It's called the "long s" or "medial s".

"An alternative form of s, ſ, called the long s or medial s, was used at the beginning or in the middle of the word; the modern form, the short or terminal s, was used at the end of the word. For example, "sinfulness" is rendered as "ſinfulneſs" using the long s. The use of the long s died out by the beginning of the 19th century, largely to prevent confusion with the minuscule f."

ok I stole that from wikipedia

I am reminded of a scene from Stan Freburg's United States of America, wherein Tom Jefferson is trying to drum up signatures for his little petition to the King, that he calls a Declaration of Independence. He approaches Ben Franklin for his signature, and receives this response:

Ben: Well, lets see it. When in the courfe of human events, it becomes neffefary for one party ...What's all this? All your esses look like effs?
Tom: It's stylish. It's in. It's very in.
Ben: Oh. Well. If it's in .... so so so so so SO... um,um,um,um, eh, hereby absolve themselves from all allegience to the um crown. Ah. Little overboard, don't you think?
Tom: Well - we just wanted to make a point.....
Ben: Sounds a little SUSPECT to me. You're advocating overthrow of the British Government by force and violence.