158 GREENHOUSE CONSTRUCTION AND HEATING. 
Wherever three rows of piping are placed together they 
should be arranged in a similar manner, for this reason, 
that with a single flow pipe the hot water is carried much 
more quickly to the 
S farther end of the 
2 house than in a 
| double one, while 
< the current flows 
back more slowly 
and easily through 
the double return pipes, and the heat is consequently 
more evenly distributed than it could be with a double flow 
.and single return, though this plan is not infrequently 
adopted. 
In houses of moderate width it is an excellent plan to 
carry the single flow pipe along the front or near the outside 
wall, and the single or double return in or near the path- 
way at the back of the bed or stage, 3ft., 4ft., or possibly 
a little farther still from the first. In heating a span-roofed 
structure of considerable size, all that is necessary is to 
double this system, 
placing one set on 
each side, as shown in 
Fig. 104, where the 
tworeturns are shown 
fixed one above the 
other on one side of 
the house, and side by 
side on the other. Of course, in such a case there must be 
two separate connections from the main flow pipe, each 
being fitted with a valve to open or close the current at will. 
Fie. 103. 
Fia. 104. 
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