140 GREENHOUSES 



double-strength pipe and that the weights 

 are practically the same. The price of i- 

 inch double-strength pipe averages about 25 

 per cent more than standard one and a fourth 

 inch pipe. It is evident, therefore, that for 

 long runs it is not only safer but more 

 economical to use one and a fourth-inch 

 standard pipe than i-inch double-strength. 



Generally speaking, a I5o-foot run is about 

 the limit when elbow arms are used. This 

 may be slightly increased by using the 

 double acting arms, and still further by us- 

 ing the rack and pinion system. This is 

 equivalent to saying that the ventilators in a 

 house 300 or 350 feet long may be operated 

 from one station by having machines located 

 in the center of the house and operating each 

 way. It is economy to have all ventilator 

 sash for one house operated from the same 

 station if possible. 



Sliding Shaft System. In order to enable 

 the operator to care for an extremely long 

 line of sash from one station a sliding shaft 

 system has been devised. In this case the 

 shafting is solid and square, and instead of 

 rotating it slides backward and forward, the 

 motion being given by a pinion working on 

 a screw or worm gear at one end of the shaft. 



