18 



and sun heat, yet the case is different; the sun's rays 

 have then much greater power, and it is found that their 

 influence is sufficient, without at all times admitting them 

 directly on the plants growing in these artificial atmos- 

 pheres. 



The position of the Cucumber house, with reference 

 to the ground line, must be determined by local circum- 

 stances ; if the situation and sub-soil be dry, it may be 

 carried below the surface in the manner represented in 

 the annexed engraving, of which (a) is the ground line, 



