Eruvin, Chapter 2, Mishnah 2. In the previous Mishnah we introduced the concept of passin li'be'iros - four right angles (diyumdin) set up around springs and wells so that pilgrims traveling up to Jerusalem for the festivals could water their animals. Each board of the right angle was one amah long and ten tefachim high. We also discussed there the dispute between Rabbi Meir and Rabbi Yehudah as to how large a gap is permitted between these right angles: ten amos or thirteen and a third amos.
The question of our Mishnah:
Our Mishnah discusses how close the right angles may be brought to the well. The well may be quite large, and as a result a large gap may be created between the right angles; the question is how close they may be to the well itself.
"Mutar l'hakriv la'be'er, u'vilvad she'tehei parah rosha v'rubah bifnim v'shosah" - it is permitted to bring the boards close to the well, provided that a cow can have its head and most of its body inside while it drinks:
One may bring the diyumdin close to the well, on condition that enough room remains for a cow to put its head and most of its body inside the passin and drink. The Gemara cites a beraisa that establishes the measurement: two amos. That is, on each side there must be two amos between the place of the diyumdin and the point where the well begins, so that the animal can put its head and most of its body in and drink.
"Mutar l'harchik kol she'hu, u'vilvad she'yarbeh b'passin" - it is permitted to distance them as much as one wishes, provided that he increases the number of boards:
In the opposite direction there is no limit: one may move the passin as far from the well as he wishes and spread them out, provided that he increases the passin - that he adds the required number of boards. As we mentioned in the previous Mishnah, boards may be added: according to Rabbi Meir a board is required every ten amos, and according to Rabbi Yehudah every thirteen and a third amos, and one continues adding boards as needed.
In summary: our Mishnah establishes the two limits of distance of the passin from the well. Toward bringing them closer - one may do so, provided that two amos remain on each side, so that the cow can put its head and most of its body inside and drink; and toward moving them away - one may distance them as much as he wishes, provided that he increases the passin and adds boards according to the measurement disputed by Rabbi Meir and Rabbi Yehudah.