146 GREENHOUSE CONSTRUCTION AND HEATING. 
apparatus, excepting, of course, those constructed to burn 
gas or oil (paraffin). But these last are comparatively costly, 
both in the first outlay and in fuel, as well as being open to 
certain objections, which will be dealt with presently. In 
careful hands, and particularly where only a moderate 
degree of warmth is required, a well-constructed flue is a 
method of heating by no means to be despised, and excellent 
work may be and is done by this means. The different 
forms of flue will be fully described later on. 
Heating by means of hot water is, however, undoubtedly 
the safest and altogether best, if not, perhaps, the cheapest 
means of heating horticultural structures of all kinds, and 
those of considerable dimensions in particular, and it will 
therefore he considered first. 
A few of the advantages of heating by means of hot water 
carried in pipes of iron or some other metal—copper is the 
best conductor, but its cost renders it out of the question, 
except for quite small affairs—are (1) that the heat given 
off is so mild and gentle, it being impossible for the 
temperature of the pipes to rise above 212 degrees Fahr., 
while it usually ranges considerably less than this ; (2) the 
heat is easily carried to any point, and may be equally 
distributed over a house of any length or width merely by 
a@ proper arrangement of the pipes; (3) that with the 
boiler and furnace altogether outside the house there is 
no possibility of any smoke or fumes gaining an entrance ; 
and (4) its perfect safety. 
Heating by means of steam has been, and in some few 
eases is still adopted, especially in the United States, etc., 
but though possessing certain advantages the pipes are 
apt to become over-heated and parch or burn the air, 
