Mikvaos, Chapter 6, Mishnah 8. This mishnah, together with the one that follows it, returns to a subject we have met earlier in the masechta: iruv mikvaos, the rules governing when two bodies of water lying near one another are treated halachically as a single mikveh. The second half of the mishnah opens a further topic as well, the process known as zeriah, by which a pool may be made fit for use.
The mishnah begins with a general rule: "Metaharin es hamikvaos", one may render a mikveh kosher by means of another mikveh. And in two directions: "ha'elyon min hatachton", the upper pool by means of the lower one, "veharachok min hakarov", and a pool at a distance by means of one that is nearby.
"Keitzad?" The mishnah asks for the case. Picture a man who has two pools of water. At the top of a hill sits a pool containing the full forty se'ah of valid water. Lower down on the slope lies a pool of mayim she'uvim, drawn water, which on its own is unfit.
The Sylon
What does he do? "Meivi sylon", he takes a pipe. The mishnah describes one made of earthenware, "shel cheres", or of lead, "shel aver", though in truth any material will serve. The single requirement is that the pipe have no beis kibul: not one section of it may be shaped in a way that would hold water inside it.
He then "maniach yado o yadav tachtav", places his hand or both hands beneath the mouth of the pipe, blocking it so that the water cannot yet run out, "ad sheyismalei mayim", until the pipe is entirely filled with water from the upper pool. Once the pipe is full from end to end, "umoshcho umashiko", he draws the pipe along and brings it into contact with the lower pool. And now the striking words of the mishnah: "afilu kesa'arah dayo", even a point of contact as fine as a single hair is sufficient. The lower pool of drawn water becomes kosher, because it is mushak, it touches, it is joined by a genuine point of connection to the upper pool of forty se'ah.
A Question of the Mefarshim
The mefarshim raise an obvious difficulty. Throughout the masechta we have learned that the connection between two mikvaos must measure kishefoferes hanod, the width of the spout of a leather flask. How then can a hair's breadth suffice here?
We will present the answer of the Rosh. Our mishnah is speaking of a case where the hashakah, the joining of the two bodies of water, has already been accomplished. Once that has taken place, in order merely to preserve the connection a minimal point of contact is enough. Initially, however, the connection must indeed be kishefoferes hanod.
This approach, it should be noted, does not follow the view found in the Yerushalmi and cited in the Shulchan Aruch and in the poskim, that the hashakah must be temidis, that the connection must remain constantly as it was, nor the view that once the mikveh has been joined it requires no continuing connection at all.
Filling the Lower Pool: Nosan Venotal
The mishnah turns to a second case. "Hayah bo ba'elyon arba'im se'ah", the upper pool at the top of the hill contains forty se'ah, "uvatachton ein klum", while the lower one is completely empty. There is simply a pit in the ground, and its owner wishes to fill it and to arrange that it be a kosher mikveh. Rain is not falling. How is it to be done?
The mishnah answers: "Memalei b'ksaf", he draws water into a bucket, carries it up on his shoulder, and empties it out "venosein la'elyon", into the pool above. He keeps at this, and the water at the top brims over its edge and runs down the slope into the pit below, until forty se'ah have gathered there: "ad sheyeirdu latachton arba'im se'ah".
It is worth noting the way this is understood among the rishonim, who call the process nosan venotal, putting in and taking out. By pouring drawn water into the upper mikveh, he is displacing an equal measure of the valid rainwater already there, and it is that rainwater which spills over into the lower pit. He repeats the procedure again and again. The upper mikveh retains its kashrus throughout, and the lower pit ends up a kosher mikveh as well.
There is a discussion among the poskim whether nosan venotal is effective when one does not have rov mei geshamim, a majority of rainwater. In practice, many mikvaos today are medakdek in this: even when city water is added to the existing mikveh, they take care that the majority of the water, twenty one se'ah, remains rainwater.
The mishnah thus teaches us two very different means of transferring kashrus from one body of water to another: contact, in which the invalid water is joined to the valid water and purified by touching it, and displacement, in which the valid water itself is gradually moved to where it is needed.