In Paracelsus' "Of the Mysteries of Nature" I noticed that for the letter S two different characters are used depending on whether the letter is at the not-end of the word, or at the end. When it is the former, an f without the horizontal line is used, when it is the latter, the to us familiar s is used. Example page (on IMGUR)

The translation to English was done by R. Turner in the mid 17th century. Was using two different characters for the letter S common back then or is this a peculiarity (stylistic choice?) of R. Turner? And if it is the former, was there any reason it was that way?

Also, I am surprised by how easy it is to read the text, considering that it was translated in the mid 17th century if I understand correctly, as I was always told that English was one of the most rapidly evolving languages. At around what time did English understandable by modern (non-native) speakers emerge?

Sorry for the probably somewhat obscure questions,

Thanks in advance!

Link to the text on Archive