CIRCULATION AND CONVECTION IN GASES. 



61 



(Fig. 48&), the circulation is thoroughly established down one side and up 

 the other. The direction is easily shown by blowing out a lighted taper, 

 and holding it, while still smoking, first on one side, then on the other. 



If the candle be put at the bottom of a wide cylinder, it will usually 

 establish its own circulation, but, 

 as the direction of the currents is 

 constantly changing, the flame is 

 very unsteady. 



When a chimney with an open 

 fireplace is working properly, a 

 great amount of air in addition to 

 that used to burn the coal is 

 drawn into the chimney through 

 the open space above the grate, 

 and much of the heat of combus- 

 tion is used to warm this air and 

 send it up the chimney. At first 

 sight, this heat would appear to be 

 wasted, but the additional circula- 

 tion has one advantage, namely, 



FIG. 47. Circulation established by 

 an Open Fireplace. 



that it brings into the room a 

 greater supply of fresh air, and so 

 aids ventilation. But in the ordi- 

 nary arrangement, or rather want of arrangement, by which the cold 

 air is left to find its own way into the room, the greater circulation 

 makes itself evident by the greater draughts along the floor and near the 

 windows. The ventilation in this case is also inefficient, for much of 

 the fresh air rushes straight to the chimney, and the impure air, which 

 is warmer and damper, and therefore lighter, rises to the ceiling, and 



FlG. 48. -Candle in Flask, showing (a) no circulation, (6) circulation. 



is only renewed slowly by diffusion. It is much better to provide a 

 special passage for the air into the room by means of some such arrange- 

 ment as the Tobin ventilator, in which a pipe is brought from the out- 

 side somewhat as in Fig. 50. 



The pipe being carried up a few feet, the stream of cold air is directed 

 towards the ceiling, and is warmed there before it descends to the lower 

 part of the room. Since there is thus a more general renewal of the 

 air of the room, the ventilation is much more efficient, while cold draughts 

 are diminished. 



