Mikvaot, Chapter Two, Mishnah Eight
1) A plasterer forgot his lime-tub in a cistern and it became filled with water: if water flowed above it a little, it may be broken; and if not, it may not be broken, the words of Rabbi Eliezer.
2) But Rabbi Joshua says: in either case it may be broken.
Section one: This is a similar situation to yesterday’s mishnah. A plasterer, probably making the cistern, forgot his lime-tub in a cistern. Water then collected in the cistern, entered into the tub and then overflowed. The water in the tub now counts as if it was in the cistern. If he wants to make the cistern into a valid mikveh, he can break the tub and take it out. However, if the water has not overflowed the tub, then the water in it counts as drawn water. Therefore, he can’t break the tub and let the water into the cistern.
Section two: Rabbi Joshua again rules lenientlythe water does not count as drawn water and does not disqualify the mikveh.