Of Ch im ney Fireplaces. 537 



up the tube. The hotter the water in the bottle is, and 

 the colder that in the jar, the greater will be the velocity 

 with which the .hot water will be forced up through the 

 tube ; and the same holds of the ascent of hot smoke in 

 a chimney. When the fire is intense, and the weather 

 very cold, the ascent of the smoke is very rapid ; and 

 under such circumstances chimneys seldom smoke. 



As the cold water of the jar immediately surrounding 

 the bottle which contains the hot water will be heated 

 by the bottle, while the other parts of the water in the 

 jar will remain cold, this water so heated, becoming 

 specifically lighter than that which surrounds it, will be 

 forced upwards ; and if it finds its way into the tube will 

 rise up through it with the coloured hot water. The 

 warmed air of a room heated by an open chimney fire- 

 place has always a tendency to rise (if I may use that in- 

 accurate expression), and, finding its way into the chim- 

 ney, frequently goes off with the smoke. 



What has been said will, I flatter myself, be sufficient 

 to explain and illustrate, in a clear and satisfactory man- 

 ner, the cause of the ascent of smoke ; and just ideas 

 upon that subject are absolutely necessary in order to 

 judge, with certainty, of the merit of any scheme pro- 

 posed for the improvement of fireplaces, or to take 

 effectual measures, in all cases, for curing smoking 

 chimneys. For, though the perpetual changes and alter- 

 ations which are produced by accident, whim, and 

 caprice, do sometimes lead to useful discoveries, yet the 

 progress of improvement under such guidance must be 

 exceedingly slow, fluctuating, and uncertain. 



As to the causes of the smoking of chimneys, they 

 are very numerous and various ; but as a general idea 

 of them may be acquired from what has already been 



