VENTILATION BY OPEN FIREPLACES. (Jl 



closed. (I find by measurement that 24 inches is the usual 

 height of the upper edge of the chimney opening of an ordi- 

 nary " register" stove. Old farm-house fireplaces are open to 

 the mantelpiece.) 



Now, what happens when a heap of coal is burning in this 

 hole ? Some of the heat from 10 to 20 percent., according 

 to the construction of the grate is radiated into the room, the 

 rest is conveyed by an ascending current of air up the chimney. 

 As this ascending current is rendered visible by the smoke 

 entangled with it, no further demonstration of its existence is 

 needed. 



But how is it pushed up the chimney ? Evidently by cooler 

 air, that flows into the room from somewhere, and which 

 cooler air must get under it in order to lift it. In ordinary 

 rooms this supply of air is entirely dependent upon their defec- 

 tive construction bad joinery ; it enters only by the crevices 

 surrounding the ill-fitting windows and doors, no specially 

 designed opening being made for it. Usually the chief inlet is 

 the space under the door, through which pours a rivulet of 

 cold air, that spreads out as a lake upon the floor. This may 

 easily be proved by holding a lighted taper in front of the 

 bottom door-chink when the window and other door if any 

 are closed, and the fire is burning briskly. At the same 

 time more or less of cold air is poured in at the top and the 

 side spaces of the door and through the window-chinks. The 

 proportion of air entering by these depends upon the capacity 

 of the bottom door-chink. If this is large enough it will do 

 nearly all the work, otherwise every other possible leakage, 

 including the key-hole, contributes. 



But what is the path of the air which enters by these higher 

 level openings ? The answer to this is supplied at once by the 

 fact that such air being colder than that of the room, it must 

 fall immediately it enters. The rivulet under the door is thus 

 supplemented by cascades pouring down from the top and 

 sides of the door and the top and sides of the windows, all 

 being tributaries to the lake of cold air covering the floor. 



The next question to be considered is, What is the depth of 

 this lake ? In this, as in every other such accumulation of 

 either air or water, the level of the upper surface of the lake is 

 determined by that of its outlet. The outlet in this case is the 

 chimney hole, through which all the overflow pours upward ; 

 and, therefore, the surface of the flowing stratum of cold air 

 corresponds with the upper part of the chimney hole, or of the 

 register, where register stoves are used. 



