> Interesting. Unlike other instances of OI lenition and eclipsis,
> in which the mutated phoneme is homorganic with the original phoneme,
> /s/ and /h/ are emphatically not articulated in the same oral location.
I think that depends on the type of s you start from. If you think
specifically of s as pronounced in English or German, with the tip
of the tongue to the teeth, it is really a long way to h.
But phonological /s/ can have other places of phonetic articulation.
In languages which do not have an opposition between sibilants
and "shibilants" (as was the case with proto-Celtic and proto-
Goidelic) /s/ can be realised in a much larger region in the mouth
and still be recognised as such. So if the "average" place of
articulation actually centred around the alveoles, or even further up
towards the palate, you'd get much closer to /h/.
My own few experiences with Spanish, another language that
doesn't oppose /s/ to /š/ (or the like), was that there the s is
articulated somewhere half way between what would either be /s/
or /š/ in German.
The other extreme are of course languages like Polish, where you
have maybe 6 or 7 different sibilants and "shibilants" and the
correct place of articluation is extremely important.
> I've also been curious about the lenition of 'f', which is 0 (zero).
> (Is 0 homorganic with anything?) How did that come about?
Historically speaking it was a change of "weak" intervocalic w > 0.
This w had not been "sharpened" to /f/.
> > F to 0 is indeed a puzzle, though f has been unstable in
> > It's ended up as orthographic h (which is of course phonetic 0)
> > in Spanish: ficus-->higo, ferr(us?)-->hierro, and of course
> > Fernández/Hernández.
I'd explain this as a substratum influence of a pre-Roman language
which had no /f/ in its phonological system, like Celtiberian for
instance. But actually there were more languages around in pre-
Roman Spain which could have been responsible for that.
A different approach would be to think of a substratum language
which had no labio-dental /f/ like Latin, but rather a bilabial fricative
like Japanese, and substituted the former with the latter, in the
local variant of Latin. This sound could have been written in Roman
letters as <h>, or even developed into the sound /h/ at some time.
Some people, e.g. Peter Schrijver and Joe Eska (correct me, if I
misinterpret you), think that proto-Celtic had such a sound as well,
which continued PIE *p. I don't consider this scenario unlikely,