VENTILATION BY OPEN FIREPLACES. 337 



Siemens when he spoke; but they are quite different from 

 stove-heating or pipe-heating, which increase the vapor 

 capacity of the heated air, without supplying the demand 

 it creates. 



VENTILATION BY OPEN FIREPLACES. 



THE most stubborn of all errors are those which have 

 been acquired by a sort of inheritance, which have passed 

 dogmatically from father to son, or, still worse, from 

 mother to daughter. They may become superstitions with- 

 out any theological character. The idea that the weather 

 changes with the moon, that wind ''keeps off the. rain," 

 are physical superstitions in all cases where they are blindly 

 accepted and promulgated without any examination of evi- 

 dence. 



The idea that our open fireplaces are necessary for ven- 

 tilation is one of these physical superstitions, which is pro- 

 ducing an incalculable amount of physical mischief through- 

 out Britain. A little rational reflection on the natural and 

 necessary movements of our household atmospheres dem- 

 onstrates 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 temperature; 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 upwards. The latter postulate must be kept 

 distinctly in view, for the rising of warm air is too com- 

 monly 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- 



