180 GREENHOUSE CONSTRUCTION AND HEATING. 
directly to the fire, and to each square foot of such surface 
allowing 50fé. run of 4in. piping. But as few if any boilers 
will heat this quantity properly except under the most 
favourable conditions all round, this estimate, which is 
known by engineers as the approximate heating power, 
is practically excessive, and not more than about two-thirds 
of this amount of piping, or about 32ft. instead of 50ft., 
to each square foot of boiler surface, can be safely allowed. 
This is termed by engineers the actual heating power. And 
even then the boiler ought in all cases to be well above 
its work. 
The plain saddle is a 
very favourite type of 
boiler, and for heating 
moderate lengths of 
piping, say from 150ft. 
to about 1200ft., is still 
one of the best and most 
Fig. 119. effective. This boiler, 
which is made in various 
sizes from 18in. in length by 10in. by 1lin. (inside the arch) 
up to 5ft. or 6ft. in length, and proportionately wide and deep, 
is shown in Fig. 119. If properly set this boiler is fairly 
economical in fuel, as well as easily managed, and the 
construction being quite simple, they cannot well get out 
of order. This class of boiler will also consume almost 
anything in the way of fuel ; coke in moderate sized pieces, 
or anthracite coal, should be used when the full heat is 
required, but for banking up at night, and on warm days, 
cinders, dusty coke or coal, ete., may be freely employed. 
The setting of all boilers of this type is a matter of the 
