S/K WILLIAM SIEMENS, F.R.S. 163 



steam without expansion, and the result was found to be that 

 \\ hrivver this rectangle was taken its area amounted to only -j^th 

 of the area of the curve expressing the power due to the ultimate 

 usion of the same steam ; but a good high-pressure expansive 

 engine using steam of four or five atmospheres total pressure, 

 cutting off at about one tenth of the stroke and working down to 

 a good vacuum, realised a very considerable proportion, something 

 like one fourth or one fifth of the theoretical maximum which 

 could ever be obtained. The theoretical minimum consumption of 

 1'iu'l in a perfect steam engine he had calculated would be (taking 

 14,000 as the total units of heat developed by the complete com- 



33,000 ft.-lbs. x 60 mins. 

 bustion of 1 Ib. of carbon) ^ m unitg heafc x ._, ffc .^ = 02 Ib. 



or one fifth of a pound of carbon per horse power per hour ; or one 

 fourth of a pound of coal, taking into account impurities. 



In the case of the air engine, it was apparent that both Stirling 

 and Ericsson had over-estimated the real value of the regenerator 

 under a misconception of its true action ; they had imagined that 

 it was possible to absorb and give back the whole of the heat 

 originally put into the air, with the exception only of accidental 

 losses, and had overlooked the fact that a portion of the heat 

 became entirely used up by being changed into mechanical effect ; 

 both their engines had accordingly been deficient in heating power, 

 and had failed to give permanently , satisfactory results. Even 

 supposing perfect re-absorption of heat from the exhaust air, 

 theoretical considerations showed that an air engine was necessarily 

 very imperfect as a means of developing power from heat ; because 

 although a volume of highly compressed and highly heated air, if 

 expanded down to atmospheric pressure and discharged at no 

 higher temperature than that of the external atmosphere, would 

 yield the full result for the heat absorbed in expansion, yet an 

 equal weight of air would then have to be taken up again and 

 compressed to the original pressure, thereby generating a great 

 amount of heat, which would all be wasted because it was 

 generated in the air before its expansion by heat took place and 

 when it should occupy the least volume, the power of the engine 

 being dependent upon the increase of volume. In the best air 

 engines, therefore, even supposing perfect absorption of the escaping 

 heat, it would not be possible to realise anything like so much as 



