THE LOCOMOTIVE. 



long after its first adoption. In the first instance, the tubes 

 traversing the boiler were small in number, and considerable in 

 diameter, but as theif effects were rendered more and more 

 evident by experience, their diameter was diminished and their 

 number increased, and at length it was not uncommon for the 

 boiler to be traversed by one hundred and fifty tubes of one inch 

 and a half in internal diameter. 



The heat was thus, as it were, strained out of the air before the 

 latter was dismissed into the chimney. 



These tubes were necessarily kept below the surface of the 

 water in the boiler, so that they were constantly washed by the 

 water, and the heat taken up from them was absorbed imme- 

 diately by the bubbles of steam generated at their surface, which 

 bubbles continually rose to the top of the boiler and collected in 

 the steam chamber. 



It will be understood from these observations, that the evapo- 

 rating power of the locomotive boiler, is determined by the 

 quantity of surface exposed to the radiant heat in the fire-box 

 and the quantity of surface exposed to the action of the heated air 

 in the tubes. The expression of the quantity of this surface in 

 square feet is the usual test of the evaporating power of the boiler. 



19. Much of the efficacy of these boilers depends on the quality 

 of the fuel. As the engines travel through districts of the 

 country more or less populous, the evolution of smoke is inad- 

 missible in consequence of the nuisance it would produce. It 

 was, therefore, resolved to use coke as fuel instead of coal. 



Another advantage, however, attended the use of this fuel. 

 Coke being composed chiefly of carbon, to the exclusion of the 

 more volatile constituents of coal which produce flame in the com- 

 bustion, the chief part of the heat developed acts by radiation. 

 No flame issues from the furnace, and heated air only passes 

 through the tubes. It is more easy, therefore, to extract the 

 heat than would be the case if flame were developed. In short, 

 with this fuel, the portion of the heat developed in the furnace is 

 much greater than that which would be developed in the com- 

 bustion of coal. The surface of the fire-box becomes relatively 

 more efficient, and the flues less so than in stationary engines 

 where coal is used. 



Independently, therefore, of the advantage of developing no 

 smoke, the coke is a form of fuel better adapted to the condition 

 of the lomocotive engine. 



20. To sustain a rapid and intense combustion on a grate 

 necessarily small, a proportional force of draft is indispensable. 

 In stationary engines, as is well known, the draft in the furnace 

 is usually produced by a chimney of corresponding elevation ; but 



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