Mikvaos, Chapter 4, Mishnah 4. Water that was once collected in a vessel becomes mayim she'uvim, drawn water, which is unfit for a mikveh. One of the ways such water can be restored and regain the standing of valid rainwater is through hamshacha: the water is drawn along the ground rather than poured directly in. Our mishnah deals with hamshacha, and it should be said at the outset that the mishnah reflects only one opinion among the Tannaim. At the end of the lesson we will look at the other views.
Mixed Waters Before They Reach the Mikveh
The mishnah opens with a case of waters that joined together: "Mayim she'uvim u'mei geshamim shenis'arvu b'chatzer o b'ukah o al ma'alos ham'arah", drawn water together with rain water that came together in a courtyard, or inside a hollow in the ground, or upon the stone stairway that descends into a cave. The rain water spoken of here never sat in a kli at any point, and the two kinds of water have gathered into a single body at some spot outside the mikveh before making their way inside.
"Im rov min hakasher, kasher": if the majority of the mixture is the valid water, the mikveh is kasher. The rainwater portion was never disqualified to begin with, and the she'uvim portion was drawn along the ground, over earth capable of absorbing it, for the required distance of three tefachim, so it has undergone hamshacha and is reconstituted.
Two conditions emerge from the Tanna's words. First, the majority of the water must be rainwater. Second, the mixing must take place before the water enters the mikveh.
"Ve'im rov min hapasul, pasul": if the majority comes from the drawn water, hamshacha will not save it and the water remains disqualified. And the same applies mechtzah lemechtzah, half and half: it is pasul.
When Does Hamshacha Help?
The mishnah now defines the boundary: "Eimatai? Bizman shemisa'arvin ad shelo yagi'u lamikveh". Hamshacha removes the disqualification of she'uvim only when the two waters combined before reaching the mikveh.
Suppose instead that the two waters never pooled together outside, and each made its way in on its own: "Hayu mekalchin letoch hamayim", they were pouring into the mikveh, this stream from one direction and that stream from another. Everything now turns on what can be verified. "Im yadua sheba'u mayim k'sheirim", if it is known that the fit water arrived, "ad shelo yardu letocho", before there came down into it "shlosha lugin mayim she'uvim", three lugin of drawn water, then "kasher": a full forty se'ah of rain water was already in place first, and a mikveh that has its forty se'ah is fixed and stays fixed. "Ve'im lav, pasul": where there is no way to determine that the forty se'ah of rain water arrived ahead of the drawn water, the mikveh is disqualified.
The Other Opinions
As noted, the Tanna of our mishnah requires both conditions together: the waters must mix before entering the mikveh, and the majority must be rainwater. Only under those two conditions does hamshacha accomplish anything.
Other tannaic positions are preserved in the Gemara in Maseches Temurah, on amud beis of daf yud beis. Rebbi Eliezer ben Yaakov maintains that hamshacha is effective even where the two waters never joined outside the mikveh. So long as the greater part is rain water, and so long as the drawn portion was led along the ground before it arrived, the mikveh is valid even when each stream came in by its own route. The Chachamim in that discussion take a broader stance still: in their view hamshacha rehabilitates the water even if every drop of it is she'uvim, a position that does not match the ruling of our mishnah.
As for the halachah, it appears that we rule like Rebbi Eliezer ben Yaakov: where the majority is rainwater and the drawn portion was brought in by way of hamshacha, the mikveh is valid. This is not meant as a ruling for practice, but as an explanation of the various positions among the Tannaim regarding the halachah set out in our mishnah.