110 MECHANICS, 



recommended for broken-stone roads, and gravelly, or 

 dry, sandy localities. They are also much the best for 

 the wheels of sowing or drilling machines, which only 

 pass over mellowed surfaces. 



The larger the wheels are made, the more easily they 

 run ; thus a wheel six feet in diameter meets with only 

 half the resistance of a wheel three feet in diameter. 



A flat piece of wood, sliding on one of its broad sur- 

 faces, is subject to the same amount of friction as when 

 shding upon its edge. Hence the friction is the same, 

 provided the pressure be the same, whether the surface 

 be small or large.* Or, in other words, if the surfaces 

 are the same, a double pressure produces a double 

 amount of friction ; a triple pressure, a triple amount, 

 and so on. 



A narrow sleigh-shoe usually runs with least force, 

 for two reasons : first, its forward part cuts with less re- 

 sistance through the snow ; and, secondly, less force is 

 required to pack the narrow track of snow beneath it. 

 The only instance in which a wide sleigh-shoe would be 

 best, is where a crust exists that would bear it up, and 

 through which a narrow one would cut and sink down. 



Friction is entirely independent of velocity ; that is, 

 if a force of ten pounds is required to turn a carriage 

 wheel, this force will be ten pounds, whether the car- 

 riage is driven one or five miles per hour. Of course, 

 it will require five times as much force to draw five 



* Generall}' speaking, this is very nearly correct ; but when the 

 pressure is ielense, the friction is slightly less on the smaller sur- 

 face. 



