70 



GREENHOUSES 



the following reasons: (i) They are likely to 

 expand and contract considerably with 

 changes in temperature, thus loosening and 



A. 



. K G. H. 



Fig. 39. Types of wood sash-bars. E, F, and H are used 

 for butted glazing; G is used for double glazing 



often breaking the glass. (2) The extreme 

 cold to which they are subjected on the out- 

 side, as compared with the warm tempera- 

 ture on the inside of the house, has a ten- 

 dency to cause them to warp and thus break 

 the glass or cause it to fit poorly. (3) As all 

 metals are ready conductors of heat, much is 

 lost by radiation when they are used. (4) In 



