184 GREENHOUSE CONSTRUCTION AND HEATING. 
or 3,000ft. Its construction being, however, some- 
what complicated, renders it a rather more expensive 
boiler than some others. It is usually constructed 
to be fed through the crown, as shown, the fire-door 
being only used for stoking purposes, cleaning the 
bars, etc. It is best set as advised above, with narrow 
return flues and mid-feathers on each side, as shown 
in the illustration. 
The ‘‘Gold Medal” or ‘‘ Express” boiler (Fig. 125) 
is yet another form of the saddle, but with three 
flues through the crown 
instead of two, the 
draught passing forward 
through the larger central 
one first, and then 
returning through the two 
smaller ones on each 
side, to the chimney at 
back, Though a power- 
Fig. 126. ful and with some a 
favourite boiler, we con- 
sider this inferior to the Climax, as the flues in 
the crown being so small require frequent cleaning, 
while there is not so much surface exposed 
directly to the fire, and, lastly, without an unusually 
high chimney to increase the draught the considerable 
heating surface at the sides is wasted. This fault 
can be overcome by bringing the draught from the 
two side-flues in the boiler forward again by means 
of two other narrow ones, with or without mid-feathers 
constructed outside it, as shown in Fig. 120. But this 
