128 GREENHOUSES 



hinge the sash at the ridge and in houses 30 

 feet wide or more to provide ventilators on 

 both sides of the roof. 



Operating Machinery. Since the ven- 

 tilating sash are placed at the highest part 

 of the house, and as it is necessary to change 

 the size of the opening several times a day, 

 it is obvious that it is highly desirable that 

 some method be provided by which they may 

 all be opened and closed from some point 

 convenient for the operator. This is accom- 

 plished by means of various types of sash- 

 operating machiney. 



The essential features on which most 

 types of ventilating machinery depend are 

 as follows: (i) A horizontal shaft firmly 

 fastened near the line of ventilating sash; 



(2) a system of gearing, by which power ap- 

 plied at a point convenient to the operator 

 may be transmitted to and rotate this shaft; 



(3) arms or levers attached to the shaft and 

 also to the sash, and so arranged that the 

 sash are raised or lowered when the shaft 

 is rotated. 



Shafting. The shafting generally used is 

 one inch or one and a fourth inch gaspipe. 

 The lengths are either riveted or clamped 



