120 



BOARD OF AGRICULTURE. [Pub. Doc. 



there are certain general rules that should always be ob- 

 served. A round duct is preferable to a square one, as it 

 has a greater carrying ca})acity, there being no dead corners. 

 A smooth one is better than one that is rough, the velocity 

 of the current, all other conditions the s;ime, being greater 

 in the former than in the latter. 



To insure action, they should be as short and straight as it 

 is possible to have them. Those of too great length are 

 usually useless unless artificial heat be used in them to create 

 a circulation of air. Those placed on the south side of a 

 building, where they are exposed to the heat of the sun, are 

 more efficient than those placed on the north side. The in- 

 troduction of angles should be avoided as much as possible. 

 Each right angle put in reduces the velocity of the current 

 one-half (Fig. 2). When it becomes necessary, as it fre- 

 quently is, to change the direction, a rounded elbow may be 

 used to good advantaije, it being claimed that it will not 

 lessen the velocity of the current so much, there being no 

 square angle for the air to strike against. 



Every shaft or duct should be so constructed that it may 

 be easily cleaned in every part. Neglect of this precaution 

 often renders them useless. They soon 

 become stopped with collections of cob- 

 webs and dust (Fig. 3). 



There are many ways of using tubes, 

 either singly or in combinations, both as 

 inlets and outlets. They may be placed 

 under the floor, in the walls, or in the 

 ceilings. Parallel vertical tubes opening 

 on the ceiling above, leading to the outside 

 through the roof or cupola, are a frequent 

 form seen. One is supposed to act as an 

 inlet, the other as an outlet. The action 

 of this combination of tubes depends upon 

 existing conditions. 



If the building is occupied by a large 

 number of animals, so that the temperature of the air inside 

 is higher than that outside, all openings below closed, the- 

 walls of the ceilings not too high, the tube not too long, etc., 

 then both tubes would act as outlets. If the reverse of these 



Fig. 3. — Section of duct, 

 ehowing obstruction. 



