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1. A partition that is suspended from above and does not reach within three Tefachim from the ground is generally not considered a Mechitzah.
2. The Gemara relates the source for the law that a person who has no Techum on Shabbos may walk four Amos in any direction.
3. The Gemara discusses how the area of four Amos is calculated.
4. These four Amos are measured differently for different people.
5. There is a dispute about whether all of the loaves that comprise the Eruv Chatzeros must be placed in a single vessel.
A BIT MORE
1. However, the Chachamim allowed it to serve as a Mechitzah in order to draw water (such as through a window) from within walls that are hanging over the water. Those walls form a Reshus ha'Yachid that is considered to extend down to the water.
2. This is as the verse states, "Shevu Ish Tachtav" -- "a person should sit underneath [himself]." The Gemara understands that this implies that he has always four Amos of travel area on Shabbos (see #3).
3. While everyone agrees that a person's body is generally three Amos long, Rebbi Meir says that the extra Amah is added because this is the amount needed for one to stretch out his hands and feet. Rebbi Yehudah says that this is the extra amount needed to take something that is lying next to his feet and place it next to his head instead.
4. The minimum amount given to anyone, even a midget, is four average Amos. However, a very tall person whose arm (the measure corresponding to an Amah) is much longer than that of an average person, receives an area of four Amos based on the length of an Amah as measured by his arm.
5. Beis Shamai: All of the loaves must be placed in a single vessel, or otherwise the Eruv is invalid. Beis Hillel: The loaves do not need to be placed in a single vessel. However, even Beis Hillel maintains that all of the loaves must be placed in only one house.
Next Daf Index to Revach for Maseches Eruvin