112 GREENHOUSES 



it difficult to lay a tight roof. If this trouble 

 is experienced, it will be of advantage to sort 

 the glass and lay out each run on a smooth 

 floor, placing the panes having a similar de- 

 gree of curvature in the same run. By doing 

 this a tighter and more satisfactory roof can 

 be laid. 



Theoretically, the glass will resist more 

 pressure if it is placed so that the curve will 

 be up, that is, so that it will present a convex 

 surface to the weather. If, on the other 

 hand, it is placed so as to present a concave 

 surface to the weather, the water will have 

 a tendency to flow away from the sash bars 

 and putty to the center of the runs. In ac- 

 tual practice, these are relatively unimport- 

 ant considerations, but all glass in the same 

 run should have approximately the same 

 curvature. 



Liquid Putty. This is sometimes used for 

 sealing cracks in old glazing or in glazing by 

 the "butted" method. It may be made as 

 follows: Take equal parts by measure of 

 white lead, putty and boiled linseed oil. 

 First, mix the putty and oil thoroughly and 

 then add the lead. If it becomes too thick, 

 thin with turpentine. 



