Eruvin 33A

ונתכוין לשבות בעיקרו. ומאי ״למעלה״ ומאי ״למטה״⁠ — דהדר זקיף.

and he intended to establish his Shabbat residence at its base. And what is the meaning of the terms above and below, as we said that this tree extends horizontally to the side, which indicates that it remains at a uniform height? After the tree leans horizontally beyond 4 cubits from the place of its roots, it rises once again in an upright position, and therefore the terms above and below are applicable.

והא אי בעי מייתי לה דרך עליו!

The Talmud asks: Isn’t it true that even if the eiruv is above 10 handbreadths, if one wants, he can remove it from where it was deposited and bring it by way of the tree’s leaves, i.e., its branches that are above 10 handbreadths, to within 4 cubits of the place where he intended to establish his Shabbat residence? Therefore, the eiruv should be valid even though it is above 10 handbreadths.

כשרבים מכתפין עליו, וכדעולא. דאמר עולא: עמוד תשעה ברשות הרבים, ורבים מכתפין עליו, וזרק ונח על גביו ⁠— חייב.

The Talmud answers: We are dealing with a unique situation where the horizontal section of the tree is used by the masses to shoulder their burdens on it, i.e., to temporarily rest their loads on it, so that they can adjust them and easily lift them up again; and the halakha in that case is in accordance with the opinion of Ulla, as Ulla said: With regard to a pillar that is 9 handbreadths high and situated in the public domain, and the masses use it to shoulder their loads upon it, and someone threw an object from a private domain and it came to rest upon it, he is liable, as this pillar has the status of a public domain. Consequently, in the case of the tree, one may not bring the eiruv by way of the tree’s branches, as the horizontal section of the tree has the status of a public domain, and one may not carry from one private domain to another via a public domain.