VENTILATING 137 



way. It folds together when the sash is 

 closed and occupies little space, but it ex- 

 tends automatically when the shaft is turned. 

 It is especially convenient under certain con- 

 ditions, but it lacks the strength necessary 

 for long runs. 



Fig. 78. Compact machine for operating side ventilators 



In all systems the arms are clamped se- 

 curely and rigidly to the shafting, and as 

 near as possible to the hangers so as not to 

 spring the shafting when heavily loaded. 

 They are spaced about 3 feet apart along the 

 sash. If continuous sash are not used the 

 arms should be distributed as follows: For 

 sash up to 4 feet long, ojo^arm; from 4 to 7 



