Weathervane

From Thomas Jefferson Wiki

 Image of the weather vane
Image of the weather vane

On the Northeast Portico of the main house, there is a compass rose which is attached to a weathervane on the roof. This way, the Jefferson family could look out their windows from either the Entrance Hall or the North and South Square rooms to see the wind direction without having to step outside.

Further Sources

Monticello Explorer: http://explorer.monticello.org/text/index.php?id=10372&type=12