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In page Capitoline Grounds:

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Henry Chadwick wrote an account that on August 16, 1870, Fred Goldsmith demonstrated a new pitch, the curve ball, at the Capitoline Grounds.[5] Chadwick observed: "That which had up to this point been considered an optical illusion and against all rules of philosophy was now an established fact."[5] By the 1930s, the established baseball community had given the discovery credit to Candy Cummings, and due to Chapman's age, determined that his demonstration has most likely occurred a few years later.[5]