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rotating drums.With this power source, Heron constructed an auto- matic theater that presented Nauplius, a tragic tale set in the period after the Trojan War. As (presumably) amazed playgoers watched, the doors to a miniature theater swung open, and animated figures acted out a series of dramatic events, including the repair of Ajaxs ship by nymphs wielding hammers, the Greek fleet sailing the seas accompa- nied by leaping dolphins, and the final destruction of Ajax by a light- ning bolt hurled at him by the goddess Athena. Perhaps inspired by Hephaestuss obedient moving tables,Heron also made wheeled stands and used an ingenious trick to move them, apparently self-animated, around the theater.A weight rested on a hopper-full of grain, which leaked out through a small hole in the bottom.As the weight gradu- ally sank, it pulled a rope wound around an axle of the stand to turn its wheels and make it move.

Along with the power of falling weights, these figures used the basic mechanical resources of wheels, pulleys, and levers to create a variety of motion, but there were drawbacks.While a weight resting on slowly leaking grain delivers power over a relatively long period, it is not very compact, or usable on demand. And beyond repetitive actions like hammering, a system based on simple machines gives little scope for flexible and responsive motion. But better techniques to provide and control power came along, although only long after Greek times.The new power source was the coiled metal spring, and the new means of control was clockwork.

We do not know who first noted that a flexible piece of metal could store energy, but we use the method daily; for example, in the common safety pin. Early Greek artisans such as Philon and Heron understood that a springymaterial could act as a power source. Philon even designed a crossbow that used bronze springs to fling missiles. But these early springs were too weak to be useful, and it was not until the fifteenth century that good-quality coiled springs came into use.

In their time, springs played the role that electrical batteries now do in powering devices. They animated the next wave of artificial beings, once ways were found to control their stored power through their use in clocks.

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