Eruvin, Chapter 8, Mishnah 8. This mishnah deals with a balcony projecting out over water, and with two balconies one above the other, and with the law of drawing water on Shabbos from the body of water beneath them.
The text of the mishnah:
"Gezuztra she'hi lema'alah min hamayim - ein memal'in heimenah b'Shabbos, ela im kein asu lah mechitzah gevohah asarah tefachim, bein milma'lan bein milmatan" - a balcony that extends out over water, with a hole in it through which a bucket is lowered down to the water: one may not draw water from it on Shabbos unless a partition ten handbreadths high was made for it. This partition may be either above the hole or below it, and it is sufficient whether it surrounds the hole itself or whether it surrounds the entire balcony.
The basis of the leniency is the principle of 'gud' - we view the partition as though it extends downward all the way to the water. This is a special leniency stated regarding matters of water, whereby a partition suspended above an area is considered as extending down to the area beneath it.
The explanation: water is generally a karmelis, while the balcony is a private domain, and one may not transfer from one to the other. Making the partition accomplishes that the water from which one draws is also considered a private domain, and from then on drawing is permitted.
Two balconies one above the other:
"V'chein shtei gezuztra'os zu lema'alah mizu" - the same law applies to two balconies, one above the other, when they are aligned directly opposite each other and the hole in each lines up, so that the upper one draws water through the lower one and from there down to the water. In this scenario too, making the partitions as described is sufficient.
The mishnah now addresses a case in which partitions were not made for both of them: "Asu la'elyonah v'lo asu latachtonah - shteihen asuros ad she'ye'arvu" - if a partition was made only for the upper balcony, drawing water is not permitted for either of them until they make an eruv between them, assuming they belong to two different people.
The lower one: it is forbidden for the obvious reason, since it has no partition at all, neither around the balcony nor around the hole.
The upper one: the Gemara explains that this refers to a scenario in which the owners of the lower balcony contributed money toward the building of the upper partition. For that reason the upper balcony's partition does not set it apart as its own, and the owners of the upper one cannot separate themselves from them. The joint contribution creates a connection between them, and in the absence of an eruv chatzeiros the upper one is also forbidden to draw water, even though it has a partition.
And likewise the reverse: if a partition was made for the lower one and not for the upper one, both are certainly forbidden. The upper one is forbidden because no partition was built around its area; and the lower one is forbidden because of the right of passage, since the bucket lowered from the upper balcony passes through the lower one, and the lower one cannot separate itself from the upper one until they make an eruv between them.
However, if both of them made a partition, they are permitted even without an eruv, since they have thereby shown that there is no joint arrangement or partnership between them. The need for an eruv arises specifically in the two situations in which one made a partition and the other did not: when the lower one is a partner in the upper one's partition, and when the upper one is a partner in the lower one against its will, by the very fact that it needs to pass through it. But when each one makes its own partition independently, they do not need an eruv.
All this applies when the balconies are separated from each other by at least ten handbreadths. But if there are not ten handbreadths between them, they are considered completely joined as a single balcony, and therefore they must make an eruv between them: even if each one made a partition for itself it will not help, since they render each other forbidden until they make an eruv chatzeiros together.
In summary: in this mishnah we learned that drawing water from a balcony suspended above water depends on making a partition ten handbreadths high, either above or below, by force of the principle of 'gud' which brings the partition down to the water and turns the place of drawing into a private domain. With two balconies one above the other: if only one of them made a partition - both are forbidden until they make an eruv, either because of the partnership in the partition or because of the right of passage; and if both made a partition - they are permitted without an eruv. And all this applies when there are ten handbreadths between them, but with less than that they are considered a single balcony and require an eruv chatzeiros.