Mishnayos Mikvaos Perek 6 Mishnah 11

המטהרת שבמרחץ (the water pipe/filter of a bath/gutter) – the troughs/gutters (i.e., grooved stones to receive and carry off the overflow of a well/sink) that are made in a bath that channel to them cold water to rinse them off in order to purify themselves after they washed in hot water, and sometimes they are two troughs one on the side of the other, that the one is higher than its neighbor. And there is a hole in the wall that is between both of them that water goes out from this one to that one.

אם יש כנגד הנקב שלשה לוגין – that the hole/perforation is wide and long like the measurement that three LOGS will enter into it, the upper [Mikveh] is invalidated on account of the drawn water that is in the lower one, for it is considered as if the water that corresponds the cavity of the hole are drawn in the middle of the upper [Mikveh], not from the side, and three LOG of drawn water invalidate when they are in the middle, even though they do not invalidate from the side. And the remarkable thing is what it (i.e., the Tanna/teacher of our Mishnah) took that the bottom one is drawn water, and the upper one is fit/kosher water, for it is the manner of water to descend and it is not the manner of water to ascend, but nevertheless, the upper [Mikveh] is invalidated. And all the more so, that it (i.e., the Mikveh) is invalidated when the kosher/fit is to side of the lower one.

כמה יהא בנקב – now it comes to teach us what is the measurement of the hole/incision that three LOG [of water[ came out from it at one time.

אחד משלש מאות ועשרים בבריכה – that three LOG are 1/320th [it needs to say from forty Seah]. How so? The Seah is six KAB, and the KAB is four LOG, it is found that forty SEAH are nine hundred and sixty LOG. Thus three LOG are 1/320th part of a pool, meaning to say, that in a kosher Mikveh of forty Seah, and the Sages estimated the measurement of a Mikveh that holds forty Seah, a cubit by a cubit at the height of three cubits, for the measurement of the hole/incision that holds three LOG is 1/320th from a cubit by a cubit at the height of three cubits that hold forty Seah. And Maimonides wrote, that if the pool was less or more than forty Sea, we calculate the hole/incision at less or more according to this calculation, as for example, if the pool had twenty Seah [of water], the hole must be 1/160th in size.

אפילו התחתון מלאה כשרים והעליונה מלאה שאובין – that it is the manner of water to descend, nevertheless, three LOGS of drawn water that are in the hole do not invalidate the lower [Mikveh].

שלא אמרו אלא שלשה לוגין שנפלו – for the drawn water does not invalidate the Mikveh until it falls itno it. And the Halakha is according to Rabbi Yossi.