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In old english writting we see the letter "f" used in place of the "s" with internal lettering of a word, do you know the reason behind that?
flogged.
Hi flogged,
The letter you sometimes see used in place of 's' is NOT an 'f'.
It is what is called the 'long s'.
At first glance it can look rather like an 'f' but it is NOT. A more careful study of the letter shows that it either has no "cross-bar" - thus 'ſ'' - or that the "bar" is not a cross-bar at all but extends
only to the
left of the downstroke.
The 'long s' should never be transcribed as 'f' but always as 's'.
Unfortunately, it is all too often transcribed as 'f' leading to twee (and wrong) usages such as 'Mifs' instead of 'Miss', etc, etc, etc.
Such
errors are what give the impression of (as you commented) of a lisp.
As silvery pointed out in reply #4, there is a useful article in Wikipedia:
http://en.wikipedia.org/wiki/Long_sIt also includes some interesting external links.
Regards,
JAP
PS: By now you may have gathered that this is something of a hobby-horse of mine.

As is also the twee (and wrong) transcription of the thorn as 'y' (Ye Olde Curiositie Shoppe) instead of 'th'.
See:
http://en.wikipedia.org/wiki/Thorn_(letter)
PPS: ScotlandsPeople, in its glossary, gives the following explanation for "of that ilk".
Ilk Same, used after surname to indicate person is of the estate of the same name as the familyi.e. not 'of the same family' but 'of the estate of the same name as that of the family'.