James Watt jr. preserved his father's letters to him from 1784 to 1818, which have much advice on education, thrift, social and business concerns. The letters also include much about family health, the Wyeside/Welsh properties, tree planting and Watt jr.'s steamboat experiments. The incoming correspondence from various persons also survives until 1818. It includes a great variety of subjects and usually mixes business and personal concerns. There is business correspondence with Matthew Boulton, Matthew Robinson Boulton, Peter Ewart, Benjamin Gott, John Rennie, Josiah and Thomas Wedgwood, etc.; correspondence with employees such as James Lawson and John Mosley; with friends such as George Lee and John Furnell Tuffen; with friends who emigrated to America for political reasons such as Thomas Cooper and Joseph Priestley jr., with whom Watt jr. had at one time considered going into partnership; with family in Scotland - the Millers, Muirheads and Hamiltons especially. There is also correspondence concerning steamboat voyages, with Thomas Cator and Captain Wager; correspondence with scientists such as Thomas Beddoes, Thomas Henry and Humphry Davy; and there is a small bundle of letters from the Italian agents of T. & R. Walker. In addition, the letters from the suppliers of books, minerals, wine, plants, horses and furnishings reflect watt jr.'s interests at the time.
The outgoing correspondence includes a letter book of copy letters from Watt jr. in Italy to T. & R. Walker, with notes on the Italian trade; the remaining outgoing letters are mainly preserved on copy press paper. Only for the years 1794-1804 have they been pasted into a volume, so the letters from 1804-1816 are loose copy press letters. There are some letters to Ann Watt from 1819-1821, but later outgoing correspondence, if it survives, tends to be folded with the incoming correspondence on various subjects, such as the erection of a public monument to Watt etc.
James Watt jr. followed his father's advice and kept a number of notebooks on many subjects. Particularly interesting are his medical notebook and the books which have detailed lists of the flowers and fruit trees planted at Aston Hall, his home from 1819. There are also many details of his experiments with steam navigation and the notebooks also survive recording the first journey on a steam vessel, the Caledonia, down the Rhine in 1817.
Watt jr. was diligent in preserving his father's reputation and the papers include much on publications of the history of the steam engine, of memoirs of James Watt and on distribution of statues of Watt and the erection of a monument to him in Westminster Abbey. They also include drawings by different architects for the Watt Memorial Chapel, which was added to the church of St Mary's, Handsworth, in 1825.
James Watt jr. acted as executor for the estate of Ann Watt after her death in 1832, and there are some papers which concern that.
There is a series of Watt jr.'s own copies of accounts etc. relating to the business of Boulton & Watt, in which he was a partner.
His personal papers include some account books for Aston Hall, his Grant of Arms in 1826, and a register of the deaths of employees, family and friends.
The legal documents include some of the articles of partnership for the various changes to the firm of Boulton & Watt. There are a number of title deeds for land in Harborne and Smethwick: these form the deeds of three estates once owned by the Adney, Giles and Hopper families, which were purchased by Theodore Price in 1815 and then remortgaged. It has not been possible to establish exactly why these deeds are with Watt jr.'s papers. The land described is not thought to be that for Soho Foundry. Although the deeds were not listed in the 1848 Inventory, it was decided that they should remain with James Watt jr.'s papers.
The printed material largely concerns publications about James Watt and the steam engine.
The loose papers enclosed in the volume 'Letters respecting the Watt family' by Williamson [MS 3219/6/219] include a copy of a memorandum by his cousin Mrs Marion Campbell, about the early years of James Watt, written down by her daughter in 1798. The original of this was sold in the James Watt sale at Sotheby's in March 2003.