a 
164 GREENHOUSE CONSTRUCTION AND HEATING. 
of the water in the right direction. A general idea of such 
a system may be gathered from Fig. 108. 
In some few instances it is found impossible to get the 
boiler down sufficiently below the level of the pipes, owing 
to water being found at a short distance below the 
surface. Methods of ‘surmounting this difficulty are 
described under the heading of stoke-holes (p. 215), but 
supposing it to be found impossible, or inadvisable, to 
construct a water-tight hole, the only alternatives are 
to employ a boiler of shallow construction, such as the 
form of the Champion (tubular) boiler (made by the 
Thames Bank Iron Co.), the ‘‘Reliance” boiler (Temple- 
man’s patent), which is illustrated in Fig. 130, or something 
of the kind, with only a very slight rise out of the crown 
of the boiler into the pipes, such as that afforded by 
an ordinary elbow. I have both seen and had to use 
saddle and other horizontal boilers made with the flow 
connection issuing from one end or side of the crown, on 
the level, without any rise at all into the pipes, but though 
this may answer in the case of a deep or vertical type 
‘of boiler, I can, from experience, strongly advise the 
‘avoidance of any such arrangement, owing to the extreme 
slowness of the current, and the generally unsatisfactory 
results obtained. 
But where, from any cause, the boilers cannot be 
sunk deeply enough, and even the shallowest make is 
inadmissible, the only way out of the difficulty is to heat 
from a higher level, on what may be termed the ‘‘ return ” 
system of heating. In this the boiler is fixed as low down 
as can be conveniently done, and the flow is led from 
it almost directly into a raised tank, or circulating cistern, 
