The long s ⟨ſ⟩, also known as the medial s or initial s, is an archaic form of the lowercase letter ⟨s⟩, found mostly in works of the late 8th to early 19th centuries. It replaced the single s, or one or both of the letters s in a double-s sequence (e.g., "ſinfulneſs" for "sinfulness" and "poſſeſs" or "poſseſs" for "possess", but never "poſſeſſ" ).
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In general, the long s fell out of use in roman and italic typefaces in professional printing well before the middle of the 19th century. It rarely appears in good-quality London printing after 1800, though it lingers provincially until 1824 and is found in handwriting into the second half of the nineteenth century, and is sometimes seen later on in archaic or traditionalist printing such as printed collections of sermons. Woodhouse's
The Principles of Analytical Calculation, published by the Cambridge University Press in 1803, uses the long s throughout its roman text.
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The long s disappeared from new typefaces rapidly in the mid-1790s, and most printers who could afford to do so had discarded older typefaces by the early years of the 19th century. Pioneer of type design John Bell (1746–1831), who started the British Letter Foundry in 1788, is often "credited with the demise of the long s". Paul W. Nash concluded that the change mostly happened very fast in 1800, and believes that this was triggered by the Seditious Societies Act. To discourage subversive publications, this required printing to name the identity of the printer, and so in Nash's view gave printers an incentive to make their work look more modern.
https://en.wikipedia.org/wiki/Long_s
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