THE MACHINES OF THE ANCIENT WORLD 21 



The wheel and axle is something like a pulley. It 

 permits a mechanical advantage, since the distance 

 through which a point on the wheel moves may be 

 large as compared to the corresponding distance 

 through which the weight moves. 



Of course, in the case of all these machines, some 

 of the work of the operator is done against friction, 

 and appears as heat at the point where the friction 

 occurs. The work which the operator gets out of the 

 machine is then less than he puts in. The ratio of 

 the useful work to the input is the efficiency. In the 

 ideal case it is 100%, but no actual machines have 

 efficiencies near this limiting value. 



With these simple contrivances the buildings of the 

 ancient world were constructed. We do not know 

 the exact methods followed by their erectors, but it 

 is possible that they used derricks to set the stones. 

 In addition, as the building progressed, they used 

 scaffolding. For the Babylonian structures, it appears, 

 these scaffoldings were of brick. They would need 

 to be quite substantial to support the large weights 

 of men and materials necessary for such enormous 

 buildings as we have seen in the figures. To-day frame 

 scaffoldings may be made of comparatively light beams. 

 Before man learned the underlying physical laws, 

 strength and stability could be obtained only by 

 massive construction, of which the Pyramids form per- 

 haps the most striking illustration. 



The energy to operate such machines and through 

 them to rear such structures was not derived from 

 natural sources through inanimate means. Of the use 

 of winds the ancients had some knowledge, but except 



