124 



POLMAISE MODE OF HEATING GREEN-HOUSES. 



Fig. 20. — Section of a Green-House, heated by Polmaise. 



drain is laid under the walk, — the top 

 being covered with flag stones, tiles or 

 plank, as may be most convenient. At the 

 extreme ends of this drain (the coldest parts 

 of the house,) is an opening or openings, 5, 



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Fig. 21. — Ground plan of the same. 



5, formed at the level of the floor. There 

 should always be a descent in this cold-air 

 drain, from the mouth (5) to the other end 

 where it enters the air-chamber, in order 

 to facilitate the circulation of the air. 



It is easy to see, that when a fire is 

 lighted in the furnace, the air in the cham- 

 ber (2) will become heated, and will im- 

 mediately rise through the opening (4) into 

 the body of the house. To supply the 

 place of this air, that thus passes out, the 

 cold air in the drain (3) will immediately 

 flow into the hot-air chamber, rise over the 



furnace and become heated, and also pass 

 into the house. This cold-air drain being 

 emptied, the cold air at the other extremity 

 of the house will rush into the mouth of 

 the drain (5,) and move on likewise to- 

 wards the hot-air chamber. The subtrac- 

 tion of cold air at that end of the house 

 most remote from the furnace, causes a 

 movement there, and to supply its place 

 the heated air — pouring in from the hot-air 

 chamber — flows in a continuous stream to- 

 wards that part of the house (5) where the 

 mouth or mouths of the cold-air drain lie. 

 This circulation of the cold air towards the 

 warmest part of the house, (through the 

 drain,) and the warm air towards the cold- 

 est part of the house, (through the atmos- 

 phere of the house itself,) once commenced, 

 continues as long as the fire is kept up. 

 The increased health of the plants, caused 

 by this genial breeze, can scarcely be a 

 matter of dispute ; and the advocates of 

 Polmaise in England insist most strongly 

 on its great advantages in this respect. 

 By keeping the shallow iron tank, which is 

 placed just below the opening of the air 

 chamber (4,) filled with water, this hot-air 

 stream may be made more or less damp at 

 pleasure. The advantage of the small 

 ventilators (6) — apertures in the wall three 

 or four inches square, just above the floor — 

 will, we think, be found much greater in 

 this climate than in England. These open- 

 ings must be furnished with small sliding 

 doors or lids ; and by means of these, an 

 abundance of fresh air can be introduced 

 into the house without the least danger to 

 the plants themselves, — a point always 

 difficult, with us, when the exterior tempe- 

 rature is near zero of Fahrenheit. In ex- 

 tremely cold weather, the ventilators in the 

 back wall (opening into the back shed, B,) 

 may be used. 



The furnace is intended for coal, and. 



