Why did 18th-century writers use F inftead of S?
Why was it popular in the 18th century to occasionally substitute the letter f (or a reasonable facsimile) for s? What were the rules of grammar concerning this usage? Was it Noah Webster who finally put an end to this abfurd practice?
At last, a chance to show off my encyclopedic knowledge of paleography, the study of ancient writing. You have no idea how seldom this topic comes up in casual conversation.
What you take to be an f is actually the so-called long s, also known as the medial s, to be differentiated from the terminal or short or round s, which we regard today as the conventional form. Throughout its history, the long s has always looked a lot like the lowercase f, to the extent of having a little nubbin vaguely reminiscent of a crossbar appended to its middle sometimes. But the two letters are not otherwise related.
As one might deduce from the nomenclature, the long/medial s was supposed to be used in the middle of a word, while the terminal s was used to finish one off. (In practice this rule was somewhat haphazardly adhered to.) The two versions were phonetically equivalent and derived from the same Roman letterform. Why folks figured they needed two varieties when they could have scraped by with one is beyond me, but we might note that having terminal and middle letterforms is not inherently any nuttier than having every sentence start with a capital letter, a comparatively recent invention.
The use of two types of s dates back at least to the Middle Ages. The long s became especially popular during the Italian Renaissance, with the development of the various "humanistic" scripts that gave rise to our present English script. You'll notice that the long f, though not the long s, persists in many serif fonts in the type style we now call italic, although the long s was used in so-called roman (i.e., non-slanty) fonts as well.
The Italians often used the long s even when they should have used a short one because letters with long expressive strokes in them made for an artier-looking manuscript. Unfortunately, they also made for a manuscript that was near impossible to read, and it's probably for that reason as much as any that the use of the long s finally died out in the 19th century. The form survived in the formal German script Fraktur until Fraktur itself bit the dust after World War II. (The script had come to be associated with German militarism.) Can't say I'm sad to see it go. Spelling is enough of a crapshoot these days as it is.