PITS HEATED BY HOT WATER, &c. 



445 



ground-plan. I am of opinion that these liable to get out of order. The boiler has 



slides are superior to the valves com- a flue passing through its centre, and 



monly used, for they are more simple round the two sides. It is very powerful, 



in construction, easy to manage, and less and requires but little fuel to heat it. 



Fig. 619. 



" In the longitudinal section, fig. 620, circulation of air in the pit upon which 



you will see how I have attempted to it acts. The bottom of the pit has a fall 



make use of the ' Polmaise ' for the pur- of 6 inches towards the end at which the 



pose of economising heat, and causing a boiler is placed. From this lowest part 



Fig. 620. 



an air-drain is taken, first downwards, 

 then up the back of the boiler, into an 

 air-chamber above it, and is then con- 

 nected with the hollow in the back wall. 

 When the fire is lighted, and the air in 

 contact with the boiler, &c. becomes 

 heated, in obedience to a natural law by 

 which it is governed, the heated air 

 ascends, and the cold air presses up to 

 take its place, and so, in its turn, becomes 

 heated, and is again thrown into the pit, 

 at the openings in the back wall. 



" This circulation of air tends to keep 

 up a constant movement in the atmo- 

 sphere of the pit, and, at the same time, 

 conveys the heat, which would otherwise 

 be absorbed by the bricks and mortar 

 which surround the furnace and boiler, 

 into it. It is the common practice, when 

 tanks are used, to place the covers close 

 down on the top of the tanks. In these 

 pits the tanks are of ^-inch cast-iron, 

 15 inches across at the top, and 9 inches 

 at the bottom, with 3 inches of a bevel 



