FRICTION. *241 



and important agents in the mechanism of our 

 daily comforts and occupations. It is a force 

 which is called into play to an extent incom- 

 parably greater than all the other forces with 

 which we are concerned in the course of our 

 daily life. We are dependent upon it at every 

 instant and in every action : and it is not possible 

 to enumerate all the ways in which it serves us ; 

 scarcely even to suggest a sufficient number of 

 them to give us a true notion of its functions. 



What can appear more simple operations 

 than standing and walking? yet it is easy to 

 see that without the aid of friction these simple 

 actions would scarcely be possible. Every one 

 knows how difficult and dangerous they are 

 when performed on smooth ice. In such a situ- 

 ation we cannot always succeed in standing: if 

 the ice be very smooth, it is by no means easy to 

 walk, even when the surface is perfectly level ; 

 and if it were ever so little inclined, no one 

 would make the attempt. Yet walking on the 

 ice and on the ground differ only in our expe- 

 riencing more friction in the latter case. We 

 say more, for there is a considerable friction even 

 in the case of ice, as we see by the small distance 

 which a stone slides when thrown along the sur- 

 face. It is this friction of the earth which, at 

 every step we take, prevents the foot from sliding 

 back ; and thus allows us to push the body and 

 the other foot forwards. And when we come to 



