Just Wondering

The other day I finished rereading Under the Lilacs by Louisa May Alcott. At one point the book mentions that Ben, a young boy who has run away from his former life in a circus, is attempting to read a hymn book, but “the long s in the old-fashioned printing bewildered him . . . ."

When I was young they bewildered me too because they look so much like the letter f . I hadn't thought about them in a long time until last night when I was flipping through my copy of the Rambler. My eye fell on those funny s’s and I couldn’t help wondering what rules governed their use, for the regular letter s is also to be found scattered throughout the text.

After a bit of skimming I can see that the long s seems to have been used at the beginning and in the middle of words. A double s seems to have been written with one long s and one regular one. It also looks as though words ending in s use the regular letter. But why did they use the long s at all? There must have been some typographical reason, but I don’t know what it is or even how to look it up. Can anyone relieve my ignorance?

Update: The Wikipedia article to which Bill White links in the comments box gives links to two fascinating articles at BableStone:

"The Rules for Long S" which is far more detailed than than Wikipedia's description and "The Long and the Short of the Letter S" which discusses the origins of this letter form and includes lovely pictures of manuscript examples.