HEATING BY HOT WATER. 193 
of piping up to 100ft. or 200ft., these boilers ave unsuitable, 
as a rule, for any large or heavy work, and are consequently 
only made in the smaller sizes. 
Decidedly the best and most effective of this class of 
boiler, at any rate for comparatively 
small quantities of piping, is, in my 
opinion, that known almost indiffer- 
ently as the ‘‘Gem,” ‘Star,’ and 
‘* Victor” boiler, one of which (with 
top-feeder) is illustrated in Fig. 183. 
These consist of a double cylindrical 
shell, placed Qin. or 3in. apart, and 
forming a thin circular, vertical water- 
jacket surrounding the fire, the 
cylinder being either straight or 
conical in form, the latter by prefer- 
ence. There is a cover forming a 
feed-door at the top, with an exit 
into an iron smoke-pipe, and a set 
of fire-bars, a small ashpit, and a 
couple of close sliding doors with 
one or more regulators, by means 
‘of which the draught may be 
regulated to a nicety, at the bottom. 
The furnace works on what is known 
as the ‘‘slow combustion ”’ principle, Fic. 133. 
the draught being almost cut off at 
the bottom when the fire is once well alight, and a 
steady and regular heat is thus afforded for several hours, 
until the fire burns low, when it must be replenished, 
drawn up a little, and the draught again nearly stopped. 
fc) 
