204 GREENHOUSE CONSTRUCTION AND HEATING. 
feet from it, especially where the furnace is large, and 
it is a very usual and good plan to make a hotbed of 
sand over the furnace, for propagating purposes. Tiles 
about 1din, thick, set on edge, are sometimes employed 
-in building flues, but these make a very flimsy job, to 
my mind, and bricks (good kiln or pressed bricks are best), 
which if they do not transmit the heat quite so quickly, 
retain it longer when once heated, are decidedly preferable. 
A bottom, or bed, of flat bricks or tiles must be put 
down first, of course, and should be about 18in. wide. 
If a gentle upward incline or slope, from the furnace, 
can be afforded, the fire will draw all the better; a rise 
of about lin. per yard will do nicely. In some cases 
the base of the flue, formed of flat tiles, is raised off the 
_ground, or floor, by means of bricks or tiles set on edge 
beneath every joint, but this is really not necessary, and 
a flue laid on a solid bottom answers perfectly well, 
while being much more substantial. 
Flues consisting of ordinary (glazed) earthenware or 
sanitary pipes, with sockets, carefully jointed with a 
mixture of mortar and Portland cemeni, are not infre- 
quently employed, and possess the advantages of being 
at once inexpensive, easily fixed, neat, and effective. 
The usual sizes of pipes employed are the 6in., 9in., and 
12in., the first being most suitable for all ordinary 
purposes. Pipes only 4in. in diameter are sometimes used 
for quite small structures, but these are not to be 
recommended, as not affording sufficient sectional area 
to provide a good draught for even a small fire, and 
requiring to be very frequently cleaned also. 
Note, that wherever these earthenware pipes are 
