THE CONQUEST OF NATURE 



with indentations and rugosities. This granular sur- 

 face of even seemingly smooth objects, is easily visu- 

 alized through the analogy of numberless substances 

 that are visibly rough. Yet the vast practical impor- 

 tance of this roughness is seldom considered by the 

 casual observer. In point of fact, were it not for the 

 roughened surface of all materials with which we come 

 in contact, it would be impossible for any animal or 

 man to walk, nor could we hold anything in our hands. 

 Anyone who has attempted to handle a fish, particu- 

 larly an eel, fresh from the water, will recall the diffi- 

 culty with which its slippery surface was held; but it 

 may not occur to everyone who has had this experience 

 that all other objects would similarly slip from the hand, 

 had their surfaces a similar smoothness. The slippery 

 character of the eel is, of course, due in large part to 

 the relatively smooth surface of its skin, but partly 

 also to the lubricant with which it is covered. Any 

 substance may be rendered somewhat smoother by 

 proper lubrication; it is necessary, however, that the 

 lubricant should be something which is not absorbed 

 by the substance. Thus, wood is given increased 

 friction by being moistened with oil, but, on the other 

 hand, is made slippery if covered with graphite, soap, 

 or any other fatty substances that it does not absorb. 



Recalling the more or less roughened surface of all 

 objects, the source of friction is readily understood. 

 It depends upon the actual jutting of the roughened 

 surfaces, one upon the other. It virtually constitutes 

 a force acting in opposition to the motion of any two 

 surfaces upon each other. As between any different 



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