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Commercial Gardening 



The colder water in the return pipe c, being heavier than the heated 

 water, rushes in at the bottom of the boiler, and thus pushes the warmer 

 water upwards, and forces it through the flow pipe to fill the space 

 caused by the flow of water from c to the boiler. The greater the heat 

 applied the quicker the circulation. Now, as the heated water travels 

 along the flow pipe B it is gradually losing its heat, and its colder 

 particles begin to sink to the bottom. It cannot, however, return the 

 way it came, because it is being pushed forward by the hotter water 

 coming from the top of the boiler, which m its turn is forced up by the 

 colder water entering the boiler by the return pipe c at the base. Thus, 

 while the water in the pipes is gradually losing its heat, that in the 



Fig. 179. Diagram showing the Circulation of Hot Water in Greenhouse Pipes 



boiler is constantly rising in temperature, and rushes to occupy the space 

 that is being constantly vacated by the colder water. 



If by any chance the water in the flow pipes and in the return pipes 

 and boiler was of the same temperature, circulation would cease alto- 

 gether, as when the fire goes out and the water cools. There must there- 

 fore be a difference in the balance between the hot and cold water to 

 maintain a regular circulation. In other words, one column of water 



o 



must be heavier than the other. This is secured by having the flow and 

 return pipes at different heights, and the boiler at a lower level than 

 either. When pipes are being set there is always a very slight rise in 

 the flow pipe from the boiler to the end of the housCj and a corre- 

 sponding fall in the return pipe to the boiler. In this way a difference 

 is secured in the two columns of water. And this difference is accen- 

 tuated by having water in the supply cistern, which is placed several 

 feet higher than the highest point of the flow pipe and is connected 

 with a pipe to the return pipe. This supply cistern should always be 

 kept filled with water, and as there is a pressure of about | Ib. to every 

 square inch of its surface for every foot in height, it will be realized at 



