(R. Ba citing Rav Yehudah citing Rav): If a city is shaped like a bow - if there are 3999 Amos between this [end] and this, [we view it as if it were full of houses, and] he may traverse all of it and 2000 Amos outside of it;

If 4000 Amos [separate the ends], he has only 2000 Amos from his Eruv.

(R. Acha): R. Shmuel explained, [in the former case it is permitted because] if someone [went along the Yeser, i.e. the string between the ends of the bow, and] put his sandals here [past 2000 Amos], he may go outside (back to the end of the bow, and along the bow to the other end, and to where he put them) and take them (for they are within 2000 Amos), and if he put his sandals here [past 2000 Amos from the other end], he may go outside (back to the bow and along the bow...) and take them;

From here, he can go outside (via the bow), and from here, he can go outside (via the bow). Will you say that from one direction it is forbidden and from two directions it is permitted?! (Rather, surely it is permitted even from one direction - SHA'AREI TORAS ERETZ YISRAEL.)

(R. Ba citing Rav Yehudah): If one city was big, and it has a trench - if it is not 10 deep and four wide, you view it as if it were full of earth and pebbles (so the entire city may make one Eruv). If not (rather, it is 10 deep and four wide), you view it like Mefulash (each side of the city must be Me'arev by itself. We explained this essentially like HA'GAON RAV C. KANIEVSKY, SHLITA.)

(R. Ba citing Ami bar Yechezkel citing Rav): If a city was built on the bank of a river - if there are four Tefachim between it and the river you measure [2000 Amos] from the wall. If there are three [Tefachim in between], you measure from the outer bank of the river, as long as the river is not more than 70 Amos and a fraction [wide].

Question: What is the case?

If [the river bank] slopes 10 Tefachim amidst three Amos, [even if there are only three Tefachim in between the wall and the river, do you measure] from the outer bank of the river?! (It is hard to walk there. Surely it is not Batel to the city!)

If [the river bank] slopes 10 Tefachim amidst four Amos, [even if there are four Tefachim in between it, do you measure from the wall?! One can traverse such an incline. Surely it is Batel to the city!]

Answer: [It slopes 10 Tefachim amidst] between three and four [Amos].

(Rav): A city consisting of tents [is not considered a city, for they are not fixed]. Everyone measures [his Techum] from his tent;

If there were there three Tzerifin or Borgenin (shacks of date or willow branches, or of people who guard Peros), he measures from the outer one.

Question #1 (Asi): It says "v'Yad Tihyeh Lecha mi'Chutz la'Machaneh'' - (when Bnei Yisrael were in the Midbar, one who needed to defecate needed to go in back of Machaneh Yisrael, which was 12 Mil by 12 Mil. If everyone measures from his tent, on Shabbos,) how could people [not near the back] go outside [the Machaneh] to eliminate?!

Question #2 (R. Chiyah brei d'R. Shivsai): [If everyone measures from his tent, on Shabbos,] how could they go to Moshe's Beis Midrash?!

Answer #1 (to both questions): Moshe made for them three Tzerifin or Borgenin. (This made the entire Machaneh like one city. All the Ohalim were Batel to them.)

Answer #2 (to both questions - R. Yosah): Since they camped and traveled according to Hash-m's word, it is considered permanent.

Objection (R. Yosi bei R. Bun): Since Hash-m promised to enter them to Eretz [Yisrael], it is considered temporary! (PNEI MEIR)

Due to this, R. Avin needed to say [like the Stam Gemara said above,] that Moshe made for them three Tzerifin or Borgenin.