FRICTION. 243 



state of repose. If this were not so, if every 

 shock and every breeze required to be counter- 

 acted by the cement, no composition exists which 

 would long sustain such a wear and tear. The 

 cement excludes the corroding elements, and 

 helps to resist extraordinary violence ; but it is 

 friction which gives the habitual state of rest. 



We are not to consider friction as a small force, 

 slightly modifying the effects of other agencies. 

 On the contrary its amount is in most cases very 

 great. When a body lies loose on the ground, 

 the friction is equal to one third or one half, or 

 in some cases the whole of its weight. But in 

 cases of bodies supported by oblique pressure, 

 the amount is far more enormous. In the arch 

 of a bridge, the friction which is called into play 

 between two of the vaulting stones, may be equal 

 to the whole weight of the bridge. In such cases 

 this conservative force is so great, that the com- 

 mon theory, which neglects it, does not help us 

 even to guess what will take place. According 

 to the theory, certain forms of arches only will 

 stand ; but in practice almost any form will 

 stand, and it is not easy to construct a model of 

 a bridge which will fall. 



We may see the great force of friction in the 

 brake, by which a large weight running down a 

 long inclined plane has its motion moderated 

 and stopt ; in the windlass, where a few coils of 

 the rope round a cylinder sustain the stress and 

 weight of a large iron anchor ; in the nail or 



