46 GREENHOUSE CONSTRUCTION AND HEATING. 
less, brickwork will be found but little if any more expensive 
than the other. 
When there are any louvre boxes or ties for the plates 
(see p. 53) to be worked in, these should be thought of and 
placed in time, as any omissions of this kind means pulling 
the work to pieces and a lot of waste of time and labour 
afterwards. 
Concrete walls should also be made quite 10in. to 12in. in 
thickness—preferably the latter. The concrete is, of course, 
kept in place while setting by a lining of stout boards placed 
on edge on both sides, and supported by means of pieces of 
stout quartering and ties across the top. Where the soil is 
clayey, plenty of ‘‘ballast’’ may be made by burning a 
quantity of the former in a heap, with small coal or ‘‘slack,”’ 
and when burnt through and getting cool the whole 
| | | | { is screened, the 
[+] moderately coarse 
part being. mixed 
with plenty of 
Portland cement, 
or lime and cement 
mixed, and water, to 
form the concrete. 
The boards must 
not be removed 
until the concrete 
Fra. 29. is thoroughly set. 
Spaces for the doorways, etc., must be left beforehand 
when building withTconcrete or rough stone in particular, 
as neither of these materials can be knocked about and made 
good again like brickwork. 
v v v 
