88 MECHANICS. 



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

 sliding 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 resist- 

 ance through the snow ; and, secondly, less force is re- 

 quired 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. 



VELOCITY. 



Friction is entirely independent of velocity ; that is, if 

 a force of ten pounds is required to turn a carriage wheel, 

 this force Avill be ten pounds, whether the carriage is 

 driven one or five miles per hour. Of course, it will re- 

 quire five times as much force to draw five miles per hour, 

 because five times the distance is gone over ; but, measured 

 by a dynamometer or spring-balance, the pressure would 

 be the same. In precisely the same way, the w*eight of a 

 stone remains the same, whether lifted slowly or quickly. 

 If the friction of the wheels of a wagon on their axles be 

 equal to ten pounds, driving the horse fast or slowly will 

 not increase or diminish it. But fast driving will require 

 more strength, for the same reason that a man would need 

 more strength to carry a bag of wheat up two flights of 

 stairs than one, in one minute of time. 



FRICTION AT THE AXLE. 



A carriage wheel, or any other wheel revolving on an 



* Generally speaking, this is very nearly correct ; but when the pres- 

 sure is intense, the friction is slightly less on the smaller surface. 



