vii LAW AND PRINCIPLE 179 



steam and thus drive engines. Suppose a certain 

 amount of coal to be capable of generating 100 units 

 of heat when burnt in the furnace of an engine. About 

 25 per cent, of this quantity of heat is lost from various 

 causes, and 75 per cent, reaches the engine in the steam. 

 Only 6 of these units, however, are convertible into 

 the mechanical work represented by the driving of the 

 engine, and the remaining 69 per cent, is lost. If the 

 6 units are used to drive a dynamo for the production 

 of electric power, 1 has to be paid as commission for the 

 exchange, thus leaving us with 5 units. Let these 5 

 units be used to drive an electric motor ; the mechanical 

 energy obtained represents about 4| units, half a unit 

 being lost in the transformation. The final result, 

 therefore, of the transformation of energy from the com- 

 bustion of coal to the work obtainable from an electric 

 motor is, that of the prime value of 100 units of energy, 

 95J per cent, is lost by the various exchanges. In the 

 whole series of these or any other related changes, the 

 items on the two sides of the balance sheet give the 

 same total, the energy supplied being always equal to 

 the sum of the useful and useless energy obtained. 



Traces of the idea of the conservation of energy may 

 be found amongst the writings of ancient philosophers, 

 and Descartes held that it was a self-evident truth. To 

 establish the truth, however, precise and detailed inves- 

 tigations were necessary, and these were not made until 

 nearly the middle of the nineteenth century, when 

 interest in the question appears to have been wide- 

 spread. 



Towards the close of the eighteenth century, Count 

 Rumford and Sir Humphry Davy conceived the idea that 

 there was some correspondence between work done and 

 heat produced, but they did not realise completely that 



