HEATING BY HOT WATER. 161 
depends of course upon the purpose for which the structure 
is required. If intended for forcing, propagating, etc., a 
portion are usually placed in hollow chambers beneath 
the bed, or beds, to provide bottom-heat, the rest being 
disposed in various ways outside, in order to afford the 
top or air-warmth. But this part of the subject will be 
dealt with a little farther on. 
To return to the plain circuit system. Where one row 
of piping is not sufficient it may be doubled, putting in 
a flow and return all round, or along three or even two 
sides of the house only, with an ordinary syphon (see Fig. 
145) connecting the two at the farthest end, where also an 
air-pipe .must be fixed. In some cases three rows of 
piping—a flow and two returns—may be carried round 
three or four sides of a house, with a three-way syphon 
at the end. But, as evident, this system is only applicable 
to houses with a door at one end only, as the pipes must 
pass across the farther end. The system of piping each 
side of a house of any magnitude separately is therefore 
usually adopted, and rightly so, especially where the 
structure is of considerable length, say 50ft. or more. 
Most green- or plant-houses are heated from one end, 
for obvious reasons, and when two or more structures are 
built contiguous, or adjoining, either end to end or side 
by side, the boiler and furnace are properly placed at 
the junction, or in the latter case at any convenient point 
between or near the ends, in order that the current may 
be conducted into the different houses as directly and 
nearly as possible. Valves must of course be placed on 
the (flow) connections into each house, or set of pipes, 
to enable the current to be cut off from or turned on to 
M 
