74 



but probable; I have however vaUied the heals in pyrometric 

 degrees so tliat I think they cannot be far from the truth, and 

 the near coincidence of conclusions derived from methods so dis- 

 similar, is sufficient to shew that they are approximations to the 

 real value. If we take the mean of them and reckon 180=' to be 

 the utmost power of a good blast furnace, the maximum heat would 

 be 4320" F. not much removed from the determination of theory. 

 Leaving this question to be decided by future experiments, we will 

 proceed to observe that the temperature of a furnace is not uniform 

 throuo-hout, the heat gradually diminishes from the axis to the cir- 

 cumference of it, for which reason the crucible when intense action 

 is required should not exceed a certain magnitude, and it also varies 

 from the grate upward. When air is drawn through a body of 

 coals, its oxygen is changed into carbonic acid gradually, and at the 

 same time it is heated so that its temperature is continually augment- 

 ing as long as it can support combustion, afterwards it can acquire 

 no increase. The centre of that stratum of fuel where the air is 

 completely deoxidated I will call the focus of the furnace ; and its 

 position should be ascertained by experiment, a rod of refractory 

 clay should be fixed vertically on the grate, and its vitrification will 

 mark the limits in which the greatest action has taken place. In a 

 furnace supplied with Kilkenny coal, I found that it was 2 inches 

 above the grate, but this depends on the quantity of the fuel and 

 also the rapidity of the current of air, as when it moves with great 

 velocity, it will be less deoxidated in passing through a given quan- 

 tity of coals. I found after an unsuccessfvd attempt to ftise platina, 

 that in essaying it with a double load on the bellows, the focus rose 

 so much above its previous place that the upper part of the crucible 



