340 ECONOMY OF FUEL FOR STEAM MACHINERY. 



sides so as to allow the free escape of steam, and hence the truth of 

 the observation that " the object of the fire box is more to generate 

 heat than to absorb it, and the absorption takes place chiefly in the 

 tubes." 



In relation to the tubes Mr. D. K. Clarke says : " There is reason 

 to believe that in the upper semi-circle part of such tube, the efficien- 

 cy chiefly resides. The winding progressive motion, observable in 

 tubes of considerable diameter, confirms the conclusion, as it is with 

 much probability due to the cooling of the upper portions of the gases 

 of combustion which, as they cool, also become heavier, and descend 

 laterally to make room for the hotter smoke next the bottom of the 

 flue, the general result of which is the spiral motion of the current 

 iu its progress forwards."* 



The writer in the Railway Times has introduced most of the above 

 quotations in his article, apparently for the purpose of paving the way 

 for proposing a new plan of constructing Locomotive Boilers. If 

 Mr. Clarke reasons correctly, however — and his argument appears to 

 have been well considered and based on actual experiment— it is 

 difficult to understand what advantages are likely to arise from the 

 introduction of Montgomery's principle into the construction of the 

 Locomotive Boiler, as proposed by the writer above alluded to, for 

 though there is no doubt but an advantage in point of area would be 

 obtained by applying the heat to the outer instead of the inner sur- 

 face of the tubes, proportionate to the increased diameter, we are 

 still unable to reconcile the following with Messrs. Armstrong and 

 Clarke's experiments and deductions in relation to vertical free sur- 

 faces : 



" If the ideas of Clarke and of Overman are correct, the value 

 of vertical flues with the water inside, as compared with the horizontal 

 flues with water outside, will be as follows : neglecting the physical 

 advantage of applying heat to the convex surface so highly estimated 

 by Overman. 



One half of the surface of the horizontal tube (the upper half) is 

 available, but this half generates steam twice as fast as the vertical 

 sides of the upright tubes. Thus the amount of evaporation will 

 be the same in either position for the same absolute tube surface, not 

 considering the increase obtained by applying the heat to the increas- 

 ed surface of the outside over the inside." 



Now it appears to us that Mr. Armstrong has shown that 

 vertical surfaces are only one half as efficient as horizontal ones 



* Railway Machinery, folio 126. 



