Talmud - Eruvin 79b עירובין ע״ט ב׳

and the hay in the house is slightly more than seven handbreadths high it is considered a full-fledged partition that reaches the ceiling, as objects separated by any gap of less than three handbreadths are considered joined, based on the principle of lavud.

The Gemara comments. Granted, according to the opinion of Abaye, that is why the baraita teaches: If the height of the haystack was reduced to less than ten handbreadths. However, according to Rav Huna, son of Rav Yehoshua, what is the meaning of: Less than ten? Even at the outset it was never as high as ten handbreadths.

The Gemara answers: It means that it was reduced to less than the law of ten handbreadths. As long as the hay is slightly more than seven handbreadths high, it is regarded as ten handbreadths high, in accordance with the principle of lavud. Once its height is reduced to less than seven handbreadths, the halakha of a partition ten handbreadths high is no longer in effect.

The same baraita taught that if the height of the hay was reduced to less than ten handbreadths, it is prohibited to carry in both. The Gemara comments: Should we learn from this that residents who arrive on Shabbat prohibit the other residents from carrying? At the onset of Shabbat, both sets of residents were permitted to use the hay and the house, but once the hay was reduced on Shabbat it is as though new residents had been added to each of the courtyards, and it is prohibited for all of them to carry. Why not say that since at the beginning of Shabbat it was permitted to carry in the domain, they are permitted to do so for its duration?

The Gemara rejects this contention: Perhaps the baraita is referring to a case where the hay was already reduced on the previous day, before Shabbat began. In that case, it was never permitted to carry at all.

The baraita continues: How, then, does the resident of one of the courtyards act if he seeks to permit use of the other courtyard to its resident? He locks his house and renounces his right to carry in the courtyard in favor of the other person. The Gemara is surprised by this ruling: Does he require these two steps? One should suffice. The Gemara answers: This is what the tanna of the baraita is saying: Either he locks his house or he renounces his right to the courtyard.

And if you wish, say a different explanation instead. Actually, both actions are required in this case, even though one of them would ordinarily suffice. The reason is: Since he is accustomed to using the courtyard, he will come to carry. Consequently, the Sages were stringent with a person in this position and obligated him to implement an additional change so that he will not forget and come to carry when it is prohibited.

It was taught in the baraita: If one locks his house and renounces his rights to the courtyard, it is prohibited for him to carry, and it is permitted for the other person to carry. The Gemara raises a difficulty: Isn’t this obvious? Why was it necessary to state this halakha? The Gemara answers: It was necessary only in a case where the other person then renounced his right in favor of the first person. And the baraita teaches us that one may not renounce his rights in favor of the other, and then have the latter renounce his own rights in favor of the former.

It was further taught in the baraita: And you can say likewise with regard to a pit of hay that is situated between two Shabbat limits. The inhabitants of each area may feed their animals from the common hay. The Gemara raises a difficulty: Isn’t this obvious? The same principle that is in effect with regard to a haystack between courtyards should apply here as well. The Gemara answers: It was necessary to state this halakha only according to the opinion of Rabbi Akiva, who said that the principle of Shabbat boundaries is by Torah law. Lest you say that let us issue a decree and prohibit it, lest people come to exchange objects from one boundary to another, which would violate a Torah prohibition; therefore, the baraita teaches us that no distinction is made between the cases, and no decree of this kind is issued.

MISHNA: How does one merge the courtyards that open into the alleyway, if a person wishes to act on behalf of all the residents of the alleyway? He places a barrel filled with his own food and says: This is for all the residents of the alleyway. For this gift to be acquired by the others, someone must accept it on their behalf, and the tanna therefore teaches that he may confer possession to them even by means of his adult son or daughter, and likewise by means of his Hebrew slave or maidservant, whom he does not own, and by means of his wife. These people may acquire the eiruv on behalf of all the residents of the alleyway.

However, he may not confer possession by means of his minor son or daughter, nor by means of his Canaanite slave or maidservant, because they cannot effect acquisition, as ownership of objects that come into their possession is as if those objects came into his possession. Consequently, the master or father cannot confer possession to the slave or minor respectively on behalf of others as their acquisition is ineffective and the object remains in his own possession.

GEMARA: Rav Yehuda said: With regard to a barrel for the merging of alleyways, the one acquiring it on behalf of the alleyway’s residents must raise it a handbreadth from the ground, as he must perform a valid act of acquisition on their behalf.

Rava said: The elders of Pumbedita, Rav Yehuda and his students, stated these two matters. One was this mentioned above with regard to lifting the barrel; and the other was: With regard to one who recites kiddush over wine on Shabbat or a Festival, if he tasted a mouthful of wine, he fulfilled his obligation; however, if he did not taste a mouthful, he did not fulfill his obligation.

Rav Ḥaviva said: In addition to the aforementioned pair of teachings, the elders of Pumbedita stated this too, as Rav Yehuda said that Shmuel said: One builds a fire for a woman in childbirth on Shabbat.

The Gemara comments that the Sages thought to infer from here: For a woman in childbirth, yes, one builds a fire, due to her highly precarious state; for a sick person, no, one does not build a fire. Likewise, in the rainy season, when the danger of catching cold is ever present, yes, one builds a fire; in the summer, no, one may not.

The Gemara adds that which was stated: Rav Ḥiyya bar Avin said that Shmuel said: With regard to one who let blood and caught cold, one builds a fire for him on Shabbat, even during the season of Tammuz, i.e., the summer. Clearly, Rav Yehuda’s ruling is limited neither to a woman in childbirth nor to the rainy season.

Ameimar said: This too was stated by the elders of Pumbedita, as it was stated that the amora’im disagreed with regard to which tree is presumed to be a tree designated for idolatry [asheira], even though no one actually saw it worshipped.

Rav said: It is any tree that idolatrous priests guard.

Traduction : The William Davidson digital edition of the Koren Noé Talmud, with commentary by Rabbi Adin Even-Israel Steinsaltz.

Eruvin 79b explores how the lavud principle applies to haystack height in determining valid partitions and their impact on Shabbat carrying between courtyards.