עירובין ד׳Eruvin 4

Mishnah 1א׳

מִי שֶׁהוֹצִיאוּהוּ גוֹיִם אוֹ רוּחַ רָעָה, אֵין לוֹ אֶלָּא אַרְבַּע אַמּוֹת. הֶחֱזִירוּהוּ, כְּאִלּוּ לֹא יָצָא. הוֹלִיכוּהוּ לְעִיר אַחֶרֶת. נְתָנוּהוּ בְדִיר אוֹ בְסַהַר, רַבָּן גַּמְלִיאֵל וְרַבִּי אֶלְעָזָר בֶּן עֲזַרְיָה אוֹמְרִים, מְהַלֵּךְ אֶת כֻּלָּהּ. רַבִּי יְהוֹשֻׁעַ וְרַבִּי עֲקִיבָא אוֹמְרִים, אֵין לוֹ אֶלָּא אַרְבַּע אַמּוֹת. מַעֲשֶׂה שֶׁבָּאוּ מִפְּרַנְדִּיסִין וְהִפְלִיגָה סְפִינָתָם בַּיָּם. רַבָּן גַּמְלִיאֵל וְרַבִּי אֶלְעָזָר בֶּן עֲזַרְיָה הִלְּכוּ אֶת כֻּלָּהּ. רַבִּי יְהוֹשֻׁעַ וְרַבִּי עֲקִיבָא לֹא זָזוּ מֵאַרְבַּע אַמּוֹת, שֶׁרָצוּ לְהַחֲמִיר עַל עַצְמָן:

If one were taken [forcibly] by idolators outside [the tchum], or an evil spirit [i.e., if he were invested by a demon or he became deranged and went outside the tchum, and he regained his senses and found himself outside the tchum], he has only four cubits (of permitted movement). If they returned him [within the tchum], it is as if he had not left [and the entire city is as four cubits to him as in the beginning, and outside it two thousand cubits on every side. This, only when they took him out and returned him forcibly. But if he went out of his own will and were returned against his will, or if he were taken out against his will and returned of his own will, he has only four cubits.] If they took him to a different city [surrounded by walls] or put him in an enclosure or a corral [fenced off with a high partition] — R. Gamliel and R. Elazar b. Azaryah say: He may walk in all of it, [for since it is fenced off, it is like four cubits.] R. Yehoshua and R. Akiva say: He has only four cubits. [For since he had not dwelt in the partitioned space while it was yet day, the partitions do not avail him. Our rabbis explain that enclosure and corral are decreed against by reason of valley, where there are no partitions. And all agree that in a valley he has only four cubits.] Once they came from Prandisin and their boat put out to sea (hifligah) [leaving the shore and putting out to sea is called "haflagah," as in (Psalms 1:3): "palgei mayim" (lit., "clefts of water").] R. Gamliel and R. Elazar b. Azaryah walked all of it. [They walked along the whole boat. And even though the boat sailed on Shabbath and they left the tchum, this is comparable to leaving the tchum and being placed in an enclosure or a corral, the boat being "fenced" with partitions.] R. Yehoshua and R. Akiva did not budge beyond four cubits, desiring to be stringent with themselves. [For according to them, too, who decreed enclosure and corral by reason of valley — with a boat, so long as it is moving, it is permitted to walk along all of it. And, what is more, they dwelt within its partitioned space while it was still day. But they desired to be stringent. The halachah is in accordance with R. Gamliel and R. Elazar b. Azaryah.]

Mishnah 2ב׳

פַּעַם אַחַת לֹא נִכְנְסוּ לַנָּמָל עַד שֶׁחֲשֵׁכָה. אָמְרוּ לוֹ לְרַבָּן גַּמְלִיאֵל, מָה אָנוּ לֵירֵד. אָמַר לָהֶן, מֻתָּר אַתֶּם, שֶׁכְּבָר הָיִיתִי מִסְתַּכֵּל, וְהָיִינוּ בְתוֹךְ הַתְּחוּם עַד שֶׁלֹּא חֲשֵׁכָה:

Once, they did not enter the harbor until dark. They asked R. Gamliel: "May we go down?" [from the boat into the city, or did we come from outside the tchum after dark? (This harbor was not partitioned; for if it were, R. Gamliel has already said above that if one were placed into a (fenced-off) enclosure or corral, he may walk in all of it.] He answered: "You may do so, for I already looked [through my telescope, adjusted to a two-thousand cubit range], and (found that) we were within the tchum before it had gotten dark."