240 COSMICAL ARRANGEMENTS. 



ground which obstructs the motion. If the car- 

 riage be placed on its wheels, a much less force 

 will move it, but if moved it will soon stop : it 

 is the friction at the ground and at the axles 

 which stops it : placed on a level rail road, with 

 well made and well oiled wheels, and once put 

 in motion, it might run a considerable distance 

 alone, for the friction is here much less; but 

 there is friction, and therefore the motion would 

 after a time cease. 



The same kind of action between the surfaces 

 of two bodies which retards and stops their 

 motions when they move touching each other, 

 will also prevent their moving at all, if the 

 force which urges them into motion be insuf- 

 ficient to overcome the resistance which the 

 contact of the surfaces produces. Friction, as 

 writers on mechanics use the term, exists not 

 only when the surfaces rub against each other, 

 but also when the state of things is such that they 

 would rub if they did move. It is a force which 

 is called into action by a tendency to move, and 

 which forbids motion ; it may be likened to a 

 chain of a certain force which binds bodies in 

 their places ; and we may push or pull the 

 bodies without moving them, except we exert a 

 sufficient force to break this imaginary chain. 



1. The friction which we shall principally 

 consider is the friction which prevents motion. 

 So employed, friction is one of the most universal 



