584 MACHINES, WORK, AND ENERGY 



per ton or ^{ ct - P er Ik. The cost of 4.4 Ib. of coal would, there- 

 fore, be 4 ^foo ct - To operate this engine eight hours the cost would 

 be 8 X 0.44 ct. or 3.52 cts. per H. P. 



2. How many pounds of coal will the gas producer and gas engine 

 (Fig. 373), require to do 1 horse-power-hour of work? Ans. 1.1 Ib. 



What would be the cost of coal per horse-power-day of eight hours, 

 if bituminous coal costing $2 per ton were used? If anthracite 

 coal costing $6 per ton were used? 



Ans. 1.1 cts. for bituminous coal. 

 3.3 cts. for anthracite coal. 



3. How many pounds of gasoline would be required by the gasoline 

 engine (Fig. 374), while doing 1 horse-power-hour of work? 



Ans. About Ko Ib. 



What would be the cost for gasoline per horse-power-day of e ght 

 hours if it cost 15 cts. per gal. and each gallon weighs 6 Ib.? 



Ans. About 8.5 cts. 



(NOTE. If the engine were large it would probably burn crude 

 petroleum costing not more than one-fourth as much as the gasoline.) 



4. How many pounds of coal per horse-power-hour would be re- 

 quired to produce power by means of the electric plant (Fig. 375). 



Ans. Nearly 6.1 Ib. 



What would be the cost of running this plant per horse-power-day. 

 of eight hours if the steam engine burned soft coal costing $2 per ton? 



Ans. About 4.8 cts. 



SUMMARY OF THE FOUR POWER PLANTS 



Gasoline engine, 



, pnsrinp npr ppnt , 



per cent. ** per cent. 



Efficiency of the plant... 4.7 18.8 32.3 3.4 



1 . 1 cts. on 8.5 cts. on 



Cost for fuel per horse- soft coal gasoline 



power-day of 8 hours. .3. 2 cts. 3.3 cts. on 3 or 4 cts. on 4.8 cts 



hard coal crude petroleum 



While these figures are only approximations, they show clearly that 

 we should never confuse the efficiency of a motor with the cost of 

 doing work by means of the motor. 



