A CKNTUKY S PKodRKSS oF TlIK STKAM KXCilNK. 



;V.l7 



iH'ccssiirv to adopt, rii'st. the coniijoimd, the douMu'-cyliiuU'r ('n_i>ine; 

 then the tri|)l:\ and tiiially tlio <iiiadi-uplo-expansion inachiiu'. The 

 compound caiut' in a')out 1S54. tlie triple in l(S74, and the (juadi-uple 

 durino- the elosinu- years of the century. The demand for ini^reased 

 pi'essures also conipelled a gradual moditieation of the standard con- 

 structions of steam boilers, and finally forced the adoption of the now 

 familiar water-tul)e boiler, with its externally heated surfaces, a form 

 of boiler original with the (nirliest inventors of the steam engine of 

 modern type. 



The incr(>ase in the ratio of (expansion adopted from the tirst has 

 been in a manner fairly constant in its relation to the pressure, and 

 may be roughly taken as, for common pi-actice in condensing engines, 

 the ■* absolute" steam pressure at the boiler divided by U) pounds. 

 The terminal pressure, in good practice, has been about 1<» pounds, 

 falling, in tho engines of highest (^tiiciency and giving maxiuHini duty 



A.D. 1830 1810 1850 18G0 1870 1880 1800 1»00 



FIG. 4.— STEAM PRESSURIiS IN M.^RINE ENGINES 



for their time, to .s and occasionally to 7 or e\en ti pounds al)solute. 

 The })recise relation of the ratio of steam pr«>ssure to back pi'essure 

 to the ratio of "total" expansion in all classes of engine has necessa- 

 rily been affected \(M\ appicciably by the degree of approximation 

 secured to truly ideal thermodynamic, adiabatic exi)ansi()n. Initial 

 condensation and. later, reevaporation have a luarked (^ii'ect u])on this 

 relation, and this, in turn, is determined in amount by the character 

 of th<^ consti-uction and the "(piality" of the woi'king Huid. 



The Hnal improvement of the steam engine, mai'king the licst i)rac- 

 tice of the century, and particularly of its later years, is that which 

 reduces that variation from the thermodynamic ideal which is conse- 

 (|uent upon internal waste due to exchange of heat l)etween the steam 

 and the luetal of the working cylinder, Rankine's ideal "•cycle of the 

 nonconducting cylinder" can be secured either by actually making the 

 cylindei nonconducting or by gi\ing the steam so nearly gaseous a 



