WALLS, FOUNDATIONS, ETC. 
43 
some other solid and impervious material. These retain both 
heat and moisture in a way that no wooden walls can do, 
however carefully constructed. 
Bricks are, of course, most com- 
monly employed for all work of 
this kind, but where stone is plenti- 
ful and cheap, it may be used with 
equally good results ; and concrete, 
composed of either coarse gravel 
or burnt ‘‘ballast””’ is also a very 
suitable material. 
Fic. 26, 
As a general rule, the walls of a greenhouse of any kind, 
even if of small dimensions only, should not be less than 
Yin. in thickness, i.e., consisting of two bricks laid length- 
wise of the wall, or what bricklayers term ‘‘nine-inch work ” 
(see Fig. 26). Very low walls, of not more than about 18in. 
in height may be built of a single brick in thickness, or 
‘(43in. work,’ but in this case cement mortar should be 
used, and a pier had better be put in every 
3ft. or so, either inside or outside. Walls 
of this thickness, thus strengthened with 
piers, may be as much as 2ft. in height, 
but if more than this, Yin. work should 
be employed throughout. If the piers are 
built outside, they may be very conveniently 
utilized to carry the guttering (see Fig. 
27). Where bricks are employed, it is 
easy to calculate the number required to | 
construct a wall of any given size and thickness. An ordinary 
brick, when set, measures as nearly as possible Qin. in 
length by 43in. in breadth, and 3in. deep, so that four 
