126 MOTIVE POWER OF HEAT. 



and one hundred and eighty times less than the 

 theoretical maximum. 



We should not expect ever to utilize in practice 

 all the motive power of combustibles. The at- 

 tempts made to attain this result would be far more 

 hurtful than useful if they caused other important 

 considerations to be neglected. The economy of 

 the combustible is only one of the conditions to be 

 fulfilled in heat-engines. In many cases it is only 

 secondary. It should often give precedence to 

 safety, to strength, to the durability of the engine, 

 to the small space which it must occupy, to small 

 cost of installation, etc. To know how to appreciate 

 in each case, at their true value, the considerations 

 of convenience and economy which may present 

 themselves ; to know how to discern the more im- 

 portant of those which are only accessories ; to bal- 

 ance them properly against each other, in order to 

 attain the best results by the simplest means : such 

 should be the leading characteristics of the man 

 called to direct, to co-ordinate among themselves the 

 labors of his comrades, to make them co-operate 

 towards one useful end, of whatsoever sort it may 

 be. 



