No. 6. 



DEPARTMENT OF AGRICULTURE. 



627 



the moisture content of the air favorably: (a) Ventilation by air 

 forced through horizontal sub-earth ducts or deep vertical sub-earth 

 ducts and wells, (b) Ventilating over ice. (c) Evaporation of water. 

 Fig. B illustrates the construction of a horizontal sub-earth duct, 

 which should be twelve feet or more below the surface of the 

 ground and 100 feet or more in length. It is recommended that the 

 sub-earth duct of three rows of 10-inch drain tile laid side by 



^ 



Fig. D. Showing vertical sub-earth duet. A, brick chamber 2.5 to 30 feet below surface 

 and 40 inches inside diameter; tile or conductor pipe of galvanized iron: C. main 

 shaft of funnel ; D. brick chamber at upper end of duct. The circle and section rep- 

 resent a cast-iron plate to cover brick chamber A, and can be had of King & Walker. 

 Madison, Wis. 



(From Wis. Agr. Exp. Sta. Bui. 70.) 



