The Freeman, in December of last year, published an excellent study of a natural experiment in patents: the Steam Engine. The power of a steam engine is rated in "duty": the amount of weight it can lift. During the 42 years from 1772 to 1813 duty rose 3.8 percent per year; during the 38 years from 1814 to 1852 duty rose more than twice as fast-8.5 percent per year. The difference?

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