ON THE TIIEOUY OF THE STEAM-ENGINE. 



581 



lul the 

 C. A. 



earliei', 

 practical 



ynamics 

 priori in'fc 

 tili'iui'- et 



' Wliile 

 oinparing 

 s found a 

 team past 

 the rigbt 

 my years 



ent. may 

 abes had 

 suggested 



as nearly 



vil. The 

 ml it lias 

 long l>rac- 

 ■; can only 



be approximated to that predicted by the theory of the ideal engine, 

 by special arrangements liaving for their object the reduction of cylinder 

 waste, such as superheating, ' steam-jacketing,' and ' compounding.' 



Professor Cotterill has given more attention to this subject than any 

 writer up to the present time. Ho devotes a considerable amount of 

 space to the study of the method of absorption and surrender of heat 

 by the metal surfaces enclosing tiio steam, constructs diagrams which 

 beaatifuUy illustrate this action, and solves the problems studied by him 

 with equal precision and elogance of method. Ho summarises the ex- 

 perimental work done to the date of writing, and very fully and clearly 

 exhibits the mode of transfer of heat past the piston without transfor- 

 mation into work. Professor Cotteriil's treatise on steam, ' considered as 

 a heat-engine,' is invaluable to the engineer. 



Thus the theory of the steam-engine stands to-day incomplete, but 

 on the verge of completion, needing only a little well-directed experi- 

 mental work to supply the doubtful elements. Even these are be- 

 coming determined. Isherwood gives facts showing waste to be 

 pi'oportional, very nearly, if not exactly, to the square root of the ratio 

 of expansion; and Escher, of Zurich, has shown the loss to be also pro- 

 portional to the square root of the time of exposure, or, in other words, 

 to the reciprocal of the square root of the speed of rotation, and it 

 only remains to determine the method of variation of loss with vai-iation 

 of range of temperature to give the whole- of the necessary material for 

 the construction of a Avorking theory which will enable the engineer to 

 estimate, in advance of construction, the ecionomic performance of his 

 machine. There will ujidoubtedly be much more to bo done in con- 

 structing an exact theory involving all the physical changes occurring in 

 the working of the hcat-enginos familiar to us ; but it will yet be done, 

 and probably very soon. It is the hope of tho writer that experiments 

 made under his direction recently may furni.-h the needed data, as the 

 result of the first systematic research directed to that end; but if this 

 should prove not to be the fact, it cannot be long before direct investiga- 

 tion will secure all cisential knowledge. When this is the case, tho 

 remarks of those distinguished jihysicists and engineers, Hallauer and 

 ins great teacher Hirn, will be no longer well based upon apparent 

 (\vet. 



Says Hirn, in his mtmiorable discussion with Zenner, in regard to 

 this subject : ' Md (vniviction reste aiijnn)\r/iui' ce qii'elle ctaif il y a viiKjt mis, 

 iiiii' tJujoric proprevioif tllle ih la viachhic a vapenr cut i }i> possible ; la 

 tlu'iorie e^rperimeiitalc, rtahlie sur le vioteur lui-mcine et ilavs tmdcs les formes 

 ok il a, ete essaije, en mccanique appliiiuoc, ^^cid seule conduire a des 

 rcsultats n'rinnren.i .' 



Ai present, it seems only possible, in the absence of a complete 

 experimental examination, to do more than to base the determination of 

 the ratio of maximum efficiency upon such experience as is familiar to 

 engineers. Mr. C. E. Emery considers that, for common unjacketed en- 

 gines it is practically safe to take the ratio for maximum duty at a figure 

 expressed by an empirical formula pro[)osed by him: r = {j) +37) -j-22. 

 The writer has usually taken it, in estimates, as not far from one-half 

 the square root of the boiler pressure expressed, as before, i'l pounds 

 on the square inch. These points of cat-ofl' are reduced still further by 

 the fact that, commercially, it is better to reduce the size of engine at the 

 t'xpenso of efficiency, as the cost of fuel and of similarly variable expenses 



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