ADMISSION OF AIR OVER FUEL. 53 



according to this method, and tests Nos. 40 to 45 were con- 

 ducted with a view to determining the effect which the admis- 

 sion of air has upon the economy of the boiler, three different 

 kinds of coal being used. The coals were Cumberland, anthra- 

 cite broken and a mixture of two parts pea and dust and one 

 part Cumberland. The air was supplied to the bridge wall 

 through an opening having an area of 38.5 square inches, 

 which is 1.4 square inches for one square foot of grate surface. 

 When Cumberland coal was used the full opening of this area 

 was employed ; when anthracite coal and the mixture were 

 used the opening was contracted about one-half, thus present- 

 ing an area of 0.7 square inches for one square foot of grate. 

 There is a wide difference in the effect of admitting air in the 

 different cases. In the case of Cumberland coal, the evapora- 

 tion is increased 5.9 per cent, per pound of coal, and 6.2 per 

 cent, per pound of combustible ; that of anthracite coal is 

 decreased one per cent, per pound of combustible and is about 

 the same per pound of coal ; that of the mixed fuel is decreased 

 2 per cent, per pound of coal and 4.7 per cent, per pound of 

 combustible. The effect which the introduction of air had 

 upon the appearance of the products of combustion, as viewed 

 by an observer at the " peek hole " back of the bridge wall, 

 was very noticeable in every case. It was greatest, to be 

 sure, with Cumberland coal, but the heightened color and the 

 increased activity of the flame produced by the entrance of air 

 was plainly to be seen whichever fuel was used. 



It is to be noted that an apparent benefit was realized in the 

 case of both the anthracite coal and the mixed fuel when 

 judged by the improved state of combustion which the eye 

 beheld. In reality, however, there was a loss, and this goes 

 to show that the ocular method of judging in these matters is 

 liable to be deceptive. 



In these tests another effect was produced by the admission 

 of air. The quantity and density of the smoke issuing from 

 the chiin ney was reduced. This was most marked with Cum- 

 berland coal, which gave the most smoke. When air was 

 excluded, smoke was visible of the time ; when air was 



