FARMERS' REGISTER— CONSTRUCTION OF FiRE-PLACES. 



621 



the fuel, into which, llic fresli air rushes out of the 

 room: this being again rarified as tlic tbrmer por- 

 tion, ascends — and thus a constant current is kept 

 u[) as long as the combustion goes on. As soon as 

 hitent lieat becomes sensible, by the coIUsion of 

 flint and steel, and the compression of the air be- 

 tween them, or in other words, as soon as a spark 

 of fire is born into the world, and kept alive by 

 combustible matter, this curr.->nt is lormed around 

 it, by rarifying the air. Ontiie same principle the 

 heat of the sun produces tlie winds, (according to 

 the theory of Dr. H alley, now generally received.) 

 This is the case with the grea^ ea^t wind which 

 blows continually from east to west, in the direc- 

 tion of the apparent diurnal motion of the sun, 

 and prevails in all the extensive oceans. The heat 

 of this luminary, warming the air under it, (or 

 rather it should be said, warming the earth, and 

 that warms the air by reflected heat — for the 

 rays of the sun have no effect on that medium 

 through which they pass,) rarifies it, and a fresh 

 portion rushes in the partial vacuum, in the direc- 

 tion of the motion of the sun, and thus the cur- 

 rent is kei)t up. This is also the cause of the 

 north and south winds which blow towards the 

 equator: the heat of the sun being hottest there, 

 the air rushes in from those points to supply the 

 consequent vacuum. But to return — the current 

 or draught thus produced by the heat of the fire- 

 place, is supplied by fresh air from out of the 

 room, which if it were made air-tiglit, vrould soon 

 produce a vacuum, in which no animal could long 

 exist. But when the air of the room is exhausted, 

 fresh portions are admitted through the doois, 

 windows, aud crevices of the room: if all these 

 apertures are closed, the combustion ceases, and 

 the fire goes out. Air is no less important to keep 

 up a flame, than it is to support animal life. The 

 princii)le of the air or atmosphere which feeds the 

 fire or supports combustion, is oxj'gen, one of the 

 two gases of which the atmosphere is composed: 

 the other is azote. If the air is deprived of oxy- 

 gen, it will no longer support combustion — for if a 

 fighted taper be put into a vial of such air, it will 

 be extinguished; but if a bit of wire be dipped in 

 sulphur and lighted, and then put into a vial of 

 pure oxjgcn gas, it will be rapidly and brilliantlN* 

 consumed, and the melted iron will sink through 

 the bottom of the vial. The absolute necessity 

 of oxviren or the atmosphere to promote combus- 

 tion is forcibly presented: if a pipe be admitted 

 to the fire along the floor, having a com.munica- 

 tion with the external atmosphere, a current is 

 suddenly formed, which rushes violently with a 

 noise from the mouth of the pipe through the fire 

 vip the chimney. This shows what a powerfidde- 

 inand the fire has for air, which increases the heat 

 and combustion in pro})ortion to the rapidity of a 

 succession of particles of air introduced to fire; 

 hence, the powerful intensitj' of heat produced by 

 a blow-pipe or bellows. The combustion, and 

 consequent heat, seems to be also in proportion to 

 the smallness of the aperture through which the 

 air is admitted; but this may be still owing to the 

 rapidity of the current, lor this depends upon its 

 emalluess. From this it well appears, how much 

 the management of the draught depends upon the 

 air; and as the combustion, and consequent!}^ the 

 heat, depends upon the draught, and also the smoke 

 which is driven by the draught, the great nicety 

 in man;\ging the draught will likewise appear. 



Although air is so necessary to the fire, too 

 much air, like water, weakens combustion, and con- 

 sequently the draught, and renders the chimney 

 liable to smoke. 



These facts suggest some means for the curing 

 of smokj' chimneys. Thus, when all the aper- 

 tures of a room are closed, too little air is admitted 

 — this weakening the coudiustion, and that the 

 draught, the latter is not sufficient to drive the 

 smoke up the Hue; the consequence of this is, the 

 weight of the external atmosphere j;resscs the 

 smolce down the chinmey into the room, especially 

 if the wind blows, in which case the current of 

 wind being stronger than the draught, it puffs the 

 smoke into the room. The remedy is to open the 

 doors, £ic., and admit the air: the same happens 

 when too much air is admitted, which cools, and 

 of course weakens the draught, and not being suf- 

 ficient to pro[)cl the smoke, it is pressed back by 

 the external air in the apartment: in this case 

 the doors must be shut. The weakness of the 

 draught is the most frequent cause ofsmokj' chim- 

 neys. This is produced in a variety of ways. 

 Very often a weak fire causes the chimney to 

 smoke; for, as the heat and combustion depend on 

 the draught, so the latter depends upon the Ibr- 

 mer: so that when the fire is low, the smoke curls 

 wealvly up the chimney. The remedy suggests 

 itself — kindle up a brisk fire. 



There is another way in which the draught is 

 weakened, or rather overcome: for the want of 

 ventilation in the upper part of the room. There 

 are always two sorts of air in a room where fire is 

 kept — a ligi^t and heavy. The former occupies the 

 upper part of the room, where, if it has no vent, 

 it rushes down the wall to the mouth of the fire- 

 place, and curling round the inner side of the man- 

 tle, forces its v»-ay up, and by its superior strength 

 drives back the smoke, especially if the draught 

 happens to be weak. There are always two cur- 

 rents passing through the rnouth of the fire-place 

 if the room is not ventilated above, as well as 

 through the door; this may be seen, by holding 

 two leathers — one at the top, and the other at 

 the bottom of the mouth of the fire-place — they 

 will be drawn inward: but in the case of the door, 

 one of the currents is going out at the top, (the 

 lighter) while the other (the heavier) is coming 

 in at the bottom, which may be seen by holding 

 two candles, one in the lo\-\'cr and the other in 

 the upper part of the door. The upper one will 

 be blown outward, while the under one will be 

 blown inward. The draught is affected, when 

 several vents open into the same funnel; for the 

 fresh air rushing m through these vents out of ad- 

 jacent rooms, puti'out the smoke: this ceases, when 

 a draught is formed in every vent by kindling fire 

 in the different rooms at once. Straight funnels 

 affect the draught, because the wind is more easi- 

 ly admitted down the chimney to the fire. Large 

 fire-places with email vents affect the draught, and 

 likewise the contrary. The want of ventilation 

 affects the draught, and of course, the smoke. 

 When the draught is weakened in this way, a pipe 

 admitted to the fire, terminating at the hearth or 

 within the fire-place, having an external commu- 

 nication with the air, will f)revent smoking. The 

 pipe mav be either conducted on, or vmder the 

 floor. The draught is sometimes affected by hav- 

 ing the ventilators for letting out the foul air of a 

 room, too low down above the fire place, (wliich 



