CONSTRUCTION AND HEATING OF GLASS HOUSES. 67 



before connection. A large proportion of such joints, under the 

 varied changes and strains to which they are subjected, will open, 

 and, be it ever so little, it is fatal to that part of the roof, as 

 moisture will accumulate in the mortise, remain there, and the 

 result will be as I have stated, — premature decay. 



The next, and I believe the last, feature of the woodwork, 

 excepting the doorposts, which differs from the construction 

 already described, is the piece known as the roof ventilator 

 header ; it is really the roof bar header. The section of this is 

 shown in Fig. 2. It is securely screwed to the purlin, leaving 

 about one-fourth to three-eighths of an inch of surface on the 

 lower edge of the purlin for the support of the upper ends of the 

 roof bars, as before described. It is generally from this purlin 

 that all drip of condensation formed upon the roof-vent accumu- 

 lates, and falls to the plants below, causing considerable damage, 

 and it is to avoid this annoying drip that we use the peculiar shape 

 of header here shown. The object of the rabbet or groove on the 

 upper side is to collect all drip from the lower rail of the ventilator 

 and by means of small channels cut through to the outer air to 

 carry such water onto the roof. One would expect these channels 

 to become frozen up and useless in severe weather, but such is not 

 the case. The little current of warm air which is allowed to 

 escape through these grooves prevents them from becoming 

 clogged with ice. 



Glass Houses fok Private Estates. 



I think I have considered the construction of commercial houses 

 sufficiently for the present, and as the glazing, painting, heating, 

 and ventilating of both commercial and private structures are the 

 same, I will consider them later under their separate heads, and 

 will now pass to the construction of glass houses designed and 

 arranged for use on private estates. 



The construction of such houses differs very slightly from that 

 of the iron frame commercial house just described, the principal 

 difference lying in the fact that they are usually built upon brick 

 or stone walls instead of having posts set in the ground, with the 

 double-boarded sides. 



In the past many styles of both solid and hollow walls have 

 been tried, but it is now an established fact that the only wall 

 suitable for greenhouse foundations is a solid one from twelve to 

 sixteen inches thick, according to the weight or thrust it is 



