Mikvaos 6:8-9

Mikvaos 6:8

Mikvahs are purified a higher from a lower and a distant from a close, as follows: One brings an earthenware or lead pipe and puts his hand under it until it is filled with water. He then he draws it along until the water of the two mikvahs touch; if it’s even by a hair's breadth, it’s enough. If the higher mikvah contains 40 seah and the lower mikvah is empty, one can carry water on his shoulder and put it in the higher mikvah until 40 seah flow into the lower mikvah.

Mikvaos 6:9

If the wall between two mikvahs has a vertical crack, their waters are considered mixed; if a horizontal crack, they aren’t considered mixed unless there's a hole the size of a waterskin’s spout in one place. Rabbi Yehuda says the opposite (i.e., a horizonal crack combines mikvahs and a vertical crack doesn’t). If the waters of two mikvahs mingle via a breach, they are considered mixed if the height is as thick as a garlic skin and the breadth is the size of a waterskin’s spout.