5 t BOILEE TESTS. 



admitted, the smoke was visible 70 of the time. In the 

 case of the mixed fuel little smoke was produced in either 

 case. 



A second method of admitting air which, though not so 

 simple, is rather more effective than that just named, is the one 

 applied to Boiler No. 9. Here the air is supplied first to a 

 pipe laid in the bridge wall, and then to perforated cast-iron 

 globes which rest upon the top of the wall. This method is 

 rendered more effective than the first by the more thorough 

 mixture which it secures between the incoming air and the 

 burning gases. The quantity of air supplied is increased 

 above that naturally drawn in by means of a jet of steam. 

 The steam thus supplied mingles with the air. In this case the 

 effect of admitting air above the fuel, when Cumberland coal 

 was burned, was to increase the evaporation per pound of coal 

 8.4 per cent., and per pound of combustible 8 per cent. It 

 may be added that a test made with Lehigh coal, not recorded, 

 showed that with this fuel the admission of air was attended 

 by an increase of evaporation of 1.9 per cent, per pound of 

 coal and 3.7 per cent per pound of combustible. 



A third method of admitting air, which is more efficient 

 than either of the first two, is that applied to Boiler No. 40. 

 It consists in supplying one current of air through the bridge 

 wall by means of perforations in the rear face of the wall near 

 the top, and another current of air through a secondary wall 

 placed a short distance to the rear of the bridge wall, and 

 elevated so as to close up the passage between it and the shell 

 of the boiler, and make the products of combustion descend 

 and pass beneath it. The second current of air discharges 

 through perforations in the front face of the hanging wall. By 

 these provisions the entering air is thoroughly diffused through 

 the whole volume of burning gas. The only test made on this 

 boiler was with the air admitted, and there is consequently no 

 positive information as to the economy of this method. There 

 is indirect evidence, however, pointing to its superiority, in the 

 fact that the result of the test was most excellent, being 12.47 

 pounds of water from and at 212 degrees per pound of com- 



