66 AGRICULTURE. 



(c) Gradually pour water into the center of the keg — 

 where the soil should be, perhaps, a little lower — until it 

 runs out of some one of the holes. 



According to your result, which would carry off the 

 water first — a shallow or a deep drain ? 



In shallow drains there is danger that the tile 

 may be injured by frost. The depth to which 

 a drain should be laid depends upon the char- 

 acter of the soil; the more compact soil requires 

 more numerous and shallower drains. Three or 

 four feet deep and one hundred feet apart are 

 sufficient for ordinary farm crops. " The carry- 

 ing capacity of tile varies with the square of the 

 diameter." ''^' In every drain the tile should in- 

 crease in size as the quantity of water increases. 

 Tile varying from three to six inches, with 

 larger size for mains, are generally used. 



Since tile-drains admit more or less atmos- 

 pheric air, as the temperature and pressure 

 of the atmosphere rise and fall, the circula- 

 tion of the air is produced below the roots of 

 plants as well as above them. 



II. Irrigation. 



Experiment 13. — (a) Procure a box about three feet 

 long, one and one-half wide, and one foot deep. 



(d) In the center of one side, near the bottom, bore a 

 hole, and fit into it a cork (Fig. 20). 



{c) Nearly fill the box with dry, pulverized soil, and 

 shake it down well. 



(d) Now make a shallow trench in the soil, across the 



Soils a7td Crops, Morrow and Hunt, p. 66. 



