VENTILATING 139 



feet long, two arms; and from 8 to n feet 

 long, three arms, etc. 



Capacity of Ventilating Apparatus. The 



capacity of ventilating apparatus depends 

 largely upon the size and method of manu- 

 facture, but the length of run is limited to 

 the torsional strength of the shafting. In 

 long lengths there is always more or less tor- 

 sion, so that the ventilators at the extreme 

 end do not open as wide as those close to 

 where the power is applied. This is of little 

 consequence in summer when the ventilators 

 are wide open, but in winter, when only 

 slight ventilation is required, it may result in 

 the sash at the end of the shaft not open- 

 ing at all and the ventilation will thus be un- 

 even and unsatisfactory. Moreover, the sash 

 are likely to be frozen down in winter and 

 the tendency for the shafting to twist is thus 

 increased. It is wise to have a wide margin 

 for safety. 



An indication of the length of shafting 

 that may be used with safety is given on 

 page 130. Tests show that one and a 

 fourth-inch standard pipe has a torsional 

 strength 42 per cent, greater than i-inch 



