672 
Report on the Trials of Portable 
All loss of heat-energy means loss of mechanical energy ; 
and the steam-engine is an apparatus having many heat-energy 
leaks, which the most skilful engineer can only ])artially stop, 
and the unskilful man altogether fails to find. Energy is dis- 
sipated with every puff of exhaust-steam into the atmosphere. 
It disappears with every molecule of water condensed or re-eva- 
porated, whether in the cylinder, the jacket, or the boiler. It 
slips at every stroke through that excellent heat-carrier, the 
piston, from the hotter steam-side, to the colder exhaust-side of 
the cylinder. It pours, as " waste heat," up the chimney. It is 
reduced by every atom of atmospheric air admitted to the 
furnace over and above the quantity required for perfect com- 
bustion of the fuel, and it radiates from every square inch of 
exposed surface. These losses in the best engines may be dis- 
tributed as follows : — seventy per cent, of the total heat-energy 
derived from the fuel, including that which is lost in " steam- 
making," is rejected in the exhaust-steam ; twenty per cent, is 
lost by radiation, conduction, and faults of mechanism ; and 
only the remaining ten per cent, is converted into mechanical 
effect. 
Reverting now to the original question — " What are the 
relative merits of Simple and Compound engines ? " it is 
evident that this enquiry may be better stated in another form ; 
viz. " What leak, or leaks, of heat-energy in the simple engine 
can be stopped by compounding?" 
It has been shown that expansive working in Simple engines 
results in losses by condensation and re-evaporation which, 
increasing with the rate of expansion, are only partially com- 
pensated by the use of the steam-jacket. The greater the dif- 
ference between the temperature of the incoming and the out- 
going steam in a cylinder, the greater the loss by cooling and 
consequent condensation. If, without sacrificing the advantages 
to be gained from its expansion, steam could enter and leave a 
cylinder without any reduction of its temperature, all losses due 
to condensation and re-evaporation would be avoided. 
But no approximation to this state of things is possible in 
the simple engine, as best becomes apparent by an instance. 
Steam, entering the unjacketed cylinder of such an engine at a 
working pressure of 100 lbs., and a temperature, therefore, of 
328°, is rejected into the air at a temperature of 212°, repre- 
senting a drop of 116°, as between one end of the stroke and the 
other. The greater this fall in temperature, the greater the 
condensation and consequent re-evaporation taking place in 
the cylinder, processes which the steam-jacket (itself, be it 
remembered, a coal consumer), cannot entirely check. It is 
clear, under these circumstances, that some device is wanted 
