Mikvaot, Chapter Six, Mishnah Eleven
Today’s mishnah deals with a pool of cool water that people were accustomed to immerse in after bathing in the hot waters of the bathhouse. The waters in the bathhouse were definitely drawn water, so one would need to purify after emerging from the hot bath.
1) The purifier’ in the bathhouse: the bottom was full of drawn [water] and the top full of valid [water], if [the space] in front of the hole can contain three logs it is invalid [as a mikveh].
a) How large must the hole be to contain three logs? 1/320th of the pool, the words of Rabbi Yose.
2) But Rabbi Elazar says: even if the bottom [pipe] was full of valid [water] and the top [pipe] full of drawn [water] and by the hole’s side were three logs, [the bath is] valid, for they have only said: “if three logs fell in.”
Section one: There are two pools next to each other, one elevated above the other. The bottom one contains drawn water and the top is full of valid water. The two pools are connected by a hole. If the hole can contain three logs (the amount of drawn water that invalidates a mikveh) then we look at it as if the water from the lower pool has gone into the upper pool and its waters are invalid to use as a mikveh.
If a normal pool has forty seahs, which is equivalent to 960 logs of water (1 seah=24 logs) then three logs are 1/320th of the size of a normal mikveh.
Section two: Rabbi Elazar says that three logs invalidate a mikveh only if they fall in. They do not invalidate a mikveh in a case such as we have here, because in this case there was invalid water standing next to valid water and connected by a hole. This same literal reading of the older tradition was found above in mishnah four.