5b ON THE LOSS OP HEAT 



the point of ignition. In chimney fire-places, it is geueralh 

 observed, that wood fires burn most rapidly in cold weather : 

 and, even while the air of the room is quite cold, they are 

 known to burn very well. This fact will probably be urged, 

 to disprove the necessity of heating the air, to produce more 

 complete combustion in anthracite coal. It should be recol- 

 lected, however, that wood ignites at a much lower tempera- 

 ture, and, that in very cold weather, a much larger quantity is 

 required to be in combustion at one time, than in moderate 

 weather ; and, consequently, that the air within a few feet of 

 the fire, and before it comes in contact with it, is more heated 

 than it is at the same distance in moderate weather, when less 

 fire is required. 



The intense heat produced by an air furnace, does not appear 

 to be in consequence of an increase in the volume of air, as 

 those furnaces which are said to have the strongest "draught," 

 will be found to have the most contracted throats. But, by 

 thus contracting the throat, the friction of the air is increased, 

 and its velocity being also increased, the sound which is said 

 to denote a strong "draught," follows, as a necessary conse- 

 quence. The air being very much expanded from its increase 

 in temperature, and its rapid escape in large volume, being 

 prevented by the contraction of the throat, the contact with 

 the combustible is not only prolonged, but the real quantity in 

 contact, at any one time, may be supposed to be considerably 

 diminished; yet, this being more frequent and rapid, the union 

 is more perfect, and, consequently, more intense heat is pro- 

 duced. 



The superior light of an Argand lamp, is, probably, in con- 

 sequence of surrounding the burner with a glass chimney, by 

 which the current of air is considerably elevated in its tempera- 

 ture, and the volume admitted is diminished, and not increased, 

 as is generally supposed. Whether its increased velocity 

 through the chimney is advantageous in the process of com- 

 bustion, when abstractly considered, may be questionable; 

 but, it is evidently advantageous in dissipating the products of 

 combustion, or rather, imperfect combustion, which would 



