\»o$i\j>N Theory of Heat to the Steam-engine. 265 



For instance, machines driven by heated air will only attain a 

 decided advantage over steam-engines when a method is found 

 of allowing them to work at a far higher temperature than steam- 

 engines, in consequence of the danger of explosions, can bear. 

 The same advantage, however, could be attained with over-heated 

 vapour ; for as soon as the vapour is separated from the liquid, 

 it is just as safe to heat it further as to heat a permanent gas. 

 Machines employing vapour in this condition may possess many 

 of the advantages of the steam-engine besides those of air- 

 machines, so that a practical improvement may sooner be ex- 

 pected from these than from air-machines. 



In the machines above mentioned, where, besides water, a 

 second more vaporizable substance was employed, the interval 

 Tj — Tq is increased by lowering Tq. It has already been sug- 

 gested, that this interval might be increased in a similar manner 

 on the upper side, by the addition of a third liquid less vapor- 

 izable than water. In such a case the fire would be immediately 

 applied to the evaporation of the least vaporizable of the three 

 substances, the condensation of this to the evaporation of the 

 second, and the condensation of the second to the evaporation of 

 the third. Theoretically, there is no doubt that such a combi- 

 nation would be advantageous; the practical difficulties^ how- 

 ever, which would have to be overcome in realizing such a scheme 

 cannot of course be predicted. 



26. Besides the above-mentioned defect, arising out of the 

 very nature of our ordinary steam-engines, these machines suffer 

 from many other imperfections, which may be ascribed more 

 immediately to defective construction. 



One of these has already been considered in the foregoing 

 development, and allowed for in equation (X), that is to say, the 

 expansion cannot be continued nearly far enough to allow the 

 vapour in the cylinder to reach the temperature of the condenser. 

 If, for example, we assume the temperature of the boiler to be 

 150°, and that of the condenser to be 50°, then the Table in 

 § 16 shows, that, for the above purpose, the expansion must be 

 prolonged to twenty-six times the original volume ; whereas in 

 practice, owing to many inconveniences attendinggreat expansions, 

 three or four, and at most ten times the original volume is attained. 



Two other imperfections, however, are expressly excluded in 

 the foregoing : these are, first, that the pressure of the vapour in 

 one part of the cylinder is smaller than in the holler, and in the 

 other part greater than in the condenser', and secondly, the pre- 

 sence of vicious space. 



We must consequently extend our former considerations sO as 



to include these imperfections. =:^/"? b >i'r?.,i) /s itjl.rjijih.j 



[To be continued.] '*' fr -/r^J ., wfRr.rjHrw 



