102 



VETERINARY HYGIENE 



of construction may be zinc, galvanised or black iron. The com- 

 bined sectional area of the shafts must be designed to accord with 

 the required air changes previously indicated. The following 

 points should be noted : The greater the length of the shaft the 

 greater is its extracting power. Bends in shafts retard the air 

 current, but where they are unavoidable the diameter at the bend 

 should be increased. Shafts should not be exposed to the open air 

 more than is necessary, otherwise, in cold weather, the extracting 

 power will be retarded. 



Extraction Cozvls or Ventilators. A variety of extractors are 

 made and sold as proprietary articles. The qualities claimed for 

 them are that they prevent down-draught and accentuate the aspir- 



FIG. 43. Illustration and section of a Louvre-Board Air Outlet Box 

 fitted to the roof ridge. 



ating action in the duct or shaft which they surmount. Many 

 examples of these " extractors " may be seen in the Architects' 

 Compendium. 



Many exotic forms of terminal ventilators may be found on 

 buildings, such as pivoted cowls, lobster backs, " old wives," revolv- 

 ing cowls and cowls fitted with gas jets or hot water pipes. Some 

 are effective, but most are to be condemned in consequence of the 

 amount of nursing they require to keep them in life. 



Louvre-board Ventilators. This type of air outlet terminal is 

 very old but under many conditions, if properly constructed, is 

 good and effective. Probably the neglect of this ventilator, com- 

 bined with its improper position or design, is the cause of its fre- 

 quent condemnation. It consists of any form of covered frame or 

 box placed on the ridge of a roof, the sides of the box or framing 

 being filled with a series of superimposed sloping boards or metal 

 plates set at regular spaces in such manner as will permit of the 

 escape of foul air and prevent the ingress of rain. The louvre- 

 boards should be set at an angle of 50 or 60 degrees to the horizontal. 

 By the relative proportion of the width of board and the spacing of 



