The Mishnah (8:6) teaches that one may not draw water from a cistern that is located between two chatzeirot. The Bartenura explains that since half the cistern is in each chatzer, when one draws water they will be taking water for the other domain. As we have learnt, doing so is prohibited without an eiruv chatzeirot. The Mishnah continues that if they place a ten tephachim high mechitzah inside the cistern, whether “above” or “below”, it would permit one to draw water from the cistern.1 R’ Gamliel continues that it is the subject of debate whether the mechitzah must be below, with Beit Shammai maintaining this position and Beit Hillel aligning with the first one that the mechitza can be above or below. Finally, R’ Yehuda maintains that the existing wall the runs across the cistern dividing the two chatzerot is sufficient and there is no need for an additional mechitzah inside the cistern.
The Gemara (86a) questions the meaning of the positions “above” and “below” mentioned in our Mishnah. R’ Huna explains that above means at the top of the cistern, while below means lower down, but with the base at the surface of the water. R’ Yehuda however explains that above means above the surface of the water with one tephach of the wall immersed, while below means that the mechitzah is immersed in the water with one tephach above the surface.
Rashi explains that according to R’ Yehuda’s explanation, Beit Shammai who demands that the mechitzah be below, requires the mechitzah to extend to the base of the cistern. The Tosfot however find this explanation difficult, maintaining that the even Beit Shammai only requires a mechitzah ten tephachim high because all Beit Shammai require is a heker mechitzah (recognisable division) within the water. If however a partition was required to divide the water, then even a full length iron mechitzah would be insufficient. The purpose of the mechitzah in this case is not to divide the water, but simply as a heker and is sufficient even though the water can still flow from side to side. The Gemara explains that a mechitzah of this nature being sufficient is a special leniency afforded in the case of water alone, provided that one does not draw water directly from the other side (Ritva).
We find this leniency as well in the later Mishnah (8:9) that deals with the case where one’s balcony overhangs a river. In that case one is only allowed to draw water from the river through a hole in the floor of the balcony if a mechitzah is constructed either above or below the hole. Normally a river is considered a karmelit and one is not allowed to carry from a karmelit to a private domain (in this case the balcony). However in this case, we view the walls of the mechitzah as if they are extending below into the river thereby walling off part of the river, making it a reshut ha’yachid. Once again, even though the water in the river is flowing through this “section”, for the case of water, the Chachamim allowed this solution.
The Rashba however asks why (according to R’ Yehuda) in this case we require that the mechitzah be partially submerged, while in the case of the balcony we do not. He suggests that our case is different in that there is a separate reshut hayachid on either side of the mechitzah and we are concerned that one might draw directly from the opposite side. In the case of the balcony, the resident has some share in the karmelit beyond the mechitzah. Consequently we are more lenient.
The Ritva provides another answer. He suggests that the Chachamim were more strict in our cases since the cistern is an area shared by a number of individual. In other words, the cistern resembles a chatzer more closely than a river does.
The Bach (676:1) explains differently that we are stricter in our case since we are concerned that one might draw water directly from the opposite side. In the case of the balcony, since the bucket will be lowered directly below, there is less reason to be concerned.
1 The Aruch HaShulchan notes that this is only if one would be using a bucket from their house or wished to bring the drawn water into their house. If however the bucket was already in the chatzer and one wanted to drink the water there, then no mechitzah would be required. This is because an eiruv chatzerot is only required for transferring items from one’s house to the chazter or back. Utensils that were already in a chatzer from the onset of Shabbat may be transferred directly from chatzer to chatzer without an eiruv chatzeirot. See Eiruvin 9:1.
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