Mikvaot, Chapter Two, Mishnah Seven
This mishnah deals with rain water that collected in jars on the roof. Is this rain water considered to be “drawn water” such that it will disqualify the water in the cistern to be used as a mikveh.
1) If one had left wine-jars on the roof to dry and they became filled with water:
2) Rabbi Eliezer says: if it was the season of rain and there was [in the cistern] a little water, one may break the jars; otherwise one may not break them.
3) Rabbi Joshua says: in either case one may break them or tilt them over, but one may not empty [them into the cistern].
Section one: The person left the jars on the roof to dry out so that he could use them for wine. Had he left them there with the purpose of gathering water the collected water would count as drawn water and could not be put into the mikveh.
Another complicating factor is that it’s the rainy season so it looks like he might have left the jars up there intentionally to gather water (note that if it’s not the rainy season, this scenario simply won’t occurin Israel, when it’s not the rainy season, it doesn’t rain at all).
According to Rabbi Eliezer, if there’s a little water in the cistern already, then he may break the jars and let the water flow into the cistern and the mikveh will be valid. This is because the water in the jars is not really drawn water, it just looks like drawn water. Therefore, there is room to be lenient when there is already some water in the cistern. However, if there is no water in the cistern, then he may not do so. As he was in mishnah four, Rabbi Eliezer is stringent when it comes to adding water to a cistern at the outset.
Section three: Rabbi Joshua is more lenient. First of all, there is no difference between letting the water flow in before there is other water there and after there is other water there. In both cases, one may either break the jar or even tilt it over and let the water flow into the cistern. The one thing he may not do is pick up the jars and pour them into the mikveh. Picking up the jars turns the water into drawn water which invalidates a mikveh, unless there are already in there 40 seahs of valid water.