134 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
or closed. The outlet flues must be air-tight and protected 
against cold in exposed places. The best material for the 
construction of these flues is galvanized iron (No. 28),2x 4 
inch lumber being used in the corners and joints. Where the 
shaft is exposed, the iron should be covered with %-inch boards. 
Outlets may also be constructed of a double layer of tongued 
and grooved boards with a layer of heavy building paper be- 
tween. If tin or galvanized iron pipes are used, they should 
be surrounded by a square wood frame and the interstices filled 
Fie. 10.—An outlet flue hinged at the ceiling so that it can be drawn up out of the way. 
in with sawdust. Protection against cold is necessary because 
the air in the outlet flue must be kept warm, otherwise it will 
cease to ascend. The ceiling and walls of the stable should be 
air-tight and should be constructed with a view to preventing 
the radiation of heat as much as possible. 
The circulation of air through the stable results from the 
operation of two factors, called by Professor King aeromotive 
forces, namely: heat generated by the cattle, and wind. The 
wind drives air through the inlets on the windward side of the 
stable and thus increases the air pressure within the stable, as a 
result of which air is forced out of the stable through the outlets. 
