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The winter of 1803-1804 was particularly busy for President Thomas Jefferson. The Barbary Pirate War smoldered, the U.S. flag was raised over New Orleans for the first time, and the Lewis and Clark Expedition was preparing to launch that spring. But on January 28, 1804, Jefferson took time to write a letter about one of his clocks when he was in Washington.  His recipient was Irish émigré and “housejoiner” (or carpenter) James Dinsmore, who was largely responsible for expanding Monticello. 

Having learned that what is today known as the “Great Clock” did not fit into its allotted space in Monticello’s newly constructed entrance hall, Jefferson was faced with a dilemma.  The seven-day clock was powered by gravity, and the cannonball weights providing the power were simply too long, despite a ceiling height of more than 18 feet. As designed, the weights would hit the floor after barely five days and the clock would stop. Jefferson instructed Dinsmore to cut square holes into the floor to allow the weights to pass through to the basement. Dinsmore did as instructed, but he disobeyed in one respect: today’s holes are round to correspond to the weights’ shape.

 

The seven-day clock was powered by gravity, and the cannonball weights providing the power were simply too long, despite a ceiling height of more than 18 feet. As designed, the weights would hit the floor after barely five days and the clock would stop. Jefferson instructed Dinsmore to cut square holes into the floor to allow the weights to pass through to the basement. Dinsmore did as instructed, but he disobeyed in one respect: today’s holes are round to correspond to the weights’ shape.

This was just the clock’s latest problem. Jefferson designed the clock while serving as Secretary of State in Philadelphia, and he had commissioned Robert Leslie, an accomplished clockmaker, to construct this important timepiece. But after paying him $113.80, Leslie passed the commission to his apprentice, Peter Spurck, and Jefferson found his work deficient.

Writing to Leslie, Jefferson complained his “large clock could not be made to go by Spurck. I ascribed it to the bungling manner in which he had made it.”

While the mechanism was reworked, problems persisted. In 1795, back at Monticello where the clock was to be installed, Jefferson wrote to his agent in search of a clockmaker to clean and fix the device.

When Jefferson retired from the presidency in 1809, the clock was in place where it is seen today high above Monticello’s main door. An unusual feature is its two clockfaces. The exterior clockface overlooks the east lawn, and its single hand marks time in five-minute increments. By connecting it to a Chinese gong, Jefferson sounded the time throughout his 5,000-acre plantation.

The inside face of the clock features three hands, recording time down to the second. Jefferson designed the clock’s housing which measures nearly four feet tall and is capped by a classical pediment. Behind the clockface is the clock mechanism measuring some 15 inches wide.

Two sets of 18-pound weights power the timepiece -- they are run out to wheels in the near corners of the room. In one corner, Jefferson installed day markers on the wall at intervals. As the clock weights descend during the week, the top cannonball weight marks the day. Jefferson installed a similar calendar in his study where he kept his nearly eight-foot-tall case clock. Opening the case door reveals Jefferson’s penciled notations marking the days of the week that correspond to the position of the descending clock weight.

The Great Clock still runs and Monticello’s museum technicians wind it early every Sunday morning. Jefferson did the same: The Rev. Peter Fossett, a member of the enslaved community who returned to Monticello late in his life, recalled Jefferson himself would wind the clock using a crank-like key. Jefferson would use a folding ladder perhaps made by Fossett’s uncle, John Hemmings, that remains in place in the Entrance Hall.