BOILER No. 15. 113 



space with a perforated iron plate, which was placed on a level 

 with the top of the bridge. A damper located in the pipe, 

 was wide open when Cumberland coal was used and about 

 half-way open with the other fuels. 



The effect upon the economic result produced by the admis- 

 sion of air at the bridge was quite marked in the cases of the 

 Cumberland coal and mixed fuel, but not so in that of the 

 anthracite coal. The evaporation per pound of combustible 

 was increased 6.2 per cent, with Cumberland coal, but it was 

 decreased 4.7 per cent, with the mixed fuel. There was little 

 difference produced with anthracite coal. 



In every case the admission of air was accompanied by an 

 increased temperature of the escaping gases and a decreased 

 amount of power developed. These differences are small, but 

 they are always in the same direction. 



The character of the combustion, as seen by the eye of an 

 observer looking through a peek hole into the space behind 

 the bridge wall, was always improved by the admission of 

 air. 



The effect of admitting air upon the Cumberland coal was 

 to reduce the quantity and density of the smoke discharged at 

 the top of the chimney. There was entire absence of smoke 

 of the time when air was admitted, and of the time 



when air was not admitted. There was little smoke in either 

 case with mixed fuel. 



There was no appreciable difference in the quantity of soot 

 deposited in the tubes, as shown by examination at the front 

 ends after each test, whether air was admitted at the bridge or 

 not. 



The draught suction at the bottom of the chimney was 

 ordinarily -j^ of an inch, expressed in terms of water pressure. 

 In the boiler flue it was | of an inch with wide open damper, 

 and in the space behind the bridge wall j^r of an inch. 



Boiler No. 16. 



Kind of boiler, ...... Horizontal return tubular. 



Number used, ....... One. 



Horse-power (basis 12 square feet), . . Seventy-five. 



Age, ........ Two years. 



