28 LIFE ON THE FARM. 



quite essential, too, thus to get rid of the surplus 

 water, especially during very wet springs and sum- 

 mers, for, besides opening up a means for the free 

 circulation of the air among the spaces left by the 

 vacated water, the soil is left warmer for early 

 germination and growth. In the discussion further 

 on, it will be shown that water which has to leave 

 the soil by evaporation at the surface makes the 

 ground so cold that the life-activities of plants are 

 retarded. 



EFFECT OF AIR PRESSURE. 



The pressure of the air upon the surface at" the 

 earth constantly varies. An instrument called the 

 barometer measures the pressure, and no two days 

 have exactly the same amount; in fact, there is 

 always a slight, and often a great, variation for two 

 hours of the same day. Various causes produce 

 these changes; the main one being the unequal 

 heating of the atmosphere. 



Now, when there is great air pressure, the water 

 in the ground, if there is any near enough the sur- 

 face to be affected, is pressed further down by the 

 added force, its place being taken by air, produc- 

 ing a downward current. When the pressure of 

 the air becomes less, the force of the confined 

 water presses it back, forcing the air with it, and 

 thus is produced an upward current. Not only 

 does this change take place from above, but in 

 many places from beneath through tile drains, and 



