Mikvaos, Chapter 6, Mishnah 9. This mishnah continues the theme of iruv mikvaos, the rules that determine when two adjacent bodies of water are regarded as one. Picture two pools standing side by side, each containing only twenty se'ah, neither of which is a mikveh on its own. If they can be joined, together they form the required forty se'ah. Until now we have learned that the joining requires an opening of kishefoferes hanod, the width of the spout of a leather flask. Our mishnah presents a chiddush: there are circumstances in which such an opening is not needed.
A Crack in the Dividing Wall
The case before us: "Kosel shebein shtei mikvaos shenisdak", a partition standing between two mikvaos in which a crack has opened. Everything now turns on the crack's orientation. "Lishesi mitztaref": where the crack follows the shesi, the two pools link up and count as a single mikveh even though no opening of the full shiur exists. "Le'erev ein mitztaref": where it follows the erev, the pools do not link up, and the mishnah adds that they remain two separate bodies of water unless at some point along the crack there is an aperture measuring shefoferes hanod.
According to the first opinion in the mishnah, shesi refers to a crack running horizontally, from side to side, and erev to a crack running vertically, from top to bottom.
The mishnah then records a dissenting view: "chiluf hadvarim", the two terms are to be understood in precisely the opposite sense. As Rebbi Yehudah reads them, a crack descending from top to bottom joins the pools with no need for an opening of shefoferes hanod, whereas a crack stretching across the wall from side to side leaves them apart unless it reaches that measure.
The Logic on Both Sides
Sevara can be marshaled in either direction here, and the poskim note as much: each of the two readings rests on solid reasoning.
- One may argue that when the crack runs from side to side, the entire breadth of one mikveh stands exposed to the other, so that the two are inherently open to one another and no stronger point of connection is required. A crack running only from top to bottom leaves them less exposed, and therefore demands a fuller opening.
- One may argue the opposite. When the crack runs from side to side, the water at other heights, higher up and lower down in the pools, is not in contact across the wall. When the crack runs from top to bottom, every level of the one mikveh meets the corresponding level of the other, and the water is genuinely me'urav, mixed as a single body.
- The rishonim offer yet another consideration, based not on the water but on the wall itself. A crack running from top to bottom does not compromise the structure. A crack that runs across the wall from side to side weakens it, and a weakened wall no longer functions as a mechitzah, a genuine barrier separating the two pools.
Since there is a machlokes as to what shesi va'erev mean here, whether the reference is to a horizontal crack or a vertical one, the poskim rule that in practice we always require an opening of shefoferes hanod when joining two mikvaos.
When the Wall Is Breached at the Top
The mishnah continues: "nifretzu zeh besoch zeh", if they were breached into one another, the top of the dividing wall having fallen away so that the two pools are open to each other over its upper edge. Here the mishnah gives two separate measurements. As for the height of the water passing over the wall, "elyon keklipas hashum", a layer as thin as the outer peel of a garlic suffices: a mere film of water spanning the top of the wall is enough. "Va'orcho kishefoferes hanod", but the breach must be at least as wide as shefoferes hanod, the opening of a flask.
Why is so little demanded of the height here? Because in this case the water is joined across the top of the wall, in the open, connecting the two pools directly to one another. That is a far more substantial union than a crack confined within the body of the wall, which does not span it, and therefore a smaller point of connection suffices.
The mishnah thus closes the discussion by showing that not every connection between two pools is measured by the same standard. Everything depends on how fully the two bodies of water stand open to one another.