The Garden Under Glass

HEATING AND STOKING 257 and cold water descends. Now the water by the action of the fire will become warm and will rise to the top of the boiler, the colder water sinking to the bottom. But in due course all the water becomes warm. Then, as the water at the top becomes hot it will seek further to expand. This can only be done by working upwards through the flow pipes. In doing this it will move the cold water there, which, by the law of circulation, will flow around through the pipes to make up for the cold water which has forced its way through the return pipe to replace the heated water which has risen. In this way a constant flow is promoted, the water throughout becomes warm, and this warmth is radiated from the pipes into the atmosphere of the house. This is the principle and practice of heating, and all the difficulties involved in heating only mean inability to procure and to maintain a constant circulation. At the highest point in this pipe a small tap is inserted, so that it may be opened occasionally for the escape of air. Higher than the boiler or any other part of the heating apparatus is a small tank by which the boiler is kept well filled with water. Though often put immediately over the boiler, I prefer it set at the far end of the house and connected with the return pipe so that when the fire is driving the water does not swell over. Where there are several houses the heat is regulated by screw valves on both flow and return pipes. A little practice will soon teach one how to regulate these. How to Light a Fire It may seem unnecessary to give directions for so simple an operation, but from experience with people who have never before lit a fire in a greenhouse boiler, I know that such information is necessary. Having seen that the boiler