Rif Eruvin 17b {Eruvin 58b continues ... 59b}

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17b

{Eruvin 58b continues - Mishna continues}
And even a servant or maidservant are believed when they say, "up to here are the Shabbat bounds," for the Sages did not decree this to be stringent, but rather to be lenient.

{Eruvin 59a}
Gemara:
Mumcheh {translated "expert"} - derived from ומחה {dropping the mem} - that is to say, an even and straight place.

"To the place that it increased," yes, but to the place that it decreased, no?! Rather, say, even to the place that it increased.

"If it increased for one {expert} and decreased for another": This is the same as the first statement?! {Perhaps this refers to measures rather than experts.} This is what it means to say: if one {expert} increased and another decreased, we listen to the one who increased.
Abaye said: Provided that he does not increase more than the measure of the diagonal of the city.

Mishna:
A town of an individual and it became of many - they may make an eruv for all of it; but of many and it became of an individual they may not make an eruvfor all of it, unless he made outside it as the town of Chadashah in Yehuda{Judea}, which has fifty inhabitants; these are the words of Rabbi Yehuda.
Rabbi Shimon says: Three courtyards of two houses.

Gemara:
The Sages learnt {in a brayta}: The town {without walls} of an individual and it became of many, and the public domain passes through it, how do they make an eruv for it? They place a lechi {vertical pole} on this side {of the head of the road/public domain that passes through} and a lechi on that side; a korah {horizontal board} on this side and a korah on that side, and he may carry in the middle. And they may not combine in an eruv halfway - either the entire town, or each alleyway by itself. And if it {the town} was initially of the public {rather than the individual} and it is now of the public, and it has only one entrance, they may make an eruv for the entire town.

{Eruvin 59b}
This that we learnt {in the brayta}, "They place a lechi on this side and a lechi on that side," we establish that this is like Rabbi Yehuda, who holds the following opinion that we learnt {in a brayta}: More than this said Rabbi Yehuda, one who has two houses on opposite sides of the public domain {road} may make a lechi on this side and a lechi on that side, or a korah on this side and a korah on that side, and carry in the middle. They said to him: We do not construct an eruv of the public domain in this way.

"And they may not make an eruv in it halfway - either all of it {the town} or each alleyway individually."
Why should {making the eruv} halfway be different, they they may not? Because one {half of the town} will invalidate the other {since originally it was one, and so it is reckoned like an eruv where one member forgot to participate}. So too each alleyway individually, they should invalidate the other?