8 THE BOOK OF THE GREENHOUSE 



has almost disappeared, as these flues were almost always 

 a nuisance ; they cooled quickly if the fire went low, were 

 often difficult to heat, and, worst of all, they were prone 

 to leakage, and once the sulphurous fumes got through 

 to the house, they meant death to the plants. I should 

 not have mentioned the flue system of heating at all, did 

 I not know that flues are to some extent used even 

 now, and sometimes advocated for their cheapness. On 

 the whole, we have nothing yet to supersede hot water 

 when driven by a good boiler through a sufficiency of 

 4-in. pipes. Steam heating is quicker in action, but 

 requires more attention from the one in charge, and it 

 is not so manageable as hot water, especially for houses 

 small in size and in number. 



Dealing, then, with ordinary hot water heating, it is 

 necessary to success that the boiler be set well beneath 

 the level of the house, that the pipes be so fixed that 

 they have a continuous rise in the upper or flow pipe 

 from the point of connection with the boiler to the point 

 farthest away from it, and that the under or return 

 pipe have a corresponding fall from the highest point 

 reached back to the boiler again, which it should enter 

 near the bottom. Speaking generally, the greater the 

 rise the better the circulation must be. It sometimes 

 happens that a doorway or some other obstruction 

 engenders a wish to allow a " dip" at some particular 

 point, but to do this will be fatal to successful cir- 

 culation. "Where two or more houses are connected 

 with the same system of piping, valves should be 

 put in the pipes so that the heat may be wholly or 

 partially shut off when not wanted in a particular 

 house or compartment. An air-tap should be connected 

 to the pipe at the highest point so that any accumula- 

 tion of air that would stop the circulation may be 

 expelled at this point. 



The heating arrangements of a greenhouse must of 



