1 86 THE SEVEN FOLLIES OF SCIENCE 



lessen the friction but to prevent the metal from forming a 

 glaze on the surface of the stone. When a steel" blade is 

 rubbed on a dry whetstone the minute particles that are 

 torn from the metal attach themselves to the surface of 

 the stone and are then burnished to a smoothness which 

 greatly lessens the friction and prevents further abrasion. 

 So that in reality the application of the oil to the whet- 

 stone actually increases the friction instead of lessening it. 



Of course this does not apply to coarse-grained grind- 

 stones where the particles of metal that are removed from 

 the tool are of considerable size and are torn off with 

 great rapidity. In that case the combined friction and 

 abrasion quickly heat the article to a degree which de- 

 stroys its temper if it is made of steel, and to counteract 

 this a stream of water is applied, but not for the purpose 

 of lessening the friction. 



It is a popular impression that friction is only a source 

 of evil. It is regarded as the great agent in wasting power 

 and destroying machinery which, if there were no friction, 

 would last forever. But friction has its advantages as 

 well as its disadvantages, and the former are quite as im- 

 portant as the latter. If it were not for friction no nail or 

 screw would hold, and our buildings and machines, unless 

 constructed after methods very different from those at 

 present in use, would all fall to pieces. No knot could be 

 made to hold; the first strain would cause it to slip. With- 

 out friction no locomotive could drag its train along, and 

 even the horses would be unable to pull their loads. A 

 striking example of this may be seen any day when the 

 roads are covered with sheets of ice and men and horses 

 are falling in every direction. Even while writing these 

 lines I have received a notable object lesson in this direc- 



