(a) RASHI explains that Rav Hamnuna's question involved a mat ten-Tefachim
high which was suspended above the ground more than three Tefachim, making
it a Mechitzah Teluyah. The open space on bottom was smaller than the height
of the mat itself.
Rashi's explanation is difficult to understand. Since the case he is
explaining is a case of Omed Merubah, why does he say that the mat was "ten
Tefachim" high? It would have made more sense to explain Rav Hamnuna's
question as referring to a case where the mat itself was a little more than
*five* Tefachim, and the open space on bottom was a little less than five
Tefachim, making this a similar case of "Omed Merubah al ha'Parutz" to the
cases that the Gemara has discussed until now!
TOSFOS HA'ROSH explains that the Gemara had previously suggested that the
question of Rav Hamnuna referred to a case of "Omed Merubah Al ha'Parutz"
when there were six Tefachim of partition on top and three and a half
Tefachim open on bottom. The Gemara rejected that possibility because
"Gedi'in Bok'in Bo," young goats can walk underneath the partition, thus
invalidating it. If this is an accepted axiom, how, then, can the Gemara
suggest now that this is the question of Rav Hamnuna?
For this reason, Rashi explains that the mat itself is ten Tefachim. The
Gemara before was referring to a mat *less* than ten Tefachim, in which case
the space underneath the mat had to join with the mat in order to make a
Mechitzah with a height of ten Tefachim. If young goats can walk through the
open part, it is clear that the open part cannot be considered as part of
the Mechitzah. In the Gemara's conclusion, on the other hand, there is
already a Mechitzah of ten Tefachim (that is, the mat). The only question is
whether we view that Mechitzah as reaching the ground through "Gud Achis" or
not. In such a case the fact that goats walk though the open part on bottom
might not invalidate the Mechitzah, since there is (1) 10 Tefachim of
Mechitzah and (2) more Mechitzah than open space under it ("Omed Merubah Al
ha'Parutz").
That is, Rav said that Mechitzah Teluyah is not considered a Mechitzah when
it is hanging over dry land. Rav Hamnuna was asking whether Omed Merubah
will change that, and make the Mechitzah work even over dry land.
(b) The SEFAS EMES explains the intention of Rav Ashi differently. According
to Rav Ashi, Rav Hamnuna's question was referring to a case of a Mechitzah
Teluyah that is hanging over water. If the Mechitzah is six and a half
Tefachim, and it is hanging three and a half Tefachim above the water, will
Omed Merubah Al ha'Parutz validate it or not. (The reason why the Gemara
earlier did not want to say that this is the question of Rav Hamnuna is
because it is a Mechitzah under which young goats are able to walk, but when
hanging over water Rav ruled that the problem of "Gedi'in Bok'in Bo" does
not apply.)
According to this explanation, the reason why the Gemara did not deduce the
answer to Rav Hamnuna's question from our Mishnah is because the Mishnah is
discussing a case where ropes were stretched out on top of *land*. That is
why it was impossible for the bottom rope to be three and a half Tefachim
above the ground; it had to be less than three Tefachim from the ground in
order for young goats not to be able to walk through it. Rav Hamnuna's
question, though, was that when a Mechitzah is hanging over water, and it is
not necessary to have the Mechitzah beginning within three Tefachim from the
surface, perhaps Omed Merubah Al ha'Parutz will allow us to allow the
bottommost of three horizontal ropes to be *more* than three Tefachim from
the surface.
Why did Rashi not explain this way? Perhaps, as the Tosfos ha'Rosh writes,
he inferred from the previous Gemara that *anything* less than seven
Tefachim wide and more than three Tefachim from the ground is not a valid
Mechitzah, *even if it is on top of water*. We are lenient with regard to
water (and do not invalidate the Mechitzah because of the fish traveling
under it) only where there is a full-fledged Mechitzah of *ten* Tefachim.
Where there is not a full Mechitzah but only six Tefachim, then even the
presence of fish is enough to invalidate it, and even Omed Merubah cannot
make it into a Mechitzah. (That is, only when there *already* is a
Mechitzah, such as a hanging mat ten Tefachim wide, the presence of fish
does not invalidate the Mechitzah.)