HOTHOUSES. 83 



supply cistern. Assuming now that the boiler and pipes are filled 

 from the supply cistern, which should always be left open, and fire 

 is applied to the boiler, the heated water in it rises, and passing 

 by the opening at the top, through the pipes, becomes cold, 

 and enters the boiler again at the bottom, to be again heated and 

 passed through the pipes. Air or vent tubes are fixed at the 

 highest ends of all pipes, so as to prevent the accumulation of air 

 in them. By hot-water pipes glass-houses are heated, and the heat 

 is diffused by radiation from the pipe surface. Pipes are generally 



3 in., 4 in., or 6 in. in diameter. In ordinary greenhouse work 



4 in. pipes are used for both mains and heating pipes. For long 

 distances 6 in. pipes are required for greenhouses. The connection 

 from the boiler to the heating pipes may be by 3 in. or even 2 in. 

 pipes. When speaking of heating pipes, 4 in. diameter is generally 

 understood. According to Fawkes, the length of 4 in. pipe required 

 for 1,000 cubic feet of actual atmospheric contents is : 



Greenhouses, conservatories, etc. . 35 to 40 ft. 



Vineries 45 to 55 



Plant stoves 55 to 65 



Forcing-houses, etc. . . . 60 to 70 



The boiler house and boiler should be placed, in small ranges, at 

 the end, and in very long ranges towards the middle, of the houses. 

 Mains from the boiler should run at a minimum inclination of '/, in. per 

 9 ft.; but the greater difference we have in the temperature between 

 the incoming and outgoing water the shorter may be the column of 

 water to produce circulation, and the higher the column the longer 

 may be the length of pipe. The main should be kept in trenches 

 to prevent radiation, and the pipes for heating the different houses 

 should be taken from the mains at the different points and conducted 

 where required. The heating pipes, once they rise from the mains, 

 should never be allowed to dip. If the pipes must be above the floor- 



