Eruvin, Chapter 4, Mishnah 2. In the previous mishnah we examined the dispute among the Tannaim - Rabban Gamliel and Rabbi Elazar ben Azaryah on one side, and Rabbi Yehoshua and Rabbi Akiva on the other - as to whether someone who was taken beyond the Shabbos boundary and brought into an area enclosed by partitions may walk about that entire area, and we discussed the incident that occurred with these four Tannaim. Our mishnah brings a further incident.
The words of the Mishnah:
"Pa'am achas lo nichnesu la'namal ad she'chashcha" - the ship reached the harbor only after dark, that is, on Shabbos night.
"Amru lo l'Rabban Gamliel: mah anu leired?" - the Sages asked Rabban Gamliel whether they were permitted to disembark from the ship.
Why was the question asked?
The passengers thought that they may have reached the boundary of the harbor while it was still day, before Shabbos began. However, the harbor was not an area enclosed by partitions and walls, and that is precisely where the question lies: they already knew Rabban Gamliel's opinion, that one who is inside an area enclosed by partitions when Shabbos begins may walk about that entire area. Had the place been enclosed by partitions, they would not have bothered to ask. Once they understood that the place had no partitions, they wanted to know something else: were they inside the boundary at the moment Shabbos began?
Rabban Gamliel's answer:
"Amar lahem: mutarin atem, she'kevar hayisi mistakel, v'hayinu b'soch ha'techum ad she'lo chashcha" - Rabban Gamliel answered that he had looked and checked, and found that they were inside the boundary before it grew dark, that is, before Shabbos began. For that reason they were permitted to disembark from the ship, since at the moment Shabbos began they were within the boundary.
How did Rabban Gamliel know?
He had with him a sort of tube designed for this purpose, by means of which one can determine exactly where the two thousand cubits end. He looked through this tube, and was able to report that they were within the two thousand cubits even before Shabbos began.
In summary: our mishnah presents an incident in which the ship arrived at the harbor after dark, and the question that was asked did not concern partitions - for the harbor had no partitions - but rather whether the passengers were within the boundary at the moment Shabbos began. Rabban Gamliel answered that they were indeed within the boundary before dark, and therefore they were permitted to disembark, and his knowledge of this rested on that tube with which he measured the two thousand cubits.