GENEEAL CONDITIONS OF ECONOMY. 33 



There is a difference here of 11.6 in the ratio given, and 

 practically no difference in the character of the results. Nothing 

 seems to be gained in these cases by increasing the surface 

 above a ratio of 36.4 to 1, although the increase amounts to 

 about one-third. Carrying the inquiry farther, and taking the 

 so-called double-deck boilers, of which there are four instances 

 given in Table No. 1, the average ratio is 65.3 to 1 and the 

 average evaporation is 10.88 pounds. Here a loss is produced, 

 although the surface is increased to the enormous extent of 80 

 per cent. These comparisons are made with different kinds of 

 anthracite coals of large sizes, and with different arrangements 

 of boilers, and some allowance must be made for the possible 

 effect which variations in these conditions may have on the 

 results. But the comparisons are suggestive. The evidence 

 here given shows that a ratio of 36 to 1 provides a sufficient 

 quantity of heating surface to secure the full efficiency of an- 

 thracite coal where the rate of combustion is not more than 12 

 pounds per square foot of grate per hour. 



Individual examples are given, which furnish evidence as to 

 the extent of surface required when bituminous coal is used. 

 Boilers No. 28 and No. 29 are cases in point. Here an increase 

 in the ratio from 36.8 to 42.8 secured a small improvement in 

 the evaporation per pound of coal, and a high temperature of 

 the escaping gases indicates that a still further increase would 

 be beneficial . Among the high results produced on common hori- 

 zontal tubular boilers using bituminous coal, the highest occurs 

 in Boiler No. 40 where the ratio is 53.1 to 1. This boiler gave 

 an evaporation of 1 2 .47 pounds. The double-deck boiler, No. 42 , 

 furnishes another example of high performance, an evaporation 

 of 12.42 pounds having been obtained with bituminous coal, 

 and in this case the ratio is 65 to 1. These examples indicate 

 that a much larger amount of heating surface is required for 

 obtaining the full efficiency of bituminous coal than for boilers 

 using anthracite coal. There is sufficient reason for this re- 

 quirement in the fact that bituminous coal is of a gaseous 

 nature, and the heat generated in its combustion is spread 

 through a larger space. The temperature of the escaping 



