132 



GREENHOUSES 



ated gear; and (3) the rack and pinion gear. 

 In the column gear a post or column sup- 

 ports .the gearing and the wheel to which 

 the power is applied. One form of column 

 gear is known as an open column 

 gear, because the drive rod is not 

 inclosed in the column and there 

 is no housing about the gearing. 

 In another open column gear 

 type a chain is used to transmit 

 the power. In the closed col- 

 umn types all gearing is inclosed 

 and runs in oil, much the same as 

 in the transmission case of an 

 automobile. This insures free- 

 dom from noise and ease of 

 operation. 



In the chain type no columns 

 are required, a feature much 

 prized by growers. By this sys- 

 tem practically all the ventilators 

 in a house may be operated from 

 one point, as the chains may be 

 run almost anywhere in the house 

 by the use of pulleys. The ab- ator 

 sence of columns means less shade. 



The rack and pinion type differs from 

 the two general types mentioned above, not 

 so much in the method of applying the power 



Fig. 73.- 



