In August 1791, Thomas Jefferson and George Washington went to the farm of Samuel Powel (1739-1793) southwest of Philadelphia to see the threshing machine invented by Alexander Anderson in 1782, a drum and beater type model. By 1792, Jefferson asked Thomas Pinckney in London to get a model. Jefferson wrote in his Memorandum Book in 1793, "Gave order on bank US for 62.8 to John Vaughan for his bill fo 13-13 sterl. on Byrd, Savage & Byrd paiable to T. Pinckney, and inclosed it to T. Pinckney to pay for threshing model." Jefferson saw a model in Arthur Young's Annals of Agriculture invented by Andrew Meikle in Scotland. He wrote James Madison, "I expect every day to receive from Mr. Pinckney the model of the Scotch threshing machine...Mr. P. writes me word that the machine from which my model is taken threshes 8. quarters (64. bushels) of oats an hour, with 4. horses and 4 men. I hope to get it in time to have one erected at Monticello to clean out the present crop."
The model, a treble-geared machine, arrived in Virginia at the end of 1793; however, Jefferson did not have it built until 1796. He hired John Buck to do the work but ended up making modifications by substituting whirls and bands for the gears of the Scottish model. This modification had first been done by William Booker of Goochland County. Jefferson wrote about the machine to Edward Rutledge, "I understand you have introduced the Lieth machine into your state for threshing your rice. I have used one this year for my wheat with perfect success. Jefferson ended up with three threshing machines; two were portable, driven by horses, and one was stationary, driven by water.
Primary Source References
10 February 1793. "After many enquiries I have found one of the Threshing Machines at no great distance from this City. I went to the place where it is & prevailed on the owner to let me see it work. I liked the performance so well that I have engaged a mechanic to make a compleat model of it & hope to send it t o you in good time for you to have one erected t o thresh out your next crop. With the force of three horses t o work it & three men to feed & attend it from 8 t o 16 bushels of wheat are threshed by it and other grain in proportion. This account I received from the owner having only seen a few sheaves threshed." (Thomas Pinckney to Jefferson).
12 April 1793. "I shall thank you most sincerely for the model of the threshing machine, besides replacing the expence of it. The threshing out our wheat immediately after harvest being the only preservation against the [[weevil]] in Virginia, the service you will thereby render that state will make you to them a second Triptolemus." (Jefferson to Pinckney).
21 April 1793. "Mr. Pinkney has seen the Scotch threshing machine. He says that three men & three horses get out from 8. to 16. bushels an hour. He promises I shall have a model in time to get out the crop of this year." (Jefferson to Thomas Mann Randolph).
11 August 1793. "I found considerable hopes on the threshing machine expeced, as 4. horses suffice to work that , and I had proposed t o work my ploughs with oxen. Should that machine fail, more horses must be kept for treading wheat in the proper season." (Jefferson to Thomas Mann Randolph).
ca. 28 August 1793. "...I have just arrived here in the Ship Amsterdam Packet, after a 68 day passage from London...I shall have the honour to bring with me a box addressed to you, Sir, (of which the key is inclosed in a letter from Mr. Pinckney, which, on recollection I have thought right to transmit herewith) containing the model of a threshing m i l l. I have accidentally discovered t hat the person who made it, & who seems an ingenious millwright, came on board the same ship with myself as a Steerage passenger, with a view to settle in America...If you direct me, Sir , I shall send the model immediately, that you may be enabled to form some idea of his abilities ." (James Adair to Jefferson).
1 September 1793. "Thankful for your attention to various things and particularly so also for your attention to the threshing machine which, if it answers what I have heard of it, will be a vast acquisition t o the s t a t e s of Virginia & North Carolina. If you are not coming personally, can send it by a careful gentleman or give it t o Remsen... A very peculiar circumstance in all the country South of the Patowmac, the finest wheat country in America, renders such a machine as valuable as the discovery of the grain itself.(Jefferson to Adair).
29 January 1794. "I wish the threshing machine may answer the purpose, I have no doubt that on a proper stream of water the effect of it would be astonishingly great." (Pinckney to Jefferson).
8 September 1795. Talks about floods of rain. "This circumstance too, preventing our treading out our wheat, which is generally done in the open air, exposes that much at this moment to the weavil. It has determined me, before another harvest, to prepare a threshing machine on the model you sent me, which the variety of other things wanting in my farms on my return t o them, has as yet prevented my making." (Jefferson to Thomas Pinckney) 
5 January 1796. "Buck begins to work."
6 July 1796. "If you can bring a moveable threshing machine, constructed upon simple principles to perfection, it will be among the most valuable institutions in this Country; for nothing is more wanting, & to be wished for on our farms." (George Washington to Jefferson)
6 November 1812. "we cultivate wheat here extensively & solely, and every body is getting the Leith machine for threshing it. I mean the original double or treble geered machine, divested of all those things which have been called improvements, in which the wheat is presented to beaters revolving like the vanes of a wheat fan. I have three of them myself, one going by water, & two by horses. Many have them i n our neighborhood. Those moved by horses get out from 80. t o 150. bushels a day with from 2 , t o 5. horses, & cost from 100 t o 150 D. Those by water get out 300. bushels a day and more i f they could be attended, & cost in proportion to their geer, canals, dams &c.." (Jefferson to Dr. Cunningham Harris).
13 June 1815. "Our threshing machines are universally in England fired with Dutch fans for winnowing, but not with us, because we thresh immediately after harvest, to prevent weavil, and were our grain then laid up in bulk without the chaff in it, it would heat & rot." (C. W. Peale).
29 December 1815. "To a person having a threshing machine, the addition of a hemp break will not cost more than 12. or 15. D. You know that the first mover in that machine i s a horizontal horsewheel with cogs on it's upper face. On these is placed a wallower and shaft which give motion to the threshing apparatus." He '11 put one on other side for hemp break. (Jefferson to George Fleming).
8 May 1816. "In a former letter I mentioned to you that I had adapted a hemp break to my sawmill, which did good work. I have since fixed one to my threshing machine in Bedford, which breaks & beats about 80. lb. a day with a single horse. The horizontal horsewheel of the threshing machine drives a wallower and shaft, at the outer end of which shaft is a crank..." (Jefferson to Charles Willson Peale).
2. December 12, 1793. James A. Bear, Jr., and Lucia C. Stanton, eds. Jefferson's Memorandum Books: Accounts, with Legal Records and Miscellany, 1767-1826 (Princeton, NJ: Princeton University Press, 1997), 2:907.
6. Edwin M. Betts, ed. Thomas Jefferson's Farm Book: With Commentary and Relevant Extracts from Other Writings (Princeton: Princeton University Press, 1953. Rep. 1976, 1987, 1999), 68-77. Manuscript and transcription available online at http://www.thomasjeffersonpapers.org/farm/
7. Please note that this list should not be considered comprehensive.
9. Ibid, 536-537.
10. Ibid, 581.
11. Ibid, 26:658.
12. Ibid, 775.
13. Ibid, 27:3
14. Ibid, 28:11-12.
15. Ibid, 457-458.
19. Edwin M. Betts, ed. Thomas Jefferson's Garden Book, 1766-1824: With Relevant Extracts from His Other Writings, (1944, Rep. Charlottesville: Thomas Jefferson Foundation, 1999), 546. Manuscript and transcription available online at http://www.thomasjeffersonpapers.org/garden/
21. Ibid, 253.