Mikvaot, Chapter Six, Mishnah Eight
Today’s mishnah teaches how one can use one valid mikveh to make valid another mikveh valid.
1) Mikvaot may be made clean [by joining drawn water from] a higher [mikveh to valid water] from a lower [mikveh or drawn water from] a distant [mikveh to valid water] in a [mikveh] near at hand.
2) How so? One brings a pipe of earthenware or of lead and puts his hand beneath it till it is filled with water; then he draws it along till [the two waters] touch even if it be by a hair’s breadth it is sufficient.
3) If in the higher [mikveh] there were forty seahs and nothing in the lower, one may draw water and carry it on the shoulder and place it in the higher [mikveh] till forty seahs have flowed down into the lower [mikveh].
Section one: If there are two mikvaot on the sides of sloped ground, one can make the higher mikveh valid by having it touch (literally, kiss) the lower mikveh. If the two mikvaot are on equal ground, one can similarly make the one valid by having it touch the other mikveh. The mishnah will now demonstrate how.
Section two: He takes some pipe made of either earthenware or lead and puts one end in the higher (invalid) mikveh. Then he closes up the other end of the pipe with his hand and draws it along until the water in the pipe touches the lower mikveh. Even if only a little bit of water touches the lower mikveh, the waters of the upper mikveh are validated.
Section three: In this case there are forty seahs of valid water in the upper mikveh but nothing in the lower mikveh. He can draw more water and carry it to fill up the upper mikveh because once it has forty valid seahs, no amount of drawn water renders it invalid. Then he can use a pipe to let water flow from the upper mikveh to the lower mikveh. In this way he can essentially use drawn water to fill the lower mikveh. Pretty neat!