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I find that these two patents:

US3079851 and US3209667 unbelievably similar; they appear to be achieving the same thing by virtually the same method.How could they both have been granted patents?

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The from page of the later one references the earlier one so I imagine there is enough difference in the claims that the examiner of the second one saw it as distinct from the first one. In 1965 – George White Jan 7 '14 at 22:54
It's difficult to answer without looking carefully at the specific claims of each patent. (Reading the Title, Abstract and Specification are not sufficient to understand what has been claimed by the applicant and allowed by the USPTO). Also, as an aside, both patents have been expired for almost 30 years so it is at best an academic exercise to reconstruct which aspects of '667 were deemed novel over '851. (The newer patents cites the older patent, as George White mentions above.) – Micah Siegel Jan 7 '14 at 22:57

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