254 



MACHINERY IX CONNECTION WITH AIR. 



of rising air above may draw the air up through the 



Fig. 2T2. throat with increased velocity, as 

 shown in iig. 272. This arrange- 



Fig. 273. 



raent also allows the fire to be built 

 so as to throw the heat more fully 

 out into the room. By leaving the 

 shoulder at h square or flat, it will 

 tend to arrest any reversed or 

 downward current in a better man- 

 ner than if built sloping, as shown 

 by the dotted line at , which 

 would act like a funnel, and throw 

 the smoke into the room. The 

 throat should be about as high as 

 the extreme tip of the flame ; if 

 much higher, the chimney will not 

 draw so well, and if lower, too 

 much of the heat will be lost, 

 .g. 273 shows a fireplace without a contracted throat, 

 the current of which is comparatively feeble. Many 

 chimneys draw badly by being made too large for the fire 

 to heat sufficiently the column of air they contain. 



A hadlxj-hvUX 

 Chimney. 



CIIIMNET-CAPS. 



When wind sweeps over the roof of a high part of the 

 building, or over a hill, it often strikes the Fig. 274. 

 tops of chimneys below, and drives the smoke 

 downward. This may be often prevented 

 by placing a cap over the chimney, like that 

 represented by fig. 274, which is supported at 

 its comers, the smoke passing out at the four 

 sides just under the eaves of this cap. But 

 it sometimes happens that there is a confusion of currents 

 and eddies at the top of the chimney, over which this cap 



