Weathervane
From Thomas Jefferson Wiki
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

