Ralph,
Empirical evidence says, if it is a little house on top of a bigger house or barn, it is a cupola of some sort, if it is a metal housing for venting, then it is a ventilator or whirly bird.
Best,
Leland
I couldn't find Hamlin's work. But I called Stony Harris, in Cowtown, NJ, who has about a dozen of them on his various barns, and he calls them "ventilators".
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