HEATING GREENHOUSES. 197 



opening, but with screw-joints 3'ou can go to bed and feel safe. 

 He uses a copper boiler two and one-half feet deep, with a fire-pot 

 twent3'-two inches in diameter, and carries about ten pounds' pres- 

 sure ; and is now heating a house by means of five two-inch pipes 

 with greater ease then formerly, when he used seven four-inch 

 pipes, and it takes less coal to do it. He thought a copper boiler 

 would wear better than an iron one over a coal fire. 



Mr. Woodford said that in his plan the pipes go round the house 

 and into the boiler. 



Col. Henr}' W. Wilson said that there are certain axioms in 

 heating, and the best way known to scientific men for heating water 

 is the best for horticulturists. Great ingenuity has been expended 

 on apparatus for heating cars. The scheme proposed by the 

 essa3'ist has been gone over before, and it has been found that 3'OU 

 cannot get a draught with a large chamber and a small aperture. 

 You cannot ventilate a room b}- an opening into one above it. A 

 vertical boiler is preferable to a horizontal, for all the motive of 

 circulation is the difljerence in the specific gravity of hot and cold 

 water. If water is heated you want to give it as rapid a vertical 

 motion as possible. Friction in large pipes is less in proportion 

 than in small pipes. Iron is not so good a material as copper or 

 brass for a boiler, though its greater cheapness makes its use more 

 general. Copper is a far better conductor of heat than iron. Iron 

 is a poor conductor; if a perfect conductor be rated as 100 iron 

 will rate as 14. The thinner the pipe is the better, so far as 

 regards the conduction of heat. Cast iron, as being thicker than 

 wrought iron, is not so good, but is preferred, because less subject 

 to rust. Water itself is a ver^- poor conductor of heat, but is able 

 to absorb and retain a very large amount ; which can be carried, 

 b}' convection, to a considerable distance from the fire, — hence 

 its value as a circulating medium. Lampblack makes an excellent 

 paint for radiating pipes, causing more rapid radiation, which is 

 the reverse of absorption. The radiation of lampblack is to that 

 of iron as 100 is to 64. A tin dipper filled with hot water, if 

 black, loses heat twice as fast as if bright. 



Mr. Woodford said that he would put on a pressure of ten pounds 

 to promote the circulation. 



Col. Wilson said that the b'est car-heaters have no pressure. 

 Mr. AVoodford is right in placing the stand-pipe near the boiler. 

 The expansion of a thousand gallons of water caused by heating 

 to 212° would fill a barrel. With a vertical partition in the stand- 



