CHAPTER X. 



VENTILATION BY OPEN FIREPLACES. 



THE most stubborn of all errors are those which have been 

 acquired by a sort of inheritance, which have passed dogmati- 

 cally from father to son, or, still worse, from mother to 

 daughter. They may become superstitions without any theo- 

 logical character. The idea that the weather changes with the 

 moon, that wind " keeps off the rain," are physical super- 

 stitions hi all cases where they are blindly accepted and pro- 

 mulgated without any examination of evidence. 



The idea that our open fireplaces are necessary for ventilation 

 is one of these physical superstitions which is producing an in- 

 calculable amount of physical mischief throughout Britain. A 

 little rational reflection on the natural and necessary movements 

 of our household atmospheres demonstrates at once that this 

 dogma is not only baseless, but actually expresses the opposite 

 of the truth. I think I shall be able to show in what follows, 

 1st, that they do no useful ventilation ; and, 2d, that they 

 render systematic and really effective ventilation practically 

 impossible. 



Everybody knows that when air is heated it expands largely, 

 becomes lighter, bulk for bulk, than other air of lower tem- 

 perature ; and therefore, if two portions of air of unequal 

 temperatures, and free to move, are in contact with each other, 

 the colder will flow under the warmer, and push it upward. 

 The latter postulate must be kept distinctly in view, for the 

 rising of warm air is too commonly regarded as due to some 

 direct uprising activity or skyward affinity of its own, instead 

 of being understood as an indirect result of gravitation. It 

 is the downfalling of the cooler air that causes the uprising of 

 the warmer. 



Now, let us see what, in accordance with the above-stated 

 simple laws, must happen in an ordinary English apartment 

 that is fitted, as usual, with one or more windows more or less 

 leaky, and one or more doors in like condition, and a hole in 

 the wall in which coal is burning in an iron cage immediately 

 beneath a shaft that rises to the top of the house t the fire-hole 

 itself having an extreme height of only 24 to 30 inches above 

 the floor, all the chimney above this height being entirely 



