- A town by a ravine
Rav Yosef said in the name of Rami bar Abba who said in the name of Rav Huna עיר שיושבת על שפת הנחל – a town that is situated at the edge of a deep ravine, אם יש לפניה דקה ארבעה – if there is a thin wall, four amos high, in front of the entire length of the town to prevent residents from falling into the ravine, מודדין משפת הנחל – we measure the techum from the edge of the ravine, since the town is considered extended until there. ואם לאו אין מודדין לה אלא מפתח ביתו – If there is no wall, we measure the techum only from the door of each house. Rashi explains that since people are fearful to use the outside area, the town is not considered to be a yishuv kavua (a fixed residential area) but rather a collection of temporary dwellings, similar to the יושבי צריפין - the hut dwellers discussed on Daf 55b.
- The towns of Geder and Chamsan
Rav Yosef based his position on a Baraisa that Rebbe permitted the residents of the town of Geder to go down to the town of Chamsan but the residents of Chamsan could not go up to Geder. Rav Yosef explained that the residents of Geder had built a thin wall at the edge of their town to prevent falling which enabled them to measure their techum from there, whereas the residents of Chamsan had not and therefore had to measure their techum from their individual houses.
The Gemara presents alternative explanations for Rebbe’s ruling. Rav Dimi explained that the residents of Geder would get drunk on Shabbos and assault the people of Chamsan who visited. Rashi states that they would even kill them. Rebbe allowed the residents of Geder to visit Chamsan because, כלבא בלא מתיה שב שנין לא נבח – a dog outside of his town does not bark for seven years, meaning one naturally becomes more timid when outside his town. Therefore, Rebbe was not concerned that the residents of Geder would become aggressive in Chamsan.
- Placing the eruv in the middle of a town
The next Mishnah stated that if one was in a large town and placed his eruv in a small town, or vice versa, מהלך את כלותה וחוצה לה אלפים אמה – he may traverse the entire town in which the eruv was placed and travel two thousand amos beyond it. Rebbe Akiva said, אין לו אלא ממקום עירובו אלפים אמה – He has no more than two thousand amos from the place of his eruv. Rebbe Akiva holds that only one who actually camps in the town can treat the town as his four amos, but not one who merely places his eruv there.
Rebbe Akiva asked the Chochomim if they do not agree that if one placed his eruv in a cave that he has only two thousand amos from the place of the eruv, and not from the edge of the cave and they responded that that is only in a case, שאין בה דיורין – when there are no inhabitants in the cave, but if there were inhabitants in there, it would be treated like the city and the techum would be measured edges of the cave.
Rav Yehudah said in the name of Shmuel that a desolate town has the same status of an uninhabited cave. According to Rebbe Elazar, when the Mishnah said, אין בה דיורין – it meant it cannot be inhabited, therefore he measures from the eruv. A desolate town can be inhabited; therefore, he can measure the eruv from the edge of town.
