256 MOTIVE POWEK EMPLOYED

necessarily produce friction, and, consequently,add to the resistance ; and this will be greaterwhen the engine is heavy, than when lightlyloaded, being in proportion to the pressurethrown by the load upon all these intermediateparts.

The total amount of friction will, however,depend upon the extent and number of themoving parts, and the quantity of rubbing-surface caused by one rotation of the engine-wheels ; and this will be always the same, inone revolution of these wheels, or in onecomplete stroke, whatever be the progressivemotion of the engine in that revolution, exceptwhat arises from the rolling of the wheelsupon the rail.

Suppose, now, instead of the engine proceed-ing 9 feet during one complete stroke, or inone revolution of the wheels, the space passedover be 12 feet, then the amount of frictionarising from all the moving parts of the engineis the same in each, while the progressivemotion is as 12 : 9. The quantity of friction,therefore, compared with the progressivemotion of the engine, will then be diminishedin that ratio, by using wheels of the diameterof 3 and 4 feet respectively ; and the effect ofthe engine will consequently be proportionablyincreased.