1) WHEN MAY A "TZURAS HA'PESACH" BE USED?
QUESTION: The Mishnah says that people who were traveling when Shabbos approached and were then stranded in an uninhabited area may put up a make-shift partition around their encampment in order to carry within it on Shabbos. They may put poles in the ground and wrap three ropes around the poles to make a Mechitzah of ten Tefachim high (employing Lavud).
Why does the Mishnah not suggest a simpler way to enclose the area? If the people have poles and ropes, they should simply put a single rope on top of the poles, instead of around them, and create a Tzuras ha'Pesach around the encampment! (TOSFOS 11a, DH Ileima)
(a) TOSFOS answers that the Rabanan permit one to carry in a Chatzer with a Tzuras ha'Pesach only when the Chatzer is used for residential purposes. The Chatzer must be a place where a person lives on a permanent basis. They do not permit one to carry through the use of a Tzuras ha'Pesach in an encampment which a person inhabits for only one day.
(b) The RAMBAM (Hilchos Shabbos 16:16) writes that a Tzuras ha'Pesach helps to close a breach that is more than ten Amos wide only when the length of the Mechitzah that supports the Tzuras ha'Pesach is greater than the sum of the breaches. When the total width of the breaches is greater than the length of the Mechitzah ("Parutz Merubah Al ha'Omed"), a Tzuras ha'Pesach is ineffective. According to this, the three vertical strings are effective where a Tzuras ha'Pesach is not -- where any of the sides of the encampment are unenclosed for a distance of more than ten Amos.
(c) The MAGID MISHNEH, however, suggests that perhaps the Rambam means that even when the breach is smaller than ten Amos, a Tzuras ha'Pesach does not help when the sum of the breached parts is greater than the sum of the closed parts ("Parutz Merubah Al ha'Omed"). (This is contrary to the apparent conclusion of the Gemara on 11a.) That is, the Magid Mishneh is in doubt whether the Rambam maintains that a Tzuras ha'Pesach never works when there is Parutz Merubah Al ha'Omed, or whether the Rambam maintains that a Tzuras ha'Pesach does not work only when there are two problems -- Parutz Merubah Al ha'Omed and a breach wider than ten Amos. If the Rambam means to say the former, it is obvious that a Tzuras ha'Pesach cannot work for an encampment, since the encampment has no solid walls, and therefore it is Parutz Merubah Al ha'Omed.
HALACHAH: The SHULCHAN ARUCH (OC 362:10) rules like Tosfos (a), that a Tzuras ha'Pesach does not help in an uninhabited area such as a valley. The MISHNAH BERURAH (362:57) adds, based on the TUR, that a Tzuras ha'Pesach does not work in a valley only when all four sides of the enclosure are comprised of a Tzuras ha'Pesach. It does work in a valley, though, to close a breach on one side that is more than ten Amos wide.
The SHULCHAN ARUCH also quotes the Rambam's opinion (b), even though he does not agree with it, that a Tzuras ha'Pesach does not work for a breach wider than ten Amos. The Mishnah Berurah (362:59) says that in practice it is best to be stringent like the opinion of the Rambam and not to rely on a Tzuras ha'Pesach that covers a breach of more than ten Amos when there is Parutz Merubah Al ha'Omed. The Bi'ur Halachah writes that if the breach is smaller than ten Amos, even if the wall is Parutz Merubah Al ha'Omed, a Tzuras ha'Pesach still works. (He does not accept the Magid Mishneh's second suggestion, as quoted in (c) above.)
2) A HORIZONTAL APPLICATION OF "OMED MERUBAH AL HA'PARUTZ"
OPINIONS: Rav Hamnuna asked whether the principle of "Omed Merubah Al ha'Parutz" ("the closed part is greater than the breached part") can validate a Mechitzah when the closed part and the breach are both horizontal. The Gemara wonders how this question would be applicable. Rav Ashi explains that Rav Hamnuna was asking about a Mechitzah Teluyah, a hanging partition which is open at the bottom. Even though the bottom is open, the rest of the partition is intact and is larger than the open area on bottom. We know that Rav says that a Mechitzah Teluyah works only when it hangs over a body of water. Rav Hamnuna wanted to know if the rule of Omed Merubah Al ha'Parutz can be used to validate such a Mechitzah.
What exactly was Rav Hamnuna's question, according to Rav Ashi?
(a) RASHI explains that Rav Hamnuna's question involved a mat ten Tefachim tall which was suspended more than three Tefachim above the ground, which thus made 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" tall? 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 case of "Omed Merubah al ha'Parutz" similar to the cases that the Gemara discussed until now!
The 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 "Gediyim Bok'in Bah," young goats can walk underneath the partition and thus invalidate it. How, then, can the Gemara suggest now that Rav Hamnuna was asking about a Mechitzah of just over five Tefachim with an open space on bottom of just under five Tefachim?
If, in the case where only three and a half Tefachim are open on bottom, the Mechitzah is invalid due to "Gediyim Bok'in Bah," then certainly a Mechitzah with almost five Tefachim open on bottom should be invalid!
For this reason, Rashi explains that the mat itself is ten Tefachim tall. The Gemara before referred to a mat less than ten Tefachim tall, in which case the space underneath the mat had to join with the mat in order to create a ten-Tefach-tall Mechitzah. If young goats can walk through the open part, it is clear that the open part cannot be considered 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 consider that Mechitzah to reach 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 are both ten Tefachim of Mechitzah and more Mechitzah than open space ("Omed Merubah Al ha'Parutz") under it.
That is, Rav said that a Mechitzah Teluyah is not considered a Mechitzah when it is hanging over dry land. Rav Hamnuna was asking whether Omed Merubah can 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 refers to a case of a Mechitzah Teluyah that is hanging over water. If the Mechitzah is six and a half Tefachim tall, 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 the problem of "Gediyim Bok'in Bah" does not apply, as Rav ruled.)
According to this explanation, the reason why the Gemara did not deduce the answer to Rav Hamnuna's question from the Mishnah is because in the case of the Mishnah, 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 to remove the possibility of young goats being able to walk through it. Rav Hamnuna's question, though, was that when a Mechitzah hangs over water, and the Mechitzah does not need to begin within three Tefachim of the surface, perhaps Omed Merubah Al ha'Parutz allows 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 tall 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 that swim under it) only where there is a full-fledged Mechitzah ten Tefachim tall. Where there is not a full Mechitzah but only one that is six Tefachim tall, then even the presence of fish is enough to invalidate it, and even Omed Merubah cannot make it into a Mechitzah. (Only when there already is a Mechitzah, such as a hanging mat ten Tefachim tall, does the presence of fish not invalidate the Mechitzah.)