1)

(a)'Bitzi'asa de'Meishan' is - a small boat which is tapered to a point at its base.

(b)Rav Huna forbids carrying in such a boat - because walls that curve and join at the bottom (or at the top) are not considered Mechitzos, and the boat is therefore a Karmelis.

(c)Carrying in such a boat is permitted however - if the angle of the walls is not sufficiently acute to reach four Tefachim wide within three Tefachim from the floor, or if the bottom of the boat is filled in with thin willow twigs and canes, provided, in the latter case, the walls reach ten Tefachim above that point.

2)

(a)When Rav Nachman asks Rav Huna why we should not say 'Gud Acheis Mechitzasah' - he means that we should extend whatever wall there is, or even just the edges of the (ninety-degree) ledge of four by four Tefachim, down to the ground, and consider it as if there was a wall there.

(b)The source for this is a statement by Rebbi Yossi b'Rebbi Yehudah, who says in a Beraisa - that someone who sticks a cane with a basket perched on its top into the ground of the Reshus ha'Rabim and then throws something into it is Chayav for throwing from the Reshus ha'Rabim into a Reshus ha'Yachid.

(c)Rav Yosef rejects this proof however, on the basis of a statement of Rav Yehudah Amar Rav (or Rebbi Chiya) that the Chachamim disagree with Rebbi Yossi b'Rebbi Yehudah, and render the thrower Patur. But Abaye queries this from the case of a plank stuck in the ground in the street, which is four Tefachim wide on top but only three at the base - yet we learned in a Beraisa, that if someone throws on to it, he is Chayav.

(d)To reconcile the two rulings of the Rabbanan, Rav Yosef explains - that whereas, in Rebbi Yossi b'Rebbi Yehudah's case, due to the 'T'-shape of the pole with the basket, the kid-goats are able to squeeze past where the wall theoretically descends ('Gedayim Bok'in Bah'), they cannot do so in the case of the plank, in which case the Rabbanan will concede to the principle of 'Gud Acheis Mechitzasah'.

3)

(a)Rav Acha b'rei de'Rav Acha asked Rav Ashi, how Rav Nachman could then even suggest saying 'Gud Acheis' in the case of 'Bitzi'asa de'Meishan' - seeing as the fish swim past the foot of the boat, which ought to negate 'Gud Acheis', just like the kid-goats do in the case of the pole.

(b)Rav Ashi replied - that the concept of 'Gedayim Bok'in Bah' does not apply to fish in the water, since they are not visible.

(c)And we prove this from a 'Mechitzah Teluyah' - a hanging Mechitzah (which allows fish to swim underneath it (and which is considered a Mechitzah, as we shall now see).

(d)Rav Tivla asked Ravin whether we apply the principle of 'Mechitzah Teluyah' in a ruin (where such walls are common). He answered - that 'Mechitzah Teluyah' only applies in water ...

(e)... a proof that 'Gedayim Bok'in Bah' does not apply in water, as Rav Ashi explained.

4)

(a)To answer the Kashya that our Mishnah's ruling permitting carrying from one boat to another, when the two are tied together, is obvious, Rava suggests that the Tana is speaking about 'Bitzis she'Beinaihu', by which he means - that the Tana is speaking about carrying from one boat to another via a third boat that is not tied to the outer ones ...

(b)Which we might otherwise have forbidden - in case the boat in the middle floats away, in which case anyone who carries will be carrying from one Reshus ha'Yachid to another via a Karmelis, which is forbidden.

(c)Rav Safra objects to Rava's answer however, based the Lashon 'mi'Zu le'Zu' used by our Mishnah, implying that there are only two ships, and not three.

(d)So Rav Safra solves the problem - by establishing our Mishnah by two ships belonging to two people, who made an Eiruv, which is valid only as long as the two ships are tied together, and our Mishnah is teaching us that the moment they become untied, the Eiruv becomes nullified, and carrying from one to the other is prohibited.

5)

(a)Rav Safra's answer is based on a Beraisa, which rules in a case where the two boats ...

1. ... came apart - that the Heter to carry falls away.

2. ... were tied together again (even though it is forbidden to so) - that the Heter returns ...

(b)... irrespective of whether the tying was done Shogegin, Mezidin, Anusin or Muta'in (which means - 'Mis'asek' [where one intended to tie something else, and tied them by mistake').

(c)The Tana also discusses a case - where a series of mats were tied together to form Mechitzos, permitting those living on one side to carry to the other, be means of an Eiruv, which became Batel when the mats came apart, and which returned to their original Heter when they were tied together again (be'Isur).

(d)It is clear from what we just learned - that a Mechitzah that is put up on Shabbos does have the Din of a Mechitzah.

6)

(a)Rav Nachman rules that a Mechitzah that is erected be'Meizid on Shabbos - only has the Din of a Mechitzah le'Chumra (for someone who throws into it from a Reshus ha'Yachid), but not le'Kula (to give it a Din of a Reshus ha'Yachid, and to permit carrying inside that area).

(b)We reconcile this with the Beraisa 'Chazru ve'Niksheru, Chazru le'Heteiran ha'Rishon' - by restricting it to Shogegin, Onsin and Muta'in (even though it appears to refer to Meizidin as well).

7)

(a)Shmuel establishes our Mishnah even when the ships are tied with a string (which is normally used to tie the neck of a coat). The problem with this is that 'Mah Nafshach' - if it will hold them together, then it is obvious; and if not, then why may one carry from one to the other?

(b)We conclude that the string must certainly hold the boats together, and that Shmuel mentions string to preclude from another statement of his in connection with a Mishnah in Ohalos, which states a rope that is tied to a boat at one end and to an Ohel ha'Meis at the other - transmits Tum'ah to the boat (if it is a Sefinas ha'Yarden, as we learned in 'Amar Rebbi Akiva') and to the vessels on the boat.

(c)Shmuel comments there - that the Tana is speaking specifically about a metal chain ('Davar ha'Ma'amidah' - something with which one usually ties it), because, based on the Pasuk in Chukas "ba'Chalal Cherev", it is only metal that will render the boat an Av ha'Tum'ah and the vessels a Rishon.

(d)That is why he sees fit to establishes our Mishnah even by an ordinary piece of string - to teach us that Shabbos, unlike Tum'ah, only requires the two boats to be well tied together, but does not require metal to achieve that purpose.