PLANTS, WITH GLASS ROOFS. 219 



liable is the glass to breakage when the water so retained becomes frozen. 

 This lap is sometimes entirely, and sometimes partially, rendered air 

 and water tight by putty. In the former case it prevents the water which 

 condenses on the inside of the glass from escaping to the outside ; and in the 

 latter, while it allows the condensed water to escape, it also retains, by the 

 attraction of cohesion, as much as fills the space between the lap ; and this 

 water in severe weather is apt to freeze, and by its expansion when under- 

 going that operation, the glass is broken. By having the laps uuputtied, not 

 only is there great danger from 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 water- proof ; 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 joinings of the glass, 

 both the under lap and the over lap, and also the putty between, in the fol- 

 lowing manner : Let the upper edge of the paint on both sides of the lap 

 run in the direction of d, e, in fig. 155, thus directing all the water which 



Fig. 155. Lap of glass panes puttied and painted. 



condenses on the inside or falls on the outside down the centre of the squares. 

 The only disadvantage attending close-puttying the lap is, that the con- 

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

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

 manner w r ill 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. Rogers, viz., to fix at places where the drip will 

 do no injury, small pieces of cobblers' wax or putty, which, by interrupting 

 the descendant 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 Mr. 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 reabsorbed by the air which deposited it, somewhat in 

 the same manner that takes place in growing plants in closed 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 sus- 

 pension in the form of elastic vapours. 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- 



