228 PRINCIPLES OF GARDENING. [CH. VII. 



The above are the average of results obtainable in 

 a common well-constiTicted furnace. By a compli- 

 cated form of boiler, perhaps, a small saving of fuel 

 in obtaining the same results may be effected ; 

 but it mil be foimd, generally, that the original 

 cost of appai'atus, and the current additional ex- 

 penses for repairs will more tlian exceed the economy 

 of fuel. 



Flues for imparting heat to hot-houses are, for the 

 most part, superseded by either tanks or hot-water 

 pipes ; but where retained, the top should be formed 

 of iron plates, these admitting the heat most readily 

 into the house, and consequently requiring a less 

 consumption of fuel. If it be desirable to have a 

 covering for the flues that will retain the heat 

 longer, as when the fires are made up at night, this 

 may be readily accomplished, by putting a row of the 

 thick square paring tiles on the top of the whole 

 length of the flue, an hour or two before the houses 

 are finally closed. 



The power of retaining heat, or, in other words, 

 of cooling slowly and gradually, wliich renders the 

 covering of paving tiles desirable, renders the tank 

 system of heating by hot-water still more efficient. 

 It is a scientific operation throughout, and will 

 be best appreciated by a reference to the annexed 

 diagram, borrowed from Mr. Rendle's pamphlet, 



