98 GREENHOUSE CONSTRUCTION AND HEATING. 
grouted in with thin cement and sand, is sometimes placed 
over the concrete, but this is not always necessary— 
much depends upon the character and solidity of the 
natural bottom. 
The position of the tank, or tanks, in a plant-house of 
any kind is a matter of some moment, and needs a little 
consideration. As a rule, the larger they are the better, 
for the longer and more the water is exposed to the action 
of the atmosphere the better, and for the same reason 
open tanks should be preferred to such as are closed, or 
built underground. specially where hard water from 
a well or spring has to be employed to any extent, the 
tanks should be as large as possible, such water being: 
injurious unless well softened and chilled by exposure to 
the air. At the same time if must be borne in mind that 
very large tanks occupy a considerable area, and unless 
covered, at least in part, they reduce the space available 
for plants, etc. 
One large nurseryman in Kent, whose houses, low 
span-roofed structures, each 100ft. by 12ft., are constructed 
side by side in extensive blocks, has two concrete tanks, 
about 61t. wide by 4ft. or 6ft. in depth, constructed 
beneath each block, running at right angles to the houses, 
and placed at about 25ft. from each end. The water 
from the roofs of the houses is conducted by means of 
the wooden gutters described on p. 58 into these tanks, 
and access is obtained to them by means of trap-doors in 
the pathway of each house, but otherwise they are closed 
entirely in. 
In another block of six similar, but wider houses, 
designed by the writer, there was a 6ft. by 5ft. tank, open 
