Lottery tickets were issued to raise funds for Sunderland Bridge in 1816. The prize was a share in the toll money paid to cross the bridge or the rights to ferry people across the river. Such private lotteries were created by Acts of Parliament and were regulated by the Lottery Office. Numbered tickets in lotteries were sold to people for as much as £10 each. Each ticket holder stood a chance of winning from a range of money prizes or annuities. [From the website of the National Archives, which includes images of the tickets.]
Fuseli ironically thanks Knowles for the communication of the "'Sunderland Bridge Lottery'", which "Some beneficent Genius" had dropped on the desk in his office. He goes on to excoriate those responsible for such acts, comparing them to Falstaff's page [2 Henry IV, I ii, 32-54], and expresses his sense of provocation that Knowles's frank and seal had been used for such swindling trash. He then alters the tone of the letter by adapting a quotation from
Horace: "Sed Fas incidere Lusum" [from Epistles I, xiv, 36, 'nec lusisse pudet, sed non incidere ludum', ("Nor is it shameful once to have been foolish, but rather not to have cut folly short"), i.e., Fuseli says "But to cut short the playfulness. . ."] and tells Knowles that he may expect his wife and himself before the end of the week. Fuseli concludes by mentioning an excursion he had made in an open carriage around Kent.
Weinglass, p. 422f.