WILLIAM SIEMENS, F.K.S. 



which the double cylinder, or Woolff engine, had come largely into 

 use. It is due in great measure to the energies of one of your 

 naval constructors, the late Mr. John Elder, that a more 

 economical marine engine has been brought into use in the shape 

 of a modification of Woolff' s engine, in which the crank of a high- 

 pressure cylinder is placed at right angles to that of the low- 

 pressure or expansive cylinder, whereby the important advantage 

 is realised that a single pair of cylinders produces a continuity of 

 driving power throughout the revolution of the engine-shaft. 



The economical results obtained through the introduction of 

 these improvements are strikingly illustrated by the fact that one- 

 horse-power is now produced with an expenditure of 2 Ibs. of coal 

 per hour, whereas in the most economical marine-engines fifteen 

 years ago double the amount was employed. 



The calorific effect residing in one pound of pure carbon, if 

 burnt under the most favourable circumstances (producing carbonic 

 acid as the result of combustion), is 14,000 heat units ; but 

 common coal, containing an average amount of ash, moisture, and 

 absorbed carbonic acid, may be taken at 12,000 units with perfect 

 combustion. These represent 12,000 x 772 = 9,264,000 ft. Ibs. of 

 force ; and the two pounds of coal consumed per horse-power 

 represent twice that amount, or 18,528,000 ft. Ibs., whereas one- 

 horse-power is represented by 33,000 x 60 = 1,980,000 ft. Ibs. 

 per hour. This comparison brings us to the conclusion that' the 

 best steam-engine of the day utilises only about one-ninth part of 

 the heat producible by the combustion of the fuel employed. 



But it must be remembered that although force may be converted 

 entirely into its equivalent of heat, heat cannot be converted into 

 force without loss, through heat descending from a point of higher 

 energy or temperature to a lower point, the amount of force 

 realisable being dependent upon the range between the maximum 

 and minimum temperatures, or in the case of an elastic fluid engine, 

 the temperature before and after expansion. 



An engine capable of developing one horse-power with two 

 pounds of coal per hour, is worked with a pressure of 60 pounds 

 above the atmosphere, or 74'7 pounds on the square inch, with a 

 corresponding initial temperature of 307 Fahr., and a pressure in 

 the condenser of 1 pound on the square inch, corresponding to 

 105" Fahr. ; we find, by taking the ratio of the difference of these 



