38 THE CONSTRUCTION OF THE FORCING-HOUSE. 
smooth job. After considerable experience with butted 
glass, the writer has abandoned it. It is practically im- 
5 possible, with any ordinary grade of 
glass, to make a perfectly smooth joint 
between the panes, and at 
every irregularity or rough- 
SSS ness at the joints the water 
S will collect and drip off. 
This difficulty is particu- 
larly liable to occur if panes 
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 curvature to allow 
of making good joints ; and 
its very thinness makes it —— Ss 
impossible to secure suffi- ] 
cient contact to 4 
make a tight job. BE pS 
In the lapping BSS 
of glass, the panes <= 
ey 
Li 
then held vm >_ 
slipping down by 
a straight shoe- ZA" == 
nail at each lower 23. Flain joints at the ridge. 
22. Mortice-joint at the ridge 
