Eruvin 48 - Shabbat September 26, 8 Tishrei

Water in a ditch that is exactly in between the techum of two cities, does one need to put a partition in the water to not allow water to flow from one side to the other? Would a partition be effective? When the mishna says that one who leaves the techum gets four cubits, how do we measure those cubits? Is it four in each direction, two in each direction or can one move four in one direction? Are the cubits measured by the size of the forearm of that particular person or it by an objective measurement? Why did Rabbi Shimon compare the case of three people eating in three different techumim to the case of three courtyards? Why doesn't he view all three courtyards as being together if each side made an eruv with the middle? Rav Yehuda and Rav Sheshet each interpret the case differently in order to answer this question. Rav Acha raises a question on each opinion and Rav Ashi answers them. In what way do the rabbis disagree with Rabbi Shimon?