THERMODYNAMICS. 283 



Now, 1 H.-P. is 76 kgm. metres/sec., 



say 746 x 10 7 ergs/sec., 

 and 1 H.-P. for 1 hour is 746 x 3600 x 10 7 , 



= 2680xl0 10 ergs say. 

 This requires 2680 -f-8'27 = 324 grammes of coal, 



say 0'71 Ib. of coal. 



Actual engines have by no means reached this efficiency over such a 

 temperature range. Probably 2| to 3 Ibs. of coal per horse-power hour 

 would be a good result at present. 



It may be noted that if the whole of the heat-yield of the coal were 

 transformed, then we should have 



1 H.-P. for 1 hour requiring (2680 x 10 10 ) -r (8000 x 42 x 10 7 ), 



= 80 grammes nearly, 

 or = 0-181b. 



