38 GREENHOUSE CONSTRUCTION AND HEATING. 
hinged wooden flaps (usually the latter), to enable them to 
be opened or closed at will. (See Fig. 22.) This system has 
been almost universally adopted by the Worthing growers, 
but though inexpensive, and, to a certain extent, effective, 
the louvres possess several disadvantages. 
The usual size of these louvres is about 3ft. in length 
(equalling four bricks), by a depth or width of Yin. or 12in. 
(three or four courses respectively) over all, and in order to 
enable them to be fixed well above the ground level, the 
walls are usually built 3ft. or 4ft. in height. The “stuff” 
of which they are made should, of course, be of the same 
width as the thickness of the wall, z.¢., usually 9in., and if 
not built in at the time they may be fixed afterwards by 
means of wedges, filling and pointing them all round with 
cement subsequently. 
The disadvantages of the louvre are that when opened there 
is a draught of air opposite each, but little or no current in 
the spaces between ; secondly, they admit no light at all 
when closed, and very little when open ; thirdly, they have 
a very unsightly appearance, while lastly, where several 
long houses are fitted with them, the necessary opening and 
closing occupies a lot of time, and they cannot very well be 
fitted with gearing of any kind. 
There is yet another method of admitting air at the 
bottom or sides of a house—a kind of compromise between 
the two already described—and an excellent one where 
plenty of air is required, and yet it is desired to avoid the 
expense of sashes, etc., though in this case also a double 
‘plate’ is necessary. This is to fix a continuous row of 
comparatively narrow wooden flaps, hinged along their 
upper edges, in a suitable framework, on the top of the 
