The node formerly known as:
The Fucking Motion
So you are reading through your Dover facsimilie edition of the first English translation of William Harvey's works, and you come across a wonderful passage such as the title of this node.
These are not f's!
They may look very similar to f's, but these non-terminal s's are, in fact, not the same. Observe, on the left, the lower case f, on the right, the non-terminal s:
/ \ / \
In the old style s, the bar only goes halfway across, which means that the s would only take up half the space of an f, and about half the space of a terminal s.
This sort of space saving may seem silly now, but paper and other writing materials have not always been so cheap as they are now. In the Middle Ages, when manuscripts were written on vellum, a material made from sheep or goat hide, space was precious, so the monk copying the manuscript would use abreviations and other devices, such as this non-terminal s, to save space, and therefore, money. The manuscript non-terminal s would take up even less space - it might look something like this:
It takes up even less space than the example above - perhaps a quarter the width of a normal character.
With the transition to printing with movable type, the need to save space remained. Paper was expensive, and this style of s was employed to save paper. This use continued to some extent until the mid 19th century.
Thus, you have passages in Harvey about "the fucking motion", meaning, the sucking motion, referring to the method of the flow of blood.
The non terminal s is Unicode 017F, which displays as: ſ - many browsers do not support it. Mozilla displays this as a normal lowercase s - this is incorrect. Later versions of Mozilla (1.2.1 and after) seem to display it correctly.