What ingenious mechanism did Watt and Boulton develop to create continuous rotational power?

Answer

The sun-and-planet gear.

In 1781, Watt and Boulton developed the sun-and-planet gear, which was necessary because they faced a patent barrier for using the more obvious solution, a crank, to convert the up-and-down motion into rotational power.

What ingenious mechanism did Watt and Boulton develop to create continuous rotational power?
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