HOW WORK IS DONE 



for example, excavating a gravel bank, seems almost 

 a thing of intelligence ; as it gores into the bank scooping 

 up perhaps a half ton of earth, its upward sweeping head 

 reminds one of an angry bull. Then as it swings lei- 

 surely about and discharges its load at just the right spot 

 into an awaiting car, its hinged bottom swings back 

 and forth two or three times before closing, with curious 

 resemblance to the jaw of a dog; the similarity being 

 heightened by the square bull-dog-headed shape of 

 the scoop itself. Yet this remarkable contrivance, with 

 all its massive steel beams and chains and cog wheels, 

 employs no other principles than the simple ones of 

 lever and pulley and inclined plane that we have just 

 examined. The power that must be applied to produce 

 a given effect may be calculated to a nicety. The 

 capacities of the machine are fully predetermined in 

 advance of its actual construction. But of course this 

 is equally true of every other form of power-transmitter 

 with which the modern mechanical engineer has to 

 deal. 



FRICTION 



In making such calculations, however, there is an 

 additional element which the engineer must consider, 

 but which we have hitherto disregarded. In all methods 

 of transmission of power, and indeed in all cases of 

 the contact of one substance with another, there is an 

 element of loss through friction. This is due to the fact 

 that no substance is smooth except in a relative sense. 

 Even the most highly polished glass or steel, when 

 viewed under the microscope, presents a surface covered 



[39] 



