The Cupola atop Mt Vernon & The Potomac River

Cupola 
Source: Abraham Swan, The Carpenters Complete Instructor, 1768.

Although no exact match exists in a pattern book, the cupola is similar in form to some of the eight octagonal cupolas illustrated in Abraham Swan's The Carpenters Complete Instructor published in 1768. The Doric order of Mount Vernon's cupola conforms to the hierarchy of classical elements, which places lighter, more elaborate orders higher up the building. The rusticated Tuscan order of the Mansion is topped off by the cupola's Doric order. In addition, Washington may have been influenced by the Governor's Palace in Williamsburg, which was crowned with a cupola.

The cupola is a significant departure from orthodox classical tastes. The pattern book plates that served as models for Washington did not show roof designs with cupolas. Washington's break from architectural correctness, however, had aesthetic implications. Extending the house to the north and south during the rebuilding made it appear too long relative to its modest height. The cupola provided a strong vertical thrust that helped counter that effect. In addition, it drew attention away from the asymmetry of the facades. The cupola also had a practical function—it helped combat the heat of Virginia summers.

Bibliography
Langley, Batty. The City and Country Builder's and Workman's Treasury of Designs, 1755.

Langley, Batty. Builder's Jewel, 2nd ed., 1757.

Swan, Abraham. The Carpenters Complete Instructor, 1768.