' 283 '- 



upon 



value. In general, the 

 volume of a gas decreases 



just as rapidly as the pressure increases. Figure 283 makes 

 plain the operation of this law, which is known as BOYLE'S 

 LAW. We shall suppose that No. 1 represents a tube 4 ft. 

 long. At its top, fitting into it air-tight, is a piston. Suppose 

 that all of the atmospheric pressure is removed from the 

 piston and simply a 1-lb. weight is placed upon it. Evidently 

 the 4 ft. of air in the tube is supporting 1 Ib. of weight. If the 

 pressure be increased to 2 Ib., the volume of gas will be com- 

 pressed to a column 2 ft. in length, as in No. 2. If 4 Ib. of 

 pressure be placed upon the air as in No 3, the column will 

 become 1 ft. And so 8 Ib., 16 Ib., 32 Ib., 64 Ib., and 128 Ib., 

 will compress the air to 6 in., 3 in., 1J^ in., % m v and % in., 

 respectively. 



544. Changes in Atmospheric Pressure Cause the Earth 

 to Breathe. We have seen (Chap. IV) that the ATMOSPHERIC 

 PRESSURE varies considerably from day to day, and that it even 



