186 GREENHOUSES 



curve in the outlet pipe at c it overflows at d. 

 This allows some of the water to escape, and 

 this goes up through the pipe e to the ex- 

 pansion tank, but the mercury being heavier 

 falls back again through f to the cistern. 



This automatically keeps the pressure at 

 any predetermined point, usually about 10 

 pounds, which makes possible the heating of 

 the water to a temperature of 240 degrees. 

 This makes practical the heating of the coils 

 to a high temperature in severe winter 

 weather and at the same time permits the 

 system to be run at lower temperatures in 

 mild weather. In this respect it has the ad- 

 vantage over steam. It is claimed for these 

 mercury "generator" devices that they 

 greatly improve the circulation of the water 

 in a heating system. 



The most apparent advantage is that they 

 make possible the use of less radiating sur- 

 face, hence the first cost is less. It is but 

 fair to say that, as a rule, growers who have 

 installed them have found them satisfactory. 

 When the hot water is circulated by 

 pumps it is possible, though probably not de- 

 sirable to maintain a high pressure. Econ- 

 omy in heating by hot water lies in having 

 abundant radiating surface and rapid circu- 



