PLANTS, WITH GLASS ROOFS. 188 



rom breakage by frost, but much heated air escapes during cold 

 weather, and rain is apt to be blown into the house during high winds 

 in certain directions. It is better, therefore, in the opinion of most 

 scientific gardeners, to putty the laps and render them waterproof; to 

 accomplish which in an efficient and economical manner, Mr. Forsyth 

 proposes a lap three-eighths of an inch broad (in our opinion a greater 

 breadth than is necessary), with the space between filled in with 

 soft putty in the usual manner, and then carefully to paint the join- 

 ings of the glass, both the under lap and the over lap, and also 

 the putty between, in the following manner : Let the upper edge of 

 the paint on both sides of the lap run in the direction of d, c, in 

 fig. 150, thus directing all the water which condenses on the inside or 

 falls on the outside down the centre of the squares. The only dis- 

 advantage attending close-puttying the lap is, that the condensed 

 water, when the roof is very flat, sometimes drops on the plants ; but 



Fig. 150. 



Lap of glass panes puttied and painted. 



if the house is kept at a proper temperature, the water that drops in 

 this manner will do little injury, and will be speedily taken up by the 

 dry air which has just parted from it. In particular cases, where the 

 drip falls on a plant, it may be directed to a point where it will do no 

 injury, by a simple process pointed out by Mr. Kogers viz., to fix at 

 places where the drip will do no injury, small pieces of cobblers' wax or 

 putty, which, by interrupting the descending current, will cause it to 

 drop down. The drip, however, is much more common from the 

 bars between the glass than from the glass itself, and to these Mi*. 

 Rogers's plan is peculiarly applicable. One great argument for 

 puttying the laps is, that the moisture of the atmosphere, though it 

 may be condensed on the glass, is not, if proper means are taken to 

 retain it at the bottom of the sloping glass, allowed to escape from the 

 house, but must be re-absorbed by the air which deposited it, some- 

 what in the same manner that takes place in growing plants in close 

 glass cases. These cases being air-tight, when the temperature within 

 is greater than that without, moisture is deposited on the glass, and 

 after some time runs down and settles along the inside of the rim ; 

 whence, when the temperature within is raised to the same height as 

 before, it is again taken up and held in suspension in the form of 

 elastic vapour. In the case of air-tight stoves, nearly the same process 

 must be constantly going on; but few have hitherto been built 

 sufficiently air-tight for this purpose. One of the greatest improve- 

 ments that have taken place in the glazing of plant-structures of every 



