FIRE AND ITS USES 151 



between cold dwellings and suffocating smoke. The chimney 

 did not appear in England until the thirteenth century and then 

 it was merely a hole in the wall over the fireplace. The built- 

 up chimney with extension above the roof is a modern conven- 

 ience. The fireplace was the best means of heating the house 

 and of cooking until 100 years ago, when the iron stove came 

 into general use. Brick or tile stoves were used back in the 

 Middle Ages really a fireplace set out in the room. Cardinal 

 Polignac, of France, invented an iron stove in 1709, but it was 

 Benjamin Franklin who devised improvements that made it 

 really practicable (1745). 



We have seen in an earlier chapter why the hot-air balloon 

 rises. The heat expands the air in it so that some of it must 

 flow out. The balloon, therefore, contains less weight of air 

 than a corresponding volume of surrounding air. Since the 

 upward pressure on the underside of the balloon is greater than 

 the downward pressure on its upper surface by an amount equal 

 to the weight of the air the balloon displaces, the balloon rises, 

 provided this difference in weights is greater than the weight of 

 the balloon and its trappings. 



In a similar way the column of air in the chimney is heated 

 by the fire in the stove or the fireplace, and, expanding, it over- 

 flows. The column of air in the chimney, therefore, weighs less 

 than a corresponding column outside because there is less of it. 

 The air in the chimney is forced up and out of its top as the cool 

 and heavier air rushes in at the bottom. This air is in turn 

 heated and so the draft up the chimney is continuous. 



An efficient fireplace is built with slanting sides and rear wall 

 so as to reflect the heat out into the room, with a large smoke 

 outlet whose cross-section area is not less than one-eighth that 

 of the fireplace opening, and with the front edge of the latter 

 opening considerably below the smoke outlet so smoke will not 

 get out into the room. Its depth should be about the same 

 as the length of the rear wall and the height of the front open- 

 ing not over three-fifths of its length (see diagram, Fig. 59), 



