38 THE CONSTRUCTION OF THE FORCING-HOUSE. 



smooth job. After considerable experience with butted 

 elass, the writer has abandoned it. It is practically im- 

 possible, with any ordinary grade of 

 glass, to make a perfectly smooth joint 

 between the panes, and at 

 every irregularity or rough- 

 ness at the joints the water 

 will collect and drip off. 

 This difficulty is particu- 



. Mortice-joint at the ridge ^ liable tO OCCUr if P aneS 



are used which are over 



twelve or fourteen inches wide. It is rare, also, that the 

 panes are squarely enough cut to make perfectly tight 

 joints possible. Another serious objection to butted glass 

 is the fact that all the water of condensation which does 

 not fall as drip is carried down upon the plate, keeping it 

 constantly wet and tending to make it decay. The drip 

 from the plate is often a serious nuisance, particularly if 

 there are heating pipes directly beneath from which a con- 

 stant shower of vapor arises. In lapped glass, the con- 

 densed water follows down the pane and passes out through 

 the lap onto the roof. If glass is to be butted, only the 

 double-thick should be used. The single-thick grade is 

 too irregular and uneven in thickness and curvaturt to allow 

 of making good joints ; and 

 its very thinness makes it 

 impossible to secure suffi- 

 cient contact to 

 make a tight job. 

 In the lapping 

 of glass, the panes 

 are bedded in soft 

 putty, and are 

 then held from 

 slipping down by 

 a straight shoe- 

 nail at each lower ,, Plain J0tnts at lhe ,, dge 



