ON THE DISSIPATION OF ENERGY. 455 



law or principle in Natural Philosophy, but rather 

 on general observation of natural phenomena, on 

 experience in practical mechanics, and on experi- 

 mental investigation of properties of matter ; an 

 answer founded on knowledge acquired in what 

 may be called the " natural history stage " of 

 progress towards truth. 



That little essay was indeed an epoch-making 

 gift to science. From it we have learned that 

 heat is only available for a steam-engine, or an 

 air-engine, or a gas-engine, in proportion to the 

 excess of the temperature of the matter in which 

 it is given above the temperature of the coldest 

 matter obtainable for use in connection with the 

 engine to carry heat away from it continually 

 during the time it is working. 



Every heat motor (as for brevity we may call 

 any heat engine doing mechanical work in virtue 

 of heat supplied to it) requires difference of 

 temperature in different parts ; or in the same 

 part at different times, as in the old Newcomen 

 condensing-engine before Watt's improvement of 

 the separate condenser was introduced. Heat is 



