Eruvin 79 – Haystack

If two courtyards have between them a haystack which is ten hand-breadths high, then naturally it separates the two, and they must each make an eruv (common food) for their courtyard, to permit carrying there on Shabbat.

The residents of one courtyard can feed their animals from the haystack, and so can the residents of the other courtyard. Potentially, one might be concerned that the animals will eat the haystack up, making it less than ten hand-breadths in height, and in this way nullifying the partition. The rule above tells us exactly that: we should not be concerned about such a possibility since normally animals don't eat that much in the course of one Shabbat.

However, if the animals ate enough of the straw during the week before Shabbat, and the height of the haystack is lower than ten hand-breadths on a section that is at least ten amot (20 feet) long – the haystack no longer serves as a partition; the people in both courtyards have to join in a common eruv, and they can no longer use their individual eruv for each courtyard.

Art: Haystack At Sunset by Martin Johnson Heade 1819-1904