540 Of Chimney Fireplaces. 



A case more frequently to be met with is, where cur- 

 rents of air set down chimneys in consequence of a 

 diminution and rarefaction of the air in a room, occa- 

 sioned by the doors of the room opening into passages 

 or courts where the air is rarefied by the action of some 

 particular winds. In such cases the evil may be reme- 

 died, either by causing the doors in question to close 

 more accurately, or (which will be still more effectual) 

 by giving a supply of air to the passage or court which 

 wants it by some other way. 



Where the top of a chimney is commanded by high 

 buildings, by cliffs, or by high grounds, it will fre- 

 quently happen, in windy weather, that the eddies 

 formed in the atmosphere by these obstacles will blow 

 down the chimney, and beat down the smoke into the 

 room. This, it is true, will be much less likely to happen 

 when the throat of the chimney is contracted and prop- 

 erly formed than when it is left quite open, and the fire- 

 place badly constructed ; but as it is possible that a chim- 

 ney may be so much exposed to these eddies in very 

 high winds as to be made to smoke sometimes when the 

 wind blows with violence from a certain quarter, it is 

 necessary to show how the effects of those eddies may 

 be prevented. 



Various mechanical contrivances have been imagined 

 for preventing the wind from blowing down chimneys, 

 and many of them have been found to be useful ; there 

 are, however, many of these inventions, which, though 

 they prevent the wind from blowing down the chimney, 

 are so ill-contrived on other accounts as to obstruct the 

 ascent of the smoke, and do more harm than good. 



Of this description are all those chimney-pots with 

 flat horizontal plates or roofs placed upon supporters 



