234 On Warming and Ventilating 



fore, is put out of all doubt, since it is essential, that the iron 

 shall not be heated to a degree capable of decomposing animal 

 and vegetable matter, in order to preserve the purity of the air 

 which is warmed in contact with its surface. 



With a view to ensure the above objects, it will be necessary 

 to dispose of the heat as it is produced from the combustion of 

 the fuel, in such a way, that an extensive surface of iron 

 shall be heated uniformly without the risk of attaining a much 

 higher temperature than 300°. This can be accomplished by 

 making the fire of a size proportionate to the interior surface of 

 an iron vessel, and it is found that radiant heat is much more 

 efficacious than the heat produced by flame and conducting 

 flues. Having heated the interior surface of an iron vessel it 

 may be conceived that the exterior surface will quickly attain 

 the same degree, and that whatever heat may be carried off" from 

 the exterior will be as quickly given from the interior, and in- 

 stantly replaced by the radiant fire. 



The next material object is the means of disposing of the 

 heat from the exterior surface. If it be surrounded by an open 

 space, and that be connected with a flue or tunnel of a certain 

 height, supposing there to be no inlet at the bottom, or outlet 

 at the top, the air will commence a circulation ; that on the 

 heated surface ^would ascend, and its place be as constantly 

 supplied by the surrounding air. In this way two currents will 

 be established ; one ascending from the heated surface, and the 

 other descending on the outside of the tunnel ; and these currents 

 will go on, as long as any diflference of density exists in the air of 

 the different parts of the surrounding space. If now an opening 

 be made in the bottom of this tunn.l and another at the top, an 

 ascending current will be kept up ; which will be as the differ- 

 ence of density between external air and that of the heated 

 column, and as the square root of the height of the tunnel. 



Let D be the density of the external air ; 



d, that in the tunnel, which will be inversely as the heat 

 supplied. 



V =: the velocity which a heavy body would acquire by 



