Henry Ainsworth, one of the most famous of the Christian Orientalists, author of the celebrated Annotations on the Pentateuch, the Psalms, and the Song of Solomon, as well as the metrical version of the Psalms used by the Pilgrims, died suddenly in Amsterdam in 1622 or 1623 at around the age of 51 under mysterious circumstances. In fact, he was thought by some to have been poisoned. Daniel Neal, in his History of the Puritans, Vol. 1 (1844 ed.), p. 243, writes:
His death, was sudden, and not without suspicion of violence; for it is reported that, having found a diamond of very great value in the streets of Amsterdam, he advertised it in print, and when the owner, who was a Jew, came to demand it, he offered him any acknowledgment he would desire; but Ainsworth, though poor, would accept of nothing but a conference with some of his rabbies upon the prophecies of the Old Testament relating to the Messiah, which the other promised, but not having interest enough to obtain it, 'tis thought that he was poisoned.
Benjamin Brook, Lives of the Puritans, Vol. 2, p. 302, says that other accounts hold that the conference with the Jewish rabbis did indeed take place, that Ainsworth confounded his opponents, and that they did in revenge poison him.
Be that as it may, at his death, Ainsworth, a voluminous writer, left behind him a large body of unpublished manuscripts. The existence of these papers became the subject of interest of the British Council of State which in 1652 planned a revision of the Authorized (King James) Version of the Bible. Ainsworth's writings were intended to serve as the basis for marginal notes that would accompany this revision. A committee which included Ralph Cudworth, a Hartlibian, was entrusted with the aim of obtaining these papers. The planned revision did not materialize but Hartlib, who greatly desired to see a complete commentary on the Bible written according to Ainsworth's method, did not forget about them. He and his friend John Worthington corresponded about the project to track down these lost Ainsworth manuscripts shortly before Hartlib's death.
In 1661, Worthington wrote to Hartlib, as recorded in the first volume of Worthington's Diary and Correspondence, pp. 263-269:
There is another author, whose remains are most worthy to be retriev'd: I mean Mr. Ainsworth, whose excellent Annotations upon the Pentateuch, &c., sufficiently discover his great learning, and his most excellent observation of the proper idiom of the holy text; with every jot and tittle of which he seems to be much acquainted as any of the Masoreths of Tiberius. I have been told that there are these MSS., of his, viz., his Comment upon Hosea, Notes upon S. Matthew, and Notes upon the Epistle to the Hebrews: which latter he was the more prepared for, by reason of his former labours upon the Pentateuch; the Epistle to the Hebrews being Moses unveil'd. Mr. Cole, (a bookseller at the Printing Press, in Corn-hill,) told one, that he had once these MSS., in his keeping, and thought to have printed them; but that a kinsman (or a son, I do not so well remember,) of Mr. Ainsworth's at Amsterdam, and John Can, could not well agree, either about the right of disposing the copy, or the price for the MSS. I have heard that Mr. [Philip] Nye, or Mr. [Henry] Jessey, knew something of these MSS. If they could be recovered, so they be like the other printed works of the author, it would be a good work indeed, and might be of singular use. Nay, if they be not throughout so compleated as the author intended, yet the whole is too good to be lost or [embezzled]. Perhaps you or Mr. Dury may be acquainted with the forementioned persons in England; or could by some understanding persons enquire of this business at Amsterdam. If the MSS., can be found, and may be purchased at a fit rate, there is no fear of being a looser. His other works have always sold well, and at a good price; and were bought by men of different perswasions from him; who did esteem him for his modesty and singular learning, and were much obliged to him for his skill in Jewish antiquities, lighting their candle by his.
This business I think is worthy of consideration.
Dury did in fact track down the one who held Ainsworth's manuscripts: it was Ainsworth's son. Worthington wrote again to Hartlib later that year (ibid, p. 353) to encourage Dury in this contact with Ainsworth's son. But Dury's efforts to persuade the son to part with the manuscripts were ultimately unsuccessful. Ainsworth's son did not feel that his father's reputation would be enhanced by their publication.
As for what happened to them next, the trail grows cold, at least for this writer. It would be nice to think that they were not destroyed but tucked safely away to be discovered by some researcher centuries later, like a hidden treasure of gold. But as for now they remain a literary mystery and a lost Nachlass.