58 ANNUAL OF SCIENTIFIC DISCOVERY. 



systems practised in working out the general design, and to the neces- 

 sity for fixed principles in proportioning the locomotive to the work 

 for which it was destined. For the proper discussion of the question 

 it was indispensable to distinguish the three elements of the machine : 

 the boiler, the engine, and the carriage ; and to consider them sep- 

 arately, with respect to their proper functions, as the mixing up of 

 one with the other had caused much of the confusion with which 

 many of the recent discussions on the subject had been invested. 

 The paper was chiefly devoted to the discussion of the physiological 

 principles of locomotive boilers. It was argued, that the combustion 

 of coke in the firebox was, in practice, very completely effected ; that 

 it was quite independent of the strength of the draft, being equally 

 complete with fast and slow drafts ; that expedients for improving the 

 combustion were superfluous ; and that the combustion of coal might 

 also, in practice, be perfected by a judicious use of the ashpan, dam- 

 per, and the firectoor. The evaporation of 12 Ibs. of water per 

 pound of pure coke was found, by careful laboratory experiments, to be 

 the maximum evaporative performance ; in the best ordinary practice, 

 an actual evaporation of 9 Ibs. of water per pound of coke, or 75 per 

 cent, of the possible maximum, was readily obtained, the balance 

 being lost by leakage, of air and by waste ; and it was adopted, by the 

 author, as the ordinary standard of practical economical evaporation. 

 It was shown, by numerous examples, that the question of the rela- 

 tive value of firebox and tube surface was of no practical importance, 

 as the efficiency of boilers was not sensibly affected by their rela- 

 tive amounts ; that the superiority of firebox surface Avas due merely 

 to its greater proximity to the fire ; and that the distinction of radiant 

 and communicated heat was merely circumstantial, that what was 

 gained in radiant heat was lost in communicated heat, and that whether 

 it was all radiating, or all communicated, mattered not to the total effi- 

 ciencv of the fuel. On these grounds the author regarded with indif- 



/ O O 



ference the use of such expedients as extended fireboxes, midfeathers, 

 corrugated plates, and combustion-chambers; and it was asserted, that 

 where the addition of midfeathers had been found advantageous, 

 there had been a deficiencv or mal-arrangement of the tube-surface. 



v O 



A minute analysis was made of the results of numerous authenticated 

 experiments on the evaporative power of locomotive boilers, of very 

 various proportions, comprising several, made by the author, on the 

 engines of the Caledonian, Edinburgh and Glasgow, and Glasgow 

 and South AVestern Railways. It was concluded, that the economical 

 evaporative power of boilers was materially affected by the area of 

 the fire-grate, and by its ratio to the whole heating surface ; that an 

 enlargement of the grate had the effect of reducing the economical 

 evaporative power, not necessarily affecting the quality of combustion 

 in any way, but governing the absorbing power of the boiler, as the 

 lower rate of combustion, per foot of grate, due to a larger area, in 

 burning the same total quantity of fuel per hour, was accompanied by 

 a reduced intensity of combustion, and by a less rapid transmission of 

 heat to the water, in consequence of which a greater quantity of 



