196 GREENHOUSE CONSTRUCTION AND HEATING. 
whereas a ‘Star’ boiler 36in. in height will only heat 
about 200ft. of 4in. piping properly, a ‘‘dome-top” of 
the same height will do something like 450ft. The 
flow socket is placed on the top of the dome, and the 
return, or returns, at the bottom, at the back, or on one 
or both sides. This boiler is constructed to heat from 
about 150ft. up to fully 1,000ft. of 4in. piping, but I 
should scarcely recommend this form where more than 
about 500ft. or 600ft. have to be heated. 
Another form of these upright cylindrical boilers consists. 
of a double cylinder of sheet iron in the upper part, and 
a small coil, similar to those previously described, fixed 
in the lower part, with the fire inside it. Though useful. 
for heating very small houses these are decidedly inferior 
to the ‘Star’ or ‘‘Gem”’ boilers, especially as the thin 
metal casing soon burns through. Coils ought always to 
be set in brickwork. 
A new and distinct form of independent boiler, known 
as the ‘‘ Twelve Hours’ Syndicate,’ has recently been 
introduced, and deserves mention. One of these boilers 
is shown in Fig. 136, and will be seen to consist of a 
kind of double circular furnace, with a water-way all 
round both, at the base, and a hopper, or top-feeder, 
placed on the first and larger one, the smoke-pipe being 
fixed on the second or smaller cylinder. The fire is 
contained in the first or larger part, and there is a grating 
between the two through which the draught passes into 
the back chamber, and thence up the smoke-pipe. The 
draught is regulated by means of the (sliding) fire, and 
ash-pit doors in front, and also by means of a small damper, 
or door, placed on one side of the back chamber, which, 
