28 THE GREEN-HOUSE AND CONSERVATORY. 



In fact, the whole pipe, which rises but a few inches in 

 its course, may be considered a boiler, the only opening to 

 which is at the highest point of the pipe, e, that is, if the 

 pipes run around the houso at the end farthest from the 

 boiler, where there is an opening with a small reservoir for 

 supplying it with water. This is the simplest form of hot- 

 water heating ; it may be varied in size to any extent, and 

 will heat any pit sufficiently. 



In heating green-houses, a great saving of fuel is effected 

 by the use of wooden shutters fitted to all exposed portions 

 of the range of glass. They may be made of rough 

 boards, and should, while heavy enough to keep their 

 place by their own weight, be of manageable size. They 

 should be put on as soon as the sun leaves the house, and 

 also on windy days and nights ; some days in winter they 

 should not be removed at all. The most difficult nights to 

 keep a green-house warm are those when the mercury 

 shows from twenty to thirty degrees of frost, with a violent 

 wind sweeping over the house. The wind carries off the 

 heat very rapidly, and it is easier to maintain a high tem- 

 perature with the mercury twenty degrees below zero, and 

 no wind, than in a windy night, when the thermometer 

 only shows twenty degrees of frost. 



