Propagation by Cuttings. IgI 
where. In order that the glass may not be far above the 
plants, the side walls should not exceed five feet in height. 
These may be of any durable material, but a wall of brick, 
ten inches thick, with a two-inch air space in the center, is 
to be preferred, since it best economizes heat. The furnace 
and potting rooms obstruct the light least, and afford the 
most protection when located so as to form the north wall 
of the house. In houses extending north and south,*the 
south end is usually glazed above the height of the side 
walls. 
367. Heating Devices for the Greenhouse are of 
various kinds. The “smoke flue” is simplest, and cheapest 
in first cost. It consists of a flue extending from the far- 
nace, which is placed somewhat below the floor level, length- 
wise through the house, preferably rising gradually to a 
chimney at the opposite end. Or the flue may cross the 
farther end of the house and return at the other side, to a 
chimney built directly upon the furnace. The latter method 
usually gives better draft, since the warmth from the fur- 
nace stimulates an upward current of air through the 
chimney. The flue should be of brick for the first 25 feet 
from the furnace, as a safeguard from fire. After this, it 
may be of cement or vitrified drain pipe. 
Greenhouses of the better class are now almost invariably 
heated with steam or hot water, or with a combination of the 
two. Pipes from a boiler, located beneath the floor level, 
extend nearly horizontally about the house, beneath the 
benches, returning to the boiler; or the main feed pipe ex- 
tends overhead to the farther end of the house, where it 
connects with a system of return pipes beneath the benches. 
While the steam- or hot-water heating costs much more at 
