I haven't got a clue what the experts call it, but in northern New 
England, Nebraska & South Dakota we've always called these cupolas, 
whether round or rectilinear , and if all sides are louvered then the 
function is obvious and it is a "ventilating cupola."

If there are windows and enough room for a chair or two then its a 
"lookout cupola", if there are windows and enough room for a tea party 
it's a belvedere. These are also serving as ventilation. In the bigger 
mansions a veritable wind rises from below during the summer, and it may 
be part of a larger ventilation system included intake ports built into 
plaster ceiling decorations with wooden box ducts rising through the 
attics to the cupola, intakes on the north side of the basements, etc.

John Leeke
www.HistoricHomeWorks.com

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