134 GREENHOUSES 



to the shaft as in the method of actually 

 opening the ventilators. The chief advan- 

 tage of this system lies in the fact that there 

 is less torsional or twisting strain on the 

 shafting than when the usual method is em- 

 ployed, and they are more powerful. The 

 chief disadvantage is that provision must be 

 made for giving the shaft several revolutions, 

 while a half or two-thirds revolution is usual- 

 ly sufficient with the more common forms. 



Some practical growers claim that the rack 

 and pinion device is very subject to wear and 

 is a frequent cause of trouble. This is more 

 especially true of the older forms of this type. 

 The fact that they are not generally used 

 would seem to indicate that practical growers 

 as a rule are not yet convinced of their super- 

 iority, though they are now being installed in 

 some large houses where it is necessary to 

 operate long runs. 



Quite frequently the hand wheel and gear- 

 ing are fastened to the rafters or purlin posts 

 and no extra columns are required. 



Side Ventilating Machinery. The essen- 

 tial features of side operating machinery 

 are the same as for overhead ventilators. 

 When there are side benches a shaft is 



