MECHANICAL PHILOSOPHY. 237 



relative motion is a change of place by a body in relation to 

 some other body which is in absolute motion. 



Simple motion results from the action of a single force upon 

 a body, while compound motion is produced by two or more 

 forces acting different directions. Motion, when once attained 

 would be onward in a straight line unless changed or destroy- 

 ed by some force secondary to the one by which it was pro- 

 duced : a ball projected from a cannon, assumes a curved line 

 towards the earth, because acted upon by the attraction of 

 gravity ; and this sufficiently strong to overpower the propel - 

 ling force of the powder which gave it motion, and finally bring 

 it to rest. 



OBSTRUCTIONS TO THE ACTION OF MACHINERY. 



Friction arises mostly from the elevations of one surface 

 entering into the depressions of another; but partly also from 

 the mutual cohesion of the surfaces. 



Sliding friction is produced when pinions or axes revolve on 

 their support 



Rolling friction occurs when a round ball or wheel rolls 

 along a surface. Friction is greater between two surfaces of 

 wood where their fibres lie parallel than where they run across 

 each other: it is also greater between two surfaces of the same 

 metal than between those of different metals : two surfaces of 

 iron would produce more friction than one of copper and one 

 of iron : cast steel is said to be an exception to this rule. 



The ristance of friction may be diminished by the use of fine 

 smooth and oily substances ; the particles of which fill up the 

 cavities and lubricate the asperities of the surfaces. For this 

 purpose oil is best adapted to metals and tallow for wood. 



Extent of surface makes no difference, in a given body, in 

 regard to the amount of friction developed. Friction is in- 

 creased between two bodies by their remaining some time in 

 contact; in some cases it does not attain the maximum in four 

 or five days. In the contact of two metals, the friction attains 



