Mikvaot, Chapter Six, Mishnah Ten
Today’s mishnah deals with a bathtub that has a pipe leading out of it for drainage. The bathtub itself seems to have been filled up with collected rainwater, such that the water in it could be valid as a mikveh. However, the pipe has a stopper on the outside and putting the stopper into the end of the pipe causes the water in the stopper to be considered drawn water. The question is: does this drawn water invalidate the valid water in the tub?
1) The outlet of a bath-basin: if it is in the center, it renders [the bath] invalid [as a mikveh]; but if it is at the side, it does not render it invalid, because then it is like one mikveh adjoining another mikveh, the words of Rabbi Meir.
2) But the sages say: if the bath- basin can contain a quarter-log of [water] before it reaches the outlet, it is valid; but if not, it is not valid.
3) Rabbi Elazar bar Zadok says: if the outlet can contain any amount of [water], it is invalid.
Section one: According to Rabbi Meir, if the outlet is in the middle of the bathtub, the drawn water in it causes the entire bathtub to be invalid. However, if the outlet is at the end of the tub then the mikveh remains valid because this is considered like a mikveh of drawn water which is invalid, which is next to a mikveh of valid water. As we learned in 2:5, such a mikveh is not invalidated.
Section two: According to the sages, if there is a quarter-log of water in the basin before any water drains out the outlet, then the mikveh is valid for this is the minimum measure from the Torah (as determined by the sages) for a valid mikveh. In such a case, we would have a valid mikveh next to an invalid mikveh. But if not, then the water in the bath-basin cannot count as a mikveh.
Section three: Rabbi Eliezer considers the water in the outlet to be water in a vessel that is in a mikveh, which means that there is drawn water in the mikveh. No matter the size or position of the outlet, the mikveh is invalid.