160 GREENHOUSE CONSTRUCTION AND HEATING. 
of the same size down each side of the central pathway, as 
shown in Fig. 106. In a large nursery in Kent, where the 
houses, each 100ft. by 12ft., may be counted by the score, 
nearly the whole are heated by means of a single 2in. flow 
along each side, with a 4in. return to each, placed low down 
in the pathway, near the ground. With plenty of boiler- 
power even this small amount of piping suffices to maintain 
a genial temperature, sufficient for any of the usual run of 
greenhouse plants, in almost any weather. 
Fig. 106. 
But in wide structures, where a high temperature has 
to be maintained, it may be necessary to have four, six, 
or even eight rows of piping on each side. Cases in which 
as many as sixteen rows of 4in. piping have been fixed 
in market-growers’ forcing-houses, from 12ft. to 15ft. in 
width only, have come under my notice; in such cases 
they are usually placed under the (raised) staging, in 
double sets of one flow and two or three returns, on each 
side, or in part beneath covered beds (for bottom-heat), 
and partly exposed. The best arrangement of the pipes 
Me 
