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Designed by Thomas Jefferson as a dual-faced seven day clock with a striking mechanism to mark the hour, the "Great Clock" is located in Monticello's Entrance Hall with its Portico (exterior) face visible from the house's east front. Made in Philadelphia by Peter Spruck (variously spelled Spruck, Spurck, Spurch, and Sprunk), an apprentice to master clock maker Robert Leslie, the chiming of the "Great Clock" controlled the daily routine for both the enslaved and free.
The Entrance Hall face has a main dial with hour and minute hands with a small secondary dial with a sweeping seconds hand. The clock is powered by two sets of cannonball weights (eighteen pounds each) strung on ropes, which independently drive its ticking and the striking of a gong on the roof. As the clock winds down, the weights descend on either side of the clock via a pulley system and travel down the corners of the room, passing through holes in the floor to the cellar below. Labels are affixed on the wall along the path of the running ("ticking") weights and indicate the days of the week. The clock was wound each Sunday with a crank key using a folding ladder made in the Monticello Joiner's Shop to reach the winding mechanisms on the interior clock face.
The exterior face on the east portico has a single hand. In Jefferson's description of the clock's design, he specifies two wheels which will "turn an hour hand on the reverse face of the wall on a wooden hour plate of 12. I. [inch] radius." and notes "There need be no minute hand, as the hour figures will be 6. I. apart. But the interspace should be divided into quarters and 5. minute marks. The fore and back hour-plates will not be concentric."[1]
Jefferson began planning the clock in 1792, while in Philadelphia, writing Henry Remsen, chief clerk of the foreign desk of the Department of State:
The Chinese have a thing made of a kind of bell metal, which they call a Gong, and is used as a bell at the gates of large houses &c. ... I wish for one to serve as the bell to a clock, which might be heard all over my farm.[2]
Benjamin Franklin's use of a gong in place of a bell, may have inspired Jefferson.[3]
By the beginning of 1793, Spruck had constructed the clock to Jefferson's specifications, but his workmanship was less than satisfactory. Jefferson wrote Leslie in December 1793:
My large clock could not be made to go by Spurck. I ascribed it to the bungling manner in which he had made it. I was obliged to let him make the striking movement anew on the common plan, after which it went pretty well ....[4]
It is likely the clock was installed in Jefferson's Philadelphia house at Gray's Ferry before it was transported to Virginia.[5] It was brought to Monticello when Jefferson returned there in 1794, and he soon solicited clockworkers to undertake its repair.[6] At this same time he finally procured a gong for use with the clock.[7]
In preparation for mounting the clock in the Entrance Hall, Jefferson ordered a set of weights for the clock from the Foxall Foundry in Washington, D.C.[8] In January of that year, Jefferson realized the length of the seven-day descent of the clock' running weights (which he intended to be enclosed in a box) was greater than the height of the Entrance Hall. Writing his joiner James Dinsmore, Jefferson developed an innovative solution:
I do not approve of cutting the wall, not even the cellar wall, to make a space for the descent of the clock weights; but would have them advanced into the room so as to descend clear even of the cellar wall. should the box in this case encroach too much on the window, we may avoid the eye sore by leaving them unboxed, to descend naked till they get to the floor whence they may enter a square hole & go on to the cellar floor.[9]
The Great Clock has been repaired several times, most recently in 2020.
-Based on The Worlds of Thomas Jefferson at Monticello by Susan R. Stein, updated and amended by David Thorson, 6/24
1793. "... a toothed wheel of 2. I. [inches] on the back-end of the axis of the hour hand ... may turn an hour hand on the reverse face of the wall on a wooden hour plate of 12. I. radius. There need be no minute hand, as the hour figures will be 6. I. apart. But the interspace should be divided into quarters and 5. minute marks."[1]
1793 April 27. "Pd. R. Leslie for great clock."[11]
1795 May 23. (Jefferson to Archibald Stuart). "I think you have a watch and clock mender in Staunton. Does he ever pass this way? If he does, I should be glad he would call upon me to do a little work. I have a large clock for the top of my house which needs to be cleaned only and fixed up."[12]
1853. (Benson J. Lossing). "Over the entrance door from the portico, is a large clock, placed there by Jefferson, which by an index upon the wall, indicated the days of the week. The weight which propels it is composed of nine eighteen pound cannon balls."[13]
1872. (David M. R. Culbreth). "... over the front door built into the wall is a good-sized clock, which had to be wound standing upon a ladder—this latter being in normal position and claimed to have been made by Mr. Jefferson himself; the hands stand at 7.34 o'ck."[14]
1900. (Peter Fossett). "... As I entered the grand ballroom, there, above the door at the east, still stood the wonderful big clock, double-faced, with cannon balls of the Revolution for its weight. It marks the day of the week as well as the day time and its ponderous voice can be heard six miles away in the valley below. This Mr. Jefferson designed himself. And there, too, was the ladder he made—a folding one—to reach the lofty clock, and the big iron key he used to wind it with."[fn]Published in a Cincinnati newspaper, circa July 1900.