Watchers

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  Bert Greenleaf

  https://inventmag.com/%EB%8F%9B%EB%8B%A8%EB%B0%B0-%EC%8A%88%EC%96%B4%EB%A7%A8-%EC%9D%B8%EC%A6%9D-%EB%A9%94%EC%9D%B4%EC%A0%80%EC%82%AC%EC%9D%B4%ED%8A%B8%EC%9D%98-%EB%AA%A8%EB%93%A0-%EA%B2%83/