MECHANICS AND USEFUL ARTS. 



79 



pounds (of a somewhat inferior coal) for the same effect. There are, how- 

 ever, more than twenty large engines in Glasgow at present, which work 

 with a consumption of six and a half pounds of dross, equivalent to five 

 pounds of the best Scotch, or four pounds of the best Welsh coal, per horse 

 power per hour. The economy must be estimated from these data, as in the 

 other cases, on the assumption, which, with reference to these, is the most 

 probable we can make, that the evaporation produced by a pound of best 

 coal is seven pounds of steam. 



A cubic foot of water weighs 62*32 pounds ; allowing five pounds of coal to 

 evaporate this quantity ( =12'464 pounds per pound of coal and 12'9 has 

 been done), we get, as the theoretical duty of one pound of coal: 



It has been already said that the best expansive engines have never real- 

 ized in practice more than sixty per cent, of their theoretical duty. As re- 

 gards the composition of such loss of forty per cent., Mr. Pole found, that if 

 an engine of that kind, expanding three and a half times, were absolutely 

 perfect, each unit of heat given out by the combustion of the fuel ought to 

 develop about 134 units of work; but the amount actually produced in the 

 shape of water raised was only about eighty units, or sixty per cent, less than 

 the theoretical result. He has endeavored to discover at what parts of the 

 engine this loss occurred, and has found it might be distributed about as fol- 

 lows : 



Imperfect combustion, and other causes of waste of heat in the boiler, 

 Heat expended in raising the temperature of the feed-water to a 

 boiling-point, 



12} 



Friction, imperfect vacuum, air, pump, etc., or power wasted in 



working the engine, 15 



Useful effect realized, 



Total calorific power of the engine, 



40 

 60 



100 



The friction of the machinery of a locomotive engine has been experimen- 

 tally determined by De Pambour at -fa of the tractive force it exerts, and 

 this exactly coincides with the results of Mr. Pole's analytical investigation 

 of the friction of the direct-acting marine engine with slides. This is, of 

 course, exclusive of the resistance of the air-pump, and of the friction caused 

 by the pressure (when unbalanced) of the steam on the back of the slide- 

 Valve. Engineer and Arch. Journal. 



