VIII. NORMAN SELFE. 



h%ve already been made as to the amount of external heat which 

 would have to be supplied (or the number of thermal units that 

 would be required) at a temperature above that of the atmosphere, 

 before an equal weight of air could be liquefied, but I have seen 

 none which appear to exhaust the possibilities. It will, perhaps, 

 show why this subject is so attractive to so many minds, if for a 

 short time we leave the substantial for the visionary. 



In the steam engine we know that the useful work performed 

 generally lies between, say, 100 degrees and 300 degrees Fah. 

 One of the most useful contributions that has ever been made 

 towards a simple comprehension of the thermal efficiency of the 

 steam engine, is the report and accompanying diagrams, from 

 the specially appointed Committee, which appears in the last 

 volume of " The Proceedings of the Institution of Civil 

 Engineers." 



From these papers we see it graphically demonstrated, how 

 with a range of temperature of 259 degrees (between 100 and 359 

 degrees) the Leavitt pumping engine* (one of the highest type for 

 efficiency yet constructed) leaves in round numbers four times 

 as much of its heat energy in the condensing water, as it con- 

 verts into mechanical work. This efficiency is 67 per cent, of 

 that possible with an ideally perfect steam engine. 



Looked at in another way, this unused heat means that the 

 engines of a mail steamer which indicate 10,000 horse-power 

 will pump overboard every hour sufficient heated circulating 

 water to warm 2,300 tons of the ocean 20 degrees, four-fifths of 

 the calorific effect of the coals burnt being thus wasted. 



Now, suppose it was possible to find a medium which would 

 not only evaporate under pressure while below 32 degrees (as many 

 substances in common use actually do), but which would also 

 re-liquefy below 32 degrees when it gave up heat by expanding 

 against a piston, and doing mechanical work. What would be 

 then possible ? 



* At Louisville, Ky., U.S.A., described in the transactions of the 

 American Society of Mechanical Engineers. 



