SATUBATED AND SUPEEHEATED STEAM. 27 



large quantity of unneeded air is admitted through the brick- 

 work, not only into the furnace, but into the flue space as 

 well, which on the one hand reduces the furnace temperature, 

 and on the other hand lowers the temperature of the escaping 

 gases. In this way a direct loss is produced by cooling the 

 furnace ; and further, the loss at the chimney, measured by the 

 temperature, appears to be less than it really is. 



It is clear that, according to these tests, the vertical tubular 

 boiler in practical use, having the proportions given, is less 

 economical than the ordinary type of horizontal return tubular 

 boiler ; and this is true notwithstanding the superior value of 

 the superheated steam which they give, for which the most 

 favorable allowance has been made. 



There are two cases of vertical boilers in the paper, where 

 the proportion of water-heating surface to grate surface is fully 

 as large as in the best form of horizontal boiler, and the 

 unfavorable influence of air leakage is prevented by the use of 

 a fire-box. The boilers referred to are those numbered 58 and 

 60. The proportions of water-heating surface to grate surface 

 are respectively 35.1 and 44.5 to 1. The temperatures of the 

 escaping gases are respectively 423 and 427 degrees. The 

 evaporations per pound of combustible from and at 212 degrees 

 are both 12.29 pounds. Although these tests were made using 

 high grades of bituminous coal, and for this reason cannot be 

 fairly compared with those under consideration, the high charac- 

 ter of the two results, contrasted with those referred to, brings 

 out very clearly the fact of the inferiority of that type of verti- 

 cal boiler which is deficient in heating surface and provided 

 with a brick setting. 



It would not be fair to conclude, because the superheating 

 boilers referred to are at a disadvantage when compared with 

 those which do not superheat, that this result must always 

 follow. Were the loss from air-leakage remedied by the use 

 of a fire-box, there is good reason to expect that the vertical 

 boiler furnishing superheated steam would be as economical as 

 the horizontal boiler furnishing saturated steam, and it is not 

 improbable that with suitable arrangement of surface, it would 

 in the long run take the first place. 



