326 JOUENAL OF THE EOYAL HORTICULTUKAL SOCIETY. 
For larger quantities than 2,000 ft. I very strongly recommend a steel 
Cornish or annular riveted boiler, and where there is a reasonably good 
draught water bars are a great saving of fuel. (Fig. 167.) 
Water bars may also with advantage be used with saddle and terminal 
boilers. With these water bars a large amount of heat is got when the 
fire is banked up, which would otherwise be lost or go to burn up ordinary 
metal bars ; for it must always be kept in mind that what is wanted is a 
furnace and boiler that will go on for eight or ten hours without 
attention. 
A very powerful sectional cast-iron boiler has been introduced into 
this country from America. I cannot say I am partial to cast iron for 
boilers, as, owing to the nature of the metal, it is much more liable to 
accident than malleable iron and steel. 
The heating of a single hothouse, or even two or three when close 
together, is a comparatively easy matter, but when there are a great 
many circulations of various lengths and of various heights, there is very 
great difficulty sometimes in getting tlie water to circulate in the longer 
circulations. As an example of this I may refer to the very large appara- 
tus in use at the gardens at Sandringham, the Norfolk home of H.E.H. 
the Prince of Wales. In this case some of the houses heated are over 
400 feet away from the boilers, while there are others close at hand. 
Large mains are carried underground in a built tunnel large enough for 
a man to creep through and examine the pipes when required — all 
underground pipes should be in tunnels. The difficulty in getting the 
circulations at the extreme ends of the system to heat is caused by the 
short-circuiting of the nearer circulations — the hot water takes the line 
of least resistance. It runs round one house before it can reach another 
more distant, and when the hot water from the nearer house arrives at a 
point about midway in the main return it begins to back up in the wrong 
direction instead of going on to the boiler, and causes more or less of an 
obstruction — '* a block." This may, in some cases, be so effective as to 
stop all circulation in the furthest off parts of the apparatus. And there 
is only one way of preventing it, and that is by regulating the valves in 
houses near the boiler. The valves should be very carefully watched 
until it is found how much or how little they must be open to prevent 
Fig. 167. — Circular Eiveted Steel Boiler. 
