44 GREENHOUSE CONSTRUCTION AND HEATING. 
‘courses’ make just a foot of wall in height. Thus, a 
piece of Yin. wall, 50{t. in length by 3ft. high, would require 
50 x 4 = 662 or, say, 67 bricks in a single row ; multiply 
this by 12, the number of courses, and again by 2 for the 
double thickness, and the result is 1,604 bricks. Of course, 
a small allowance must always be made for waste and 
breakage. There are several different ways of laying bricks 
in a Qin. wall. The ‘‘old English bond” consists of a 
“header” and a couple of “stretchers” laid alternately in 
every course, but the ‘‘ stretchers’ in one course must come 
exactly over the ‘‘headers” in the one below. Then there 
is the Flemish bond, the Yorkshire bond, and others ; but 
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Fra. 28. 
for greenhouse work as good a method as any is to lay 
alternate courses of ‘‘headers”’ and “stretchers,’”’ but always 
making the top course one of ‘‘ headers,” and, if possible, 
using a little cement in the last course or two, to make a 
sound job. In all brickwork, a joint in one course must 
never occur exactly over one in the next below. 
It is, however, necessary to have a ‘‘footing’’ of some 
kind before commencing a wall of either brick or stone. 
This may consist of one, two, or more courses of rough 
bricks laid in a shallow trench, each being a brick wider 
than the one above it (see Fig. 28). Concrete footings 
made with rough gravel, stones, or ‘‘ ballast ’’ (burnt clay), 
