HOT- WATER BOILERS AND PIPES. 183 



ter at the top of his chimney, we found it rise in a few minutea 

 to 137°, after having travelled through 20 feet of flue through 

 the back wall of the house. 



Whatever apparatus be employed in heating a hot-house, the 

 flue should always be taken advantage of. It must be remem- 

 bered that smoke will not travel through a flue, — neither up 

 nor down, — without first being rarefied by heat. The smoke, 

 as already described, is, in fact, a body of gases emitted from 

 the fuel by the action of heat, and a portion of this it takes 

 along with it on leaving the furnace. In its passage, it com- 

 municates this heat to other bodies, as the flue ; and more so, as 

 the flue is in a position more or less horizontal. A flue, there- 

 fore, should, if possible, be carried the whole length before giving 

 egress to the smoke, by which a great amount of fuel may be 

 economized. 



In laying down hot-water pipes, it is necessary to allow suffi- 

 cient room for their elongation and expansion when they become 

 hot. Want of attention to this has caused several accidents ; for 

 the expansive power of iron, when heated, is so great, that 

 scarcely anything can w^ithstand it. The linear expansion of 

 cast-iron,by raising its temperature from 32° to 212°, is -0011111, 

 or about one nine hundredth part of its length, which is nearly 

 equal to If inches in 100 feet. Therefore, it is necessary to 

 leave the pipe's unconfined, so that they shall have freedom of 

 motion lengthways; and, instead of confining, as has frequently 

 been done, facilities should be provided for their free expansion, 

 by laying them on small rollers, or pieces of rod-iron, between 

 them and the bearers on which they rest ; for the contraction on 

 cooling is always equal to the expansion on heating, and unless 

 they can readily return to their original position when they 

 become cool, the joints are apt to become loose and leaky, as 

 indeed all cast-iron pipes do, that are exposed to sudden 

 extremes of temperature. 



Every hot-water apparatus should be provided with a supply- 

 cistern attached to the boiler, or the pipes; the pipe leading 

 from the supply-cistern should flow either into the return-pipe, 

 or into the boiler, near the bottom. In no case rihould it entel 

 the flow-pipe, as it is more likely to emit vapor, and the steam 



16^ 



