188 GREENHOUSE CONSTRUCTION AND HEATING. 
these boilers in pairs, with valves, etc., as described on 
p. 179, so that in case of a breakdown in one, the other 
can be put on to maintain the heat, in part if not entirely. 
The vertical tubular boiler (Fig. 128) is another powerful 
type, though more suitable for a smaller class of work 
than the last, while its form also renders it naturally more 
extravagant in fuel. Fig. 129 shows a small boiler of this 
type fitted with water-bars. 
There are several other forms of what may be termed 
Jjixed boilers (i.e., such as require to be set in brickwork), 
but with one or two exceptions it is hardly necessary to 
' refer to them here, as they are scarcely 
ever employed at the present day, and 
are indeed almost obsolete. 
A boiler known as Cannell’s Cir- 
culator may be briefly referred to as 
one of the exceptions above. In this 
the draught from the fire, which is 
placed at the bottom, in an ordinary 
Fic. 129. furnace, is made to traverse a series 
of wide but very shallow flues arranged 
between thin layers of ‘‘ water-way,” passing backwards 
and forwards several times. It is cast in flat sections, 
each consisting of a water-space and flue-space, with 
lugs on both sides of each section, both to enable 
the sections to be bolted together, and to provide a 
passage for the water to flow from one section to the 
next. This boiler is, or was, exclusively employed 
in the large establishment of the inventor at Swanley, 
but it does not appear to have ever become at all 
extensively adopted. 
