194 GREENHOUSE CONSTRUCTION AND HEATING. 
Boilers of this class are thus very regular in action, while 
consuming a comparatively small amount of fuel. 
These boilers are made in several sizes, calculated to 
heat from about 60ft. up to 200ft. or 300ft. of 4in. piping, 
but the very small sizes are not to be recommended, 
owing to the small body of fire requiring very frequent 
attention, and seldom lasting more than four or five hours. 
The largest sizes, again, are objectionable, owing chiefly 
to their great height, the most useful being those 
constructed to heat 
from 100ft. to 200ft. 
of piping, and even 
these ought always to 
be well above their 
work. A ‘‘top-feeder ” 
to hold extra fuel, is 
sometimes added to 
this form of boiler, and 
enables the fire to be 
kept alight for some 
Fic. 134. hours longer. As the 
fuel below is con- 
sumed, that in the top-feeder or hopper above drops down 
and takes its place. Coke broken small— into pieces 
about the size of hen’s eggs—forms much the best fuel 
for this class of boiler, though they may be banked up 
with cinders, etc. when the fire is to be kept low for a 
time. Coal is useless, and even the anthracite does not 
burn well in these small, close furnaces. 
No stoke-hole, or only a very shallow one, is required 
for these boilers, as a rule, as the flow connection may 
