Mikvaos, Chapter 6, Mishnah 3. This chapter deals with the ways in which separate bodies of water are joined to one another, and with the question of when water that is not valid mikvah water becomes part of a kosher mikvah. Our mishnah takes up the subject of eiruv mikvaos: when do two pools of water standing beside each other count as a single unit? The stakes are high. If each pool holds twenty se'ah of rainwater, neither one alone is a mikvah, since a mikvah requires forty se'ah. But if the two are joined into one body, we now have a complete mikvah of forty se'ah, and that complete mikvah is strong enough to render valid a neighboring pool of water that is not mei geshamim at all.
The first case: the drawn water on the side
The mishnah opens: "Shelosha mikvaos, bezeh esrim se'ah uvezeh esrim se'ah" - three pools of water collected in three adjacent pits. The first two, the one at the far right and the one beside it, each hold twenty se'ah of mei geshamim, rainwater that is fit for a mikvah, but neither one holds the full shiur of forty. "Uvezeh esrim se'ah mayim she'uvim" - the third pit, on the left, holds twenty se'ah of mayim she'uvim, drawn water, which is not valid mikvah water. "Vehasha'uv min hatzad" - the pit with the drawn water is positioned at the side, on the outside of the other two.
"Veyardu shelosha vetavlu bahen" - three people went down and immersed themselves, one in each pool. "Venis'arvu" - as they entered, the water in each pit rose up and the three bodies of water touched one another, so that at that moment all three were connected.
The ruling is: "Hamikvaos tehorin vehatovlin tehorim" - the mikvaos are valid and the people who immersed are tohor. Follow the chain. The two rainwater pools underwent hashaka, they kissed and joined into a single unit, and together they now constitute forty se'ah of kosher mei geshamim. The third pool, which contains only drawn water, is now mushak, touching and joined, to that complete forty se'ah of rainwater. Through that contact it too becomes a kosher mikvah. And so all three immersions took effect.
The second case: the drawn water in the middle
The mishnah continues: "Hayah hasha'uv ba'emtza" - what if the arrangement is different, and the drawn water sits in the middle? On the right is a pool of mei geshamim, valid water; on the left is another pool of mei geshamim; and between them lies a pool of mayim she'uvim, water that was held in a vessel before reaching the pit and is therefore unfit for a mikvah. "Veyardu shelosha vetavlu bahen venis'arvu" - again three people descended and immersed, and again the waters rose and mingled.
Here the ruling is the reverse: "Hamikvaos kemo shehayu vehatovlin kemo shehayu" - the mikvaos remain as they were and the immersers remain as they were. When the people leave and the water settles back, the two outer pools are still twenty se'ah of mei geshamim apiece, each of them lacking, each of them ready to become kosher if another twenty se'ah of rainwater were added. And the people who immersed remain exactly as they were, still tmeiyim, because the two valid pools cannot be joined to each other through the medium of drawn water sitting between them. Water that is itself invalid cannot serve as the bridge that unites two kosher bodies of water.
A question of the mefarshim
The mefarshim raise a difficulty. If the middle pool never became kosher, then its drawn water was still mayim she'uvim when the waters mingled, and some of it surely flowed into the two outer pools; at the very least we cannot guarantee that it did not. Why then does the mishnah say that the outer mikvaos remain as they were? They ought to have become pasul, since they now contain mayim she'uvim, and possibly even three lugin of it, and three lugin of drawn water entering a pool before it has been filled with forty se'ah of mei geshamim disqualifies it.
The mefarshim answer that in our case the drawn water did not pour in directly. It arrived by way of hamshacha: the water of the middle pit traveled over karka haro'uy livlo'a, ground capable of absorbing, for a distance of three tefachim before it reached the outer pools. Drawn water that comes in through such a drawing across absorbent earth does not disqualify a mikvah even with three lugin. The two outer pools therefore remain precisely as they were, valid rainwater of twenty se'ah, waiting to be completed.
The mishnah thus teaches us both sides of the same principle. A full forty se'ah of rainwater can reach out and validate drawn water that touches it from the side, but drawn water can never act as the connector that turns two deficient rainwater pools into one complete mikvah.