60 



HEAT. 



and if there is a sufficient communication with the outside, the circulation 

 thus started is maintained, the pressure due to the cold air outside the 

 chimney being always in excess of that due to the warm air and gas in 

 the chimney. In general, the communication with the outside is effected 

 through or under the door, and through the crevices between the window- 

 sashes, the air moving as represented by the arrow-heads in Fig. 47. 



October 26, 1880. 



October 27. 



October 28. October 29. 



FIG. 46. Progress of a Cyclone, October 26-29, 1880. (From the Times.) 



Sometimes this is not sufficient, and unless a passage be made for the 

 admission of cold air, the circulation is not established, and the chimney 

 smokes. 



Sometimes the chimney, if a wide one, may even establish within 

 itself a down and up current and so maintain the needed circulation. 



We may illustrate these points by lowering a lighted candle into a 

 flask (Fig. 48a). No circulation is established, and as soon as all the 

 air is used up, the candle goes out. 



If, however, a thin partition be inserted down the neck of the flask 



