620 



FARMERS' REGISTER— CONSTRUCTION OF FIRE-PLACES. 



coving A B at A% it will be reflected ogainst the 

 opposite coving C D at n; and of cour,se a person 

 Bitting in front of the fire, loses the benefit of it, 

 unless he be sitting in the comer somewhat facing 

 the coving. But if a ray of heat thrown from a 

 point in the centre of the back of the new fire- 

 place F G at s, in the same direction as in the 

 Ibrmcr case, and talis upon the coving G H at o, 

 it will rebound in the direction towards x, in front 

 — ibr the angle of reflection is always equal to the 

 angle of incidence. A person pitting, therefore, 

 before the fire, will enjoy the benefit of a ray so 

 reflected. If the covings be made of a substance 

 that absorbs more than it reflects, a great deal of 

 the heat is lost. They should be made of a ma- 

 terial that reflects more than it absorbs, and if 

 ])ossib]e, of one which does not absorb at all. 

 Dutch tiles, fire-stone, marble, and porcelain are 

 used — common brick and mortar, if it was suffi- 

 ciently durable, is the hest material; which should 

 be smoothly ])lastcred, and kept white-washed, 

 for black absorbs too much heat. The best cov- 

 ing for a fire-place, is that material which is most 

 durable, least liable to crumble, the least absorber, 

 and the greatest reflector, and that which is sus- 

 ceptible of the highest polisli, ibr the more it is 

 polished the more heat it reflects. Any one may 

 be convinced of this, if they will place the bottom 

 of a polished plate of tin (or the bottom of a cof- 

 iee-pot) by the side of the face, while standing 

 before the fire, so that the ray of reflection may 

 strike the face after it is reflected from the tin: a 

 higher degree of heat will be sensibly felt. When 

 the image of the fire is seen on the tin, then the 

 ray strikes the face. A tin plate would be a good 

 thing to regulate the position of the covings. The 

 mantle also affects the heat and smoke. If this 

 be too high, the draught is weakened, and the 

 smoke pufis out into the room before it reaches the 

 vent: if too rugged in the inner-side, it obstructs the 

 smoke: it should be smoothly and soundly plaster- 

 ed, that the smoke may curl around it on the 

 inner-side. 



If the vent is too large, too much heat escapes, 

 and the draught is also weakened — if too small, 

 the smoke is obstructed: if the fire-place is too 

 large for the vent, it takes in more air than can 

 pass up the chimney; as an animal can take in 

 its mouth more than it can swallow: the mouth 

 should ever be in ])roportion to the vent. It is an 

 establislred rule in the construction of stoves, and 

 furnaces with ffrates, that the area of the vent 

 should be e(|ual to the sums of the are;i of the 

 open spaces between the bars of the grate through 

 which the air is admitted to the fire: but the ob- 

 ject here is more to increase the heat within the 

 furnace by a rapid combustion, which is always in 

 ])roportion to the strength of the draf>, which is at 

 its smallness and swifhiess, (hence the povv^eriul 

 heat produced by the mouth of the bellows,) and 

 therefore cannot strictly apply to an open fire place, 

 where the object is not confined heat, but reflect- 

 ed. If the mouth was made as small as the vent, 

 very little heat would be reflected in the room, 

 though the combusfion would be more rapid. 

 This is well known to those who have been in the 

 habit of kindling a fire made of coal, by placijig 

 the kindler before the mouth, so that a small space 

 is left for the admittance of the air, or oxygen. 

 The same may be seen by placing a blanket be- 

 fore the fire, when the draught is suddenly increas- 



ed, accompanied with a great roaring, and the 

 combustion goes on rapidly. 



Again, if the flue is too large, the draught is 

 weakened, and a quantity of soot is formed; for 

 in this case the flue contains so much cold air, it 

 weakens the draught, and hence not being suffi- 

 cient to drive out the soot, it lets it fall on the sides 

 of the flue. The same takes place in the chan- 

 nel of a stream, (Avhich has been made use of to 

 illustrate the draught in a chinmey.) The streum 

 is alwaj's swiftest where the channel is narrowest, 

 (the volume of water being the same,) hence it is 

 able to carry the sand along: but if it suddenly 

 spreads into a large channel by which the stream 

 is weakened, it drops the sand on the sides of the 

 channel, or in the deep holes. The stream is al- 

 ways swiftest in the middle, and frequently runs 

 backwards at the sides, and hence the sand is le(\ 

 there, where the current is v/eakest. It is on this 

 principle that soot is formed in a flue, of any con- 

 struction. The draught or current of air and smoke 

 being stronger in the middle of the flue, it drops 

 the soot on the sides where it is w^eakest. On the 

 other hand, when the flue is small, it is heated all 

 the way up, and the air in it; and thus the draught 

 is made to ascend, by the heat rarifying and rais- 

 ing the air. 



If the throat is too large the soot collects in it, 

 for the same reason mentioned above. This, how- 

 ever, is an advantage; for if a deep recess be 

 made above the vent, (or a deep throat) nearly 

 all the soot formed may be collected, when it may 

 be swept out. Upon this principle furnaces have 

 been constructed so ingeniously as to clear the 

 flue of soot almost entirely. Any one may notice 

 how soot is collected in holes in the back of a fire 

 place, above the fire, where a brick happens to 

 tumble out: for this reason, the throat, vent, flue, 

 and rough places in the back, shoidd be made 

 smooth. Finally, if the shaft is too long or high, 

 the chimney is ap't to smoke, for then the flue be- 

 ing too long to be heated ail the way, tfie cold air 

 in it will cool and weaken the draught, and being 

 heavier too, wdl press the ascending smoke down 

 the chimney in the room: if too short the wind is 

 apt to blow down it. The size of the shaft should 

 be ffenerally to that of the flue as 4 to 1. 



These remarks may serve to show, how the 

 heat and smoke of a fire-place and chimney, are 

 affected by the construction of their component 

 parts. 



But the great secret in managing a fire-place, 

 so as to obtain the greatest degree of heat from 

 the smallest quantity of fuel, and to prevent smo- 

 king, is the management of the draught. This is 

 easily aflected by every part of the fire-place, as 

 well as by other external causes, as the air, winds, 

 groves of" trees, buildings, &c. 



To understand the nature of the draught — it is 

 a current of heated air, produced in the fire-place, 

 as soon as a shovel of coals or a chunk of fire is 

 introduced: which ascends the flue of the chimney, 

 carrying along with it the smoke, soot, and other 

 matter of the fuel: and is formed on the principle of 

 the expansion and rarification of that element 

 The heat of coals rarifying the air just around 

 them, divides it in into minuter particles, and con- 

 sequently becoming lighter, they ascend through 

 the denser air up the chimney: this produces a 

 partial vacuum (for there is no real vacuum in 

 nature, though one can be made by art,) around 



