Septembee 5, 1902.] 



SCIENCE. 



395 



per unit of working iluid, is secured at a loss 

 of power per unit of heat supplied. It is still 

 further to be seen that the greater, as well 



Work in Rankine Cycles. 



as the more economical, work-production is ef- 

 fected by the conversion of the so-called ' la- 

 tent ' heat of vaporization directly into 

 mechanical energy or into work. It follows, 

 still further, that the larger the quantity of 

 'latent' heat, the greater the work of a given 

 weight of fluid and the lower the weight of 

 water or other liquid per unit of power. 

 Water has thus a double advantage in low ex- 

 penditure per horse-power at a given tempera- 

 ture-range and efficiency of cycle, and in small, 

 usually insignificant, cost. 



These relations and the variations, especially, 

 of relative magnitudes of the three terms with 

 varying expansion-ratios from a given initial 

 pressure, with, as is usual, constant back-pres- 

 sure, in the ideal case, is well exhibited by the 

 accompanying figure. This set of curves in- 

 cludes those of total work, of values of the 

 several terms, and of relations of rate of 

 variation, for such a case, in which the steam- 

 pressure is about 7.5 atmospheres, the back- 

 pressure one seventh that tension, and the ra- 

 tio of expansion, ranging from unity upward. 



in an engine which would be ordinarily rated 

 at about 200 horse-power, at 85 revolutions 

 per minute with r = 4. 



It is seen that the total work, U, increases 

 rapidly from ?• ^ 1 to r = 4, passes a maximum 

 at about 5 and rather rapidly falls off again, 

 after the expansion-line begins to intersect the 

 back-pressure line (curve A). 



The work of sensible heat (curve B) in- 

 creases slowly throughout the range exhibited, 

 and substantially in proportion to r, and is, 

 throughout the whole range, small in com- 

 parison with the work of ' latent heat ' (curve 

 C). 



The work of the rectangular area below ter- 

 minal pressure (curve D) is similarly variable, 

 but becomes negative at the point of junc- 

 tion of the expansion line with the back-pres- 

 sure line. Throughout the whole usual range 

 of expansion in the real engine, this quantity 

 is small in comparison with that measuring 

 the second term. 



The value of the second term, on B, is thus 

 the principal element of the total work of the 

 cycle and is larger, relatively, as the diagram 

 approximates the form of the Carnot, rather 

 than the Rankine cycle. The deduction at 

 once follows that ' latent ' heat, and latent heat 

 only, so far as practicable, should be utilized 

 in the thermodynamic transformations of the 

 vapor-engines.'"' The ' latent heat fallacy ' is 

 thus clearly disposed of. 



A similar investigation would show that, in 

 the gas-engines, the 'latent' heat of isothermal 

 expansion, rather than the sensible heat pro- 

 ducing change of temperature, should be 

 utilized in thermodynamic transformations 

 and the production of power. 



The final conclusion is thus obvious that 

 maximum efficiency of thermodynamic engines 

 can only be secured by the utilization, solely, 

 of 'latent' heat. 



R. H. Th0rston. 



ON BACDBIRITO, THE GREAT METEORITE OF SINA- 

 LOA, MEXICO. 



For more than a century the meteorites of 

 Mexico have attracted attention and record. 



*' Manual of the Steam-Engine,' "V^ol. I., §112, 

 pp. 437-438. 



