52 BOILER TESTS. 



partially filled. Boiler No. 10, which also gave a high result 

 with anthracite coal, has a curved bridge wall with vertical 

 front, and the space behind is filled so as to conform to the 

 same curve. Boiler No. 12 is another case in point, and here 

 the bridge wall is flat with a vertical front, and the space 

 behind is filled in so as to provide an inclined bed leading 

 down to a deep chamber at the rear end. Boiler No. 35, 

 which gave a good result, if allowance is made for the high 

 temperature of the gases, has a flat bridge wall with perpendi- 

 cular front, and the space behind is filled to an even level with 

 the top. Boilers No. 13 and No. 16, using inferior grades of 

 anthracite coal, showed favorable results, and the bridge walls 

 are flat with vertical fronts and the space in the rear is open 

 in both cases. Passing to boilers using Cumberland coal, No. 



31 and No. 32, which give high results, both have vertical walls ; 

 No. 31 is curved at the top, with open space behind, and No. 



32 is flat with open space, the latter being provided with a 

 second wall at the rear end of the boiler. It would appear 

 from these examples that the general form of the setting in 

 respect to the particulars named may be one thing or another, 

 and the boiler still give the highest economy. This furnishes 

 strong ground for the conclusion that the matter is of compar- 

 atively little importance, and that the plan to be followed is 

 the one which will secure the simplest, and at the same time 

 the most convenient arrangement. 



The tests give some very pointed indications regarding the 

 eifect of admitting air into the furnace above the fire. Many 

 of the settings are arranged with a view to this special object. 

 A current of air in a finely divided state is introduced into the 

 furnace so as to mingle with the products of combustion as 

 they emerge from the burning coal. 



The simplest method followed is to conduct the air directly 

 from the outside to the interior of the bridge wall, which is 

 made hollow, and to discharge it through perforations in the 

 top covering of the wall, which may be either iron or brick. 

 The air thus supplied mingles with the lower strata of burning 

 gas as it skims over the bridge. Boiler No. 15 was arranged 



