130 LECTURES TO SCIENCE TEACHERS. 



water actually evaporated by 1 Ib. of Llangennech coal, 

 assuming the water to be at 212 which it was very nearly. 

 The other column shows the water that could have been 

 evaporated had the feed water been at the ordinary tem- 

 perature of 62. We will confine our attention to the first 

 column, and you will see there that the average obtained was 

 9-85 Ibs. of water boiled off by 1 Ib. of coal. This column 

 is sufficient to show the extreme danger of judging by an 

 average unless you know the maximum and the minimum, 

 for if you look at the second line from the top you will find 

 that as much as 11*83 was boiled off in one case, while in 

 another case it was only 4-84. The same coal was used ; the 

 difference was in the proportions of the boiler, and in the 

 ability of the stoker. With respect to the stoking on these 

 trial occasions, the stoking is* done by men, some of whom 

 are paid several hundreds a year. It is done by a hand 

 fire-shovel, more like a banker's scoop than an ordinary 

 shovel; the coal is broken into bits about the size of a 

 walnut, the fire-door is opened about once in- every two 

 minutes (which you will find very wrong, according to 

 Tredgold, and so it would be if the men were not trained to 

 do it by sleight of hand so dexterous that it is hardly possible 

 to see the operation performed) ; and in that way there is 

 kept up an absolute uniformity of combustion, a uniformity so 

 great that, during the five hours which the trial sometimes 

 lasts, I will undertake to say the pressure gauge does not vary 

 one-sixth of a pound. So accurate is the firing that whereas 

 the engine would blow off with 80 Ibs. and thus waste steam 

 which would spoil all the competitor's chance, at 77 J Ibs. 

 the engine could not run against the weight, therefore the 

 stoker's margin of possible working in these engines lies 

 within 2J Ibs. Such accuracy you cannot hope to obtain in 

 daily work. Let us see how essential good stoking is. What 

 is it that we want to do 1 We want the most perfect com- 

 bustion of the fuel, coupled with the least admixture of air, 

 for the purpose of making that combustion. Assume for the 

 sake of illustration that every pound of coal gives us 14,000 

 English heat units if we burn it to carbonic acid, and that 

 we only get in round numbers 4,000 units if we burn it 

 to carbonic oxide, that is to say, in the latter case we only 

 get -3^- and lose ^ of the effect. What we have to insure, 

 therefore, is, that sufficient air is introduced into the furnace 







