Mikvaot, Chapter Four, Mishnah Three
Today’s mishnah deals with cases where the water-spout itself is considered a vessel and therefore any water that flows from it would disqualify the mikveh.
1) If one makes a hollow in a water-spout to collect pebbles, its water disqualifies the mikveh;
a) In the case of a wooden [spout] if it holds even a little,
b) But in the case of an earthenware [spout] if it will hold a quarter-log.
c) Rabbi Yose says: also in the case of an earthenware [spout] if it holds even a little: they have spoken of “a quarter-log” only in the case of broken sherds of an earthenware utensil.
2) If the pieces of gravel moved about inside [the hollow], it disqualifies the mikveh.
a) If dirt went down into it and was pressed down, [the mikveh continues to be] valid.
3) If the spout was narrow at each end and wide in the middle, it does not disqualify [the mikveh] invalid, because it had not been made to gather anything in it.
Section one: The hollow was made to collect the pebbles and prevent them from continuing through the spout and clogging it up. However, this makes the water-spout itself into a vessel and the water that flows through it will, or at least might, disqualify the mikveh.
The size of the hollow that will turn it into a vessel depends on the material in which it was made. If it was made of wood, the receptacle can be of any size, even the smallest size, and it causes the wood to be considered a receptacle. Since wood is the more expensive material, it seems that it is easier for wood to be considered a vessel.
If it is of earthenware, it must hold at least a quarter-log. However, Rabbi Yose disagrees and says that even earthenware need hold only a minimal amount. Rabbi Yose acknowledges that there is an old halakhah concerning earthenware and the measurement of a quarter-log. However, this measurement deals with broken earthenware vesselsthey are still considered vessels as long as they can hold a quarter-log. But if the earthenware vessel is unbroken, it need hold only a minimal amount to be considered a vessel.
Section two: The mishnah now deals with the question of when is the hollow considered to be full such that it no longer turns the water-spout into a vessel. If the pieces of gravel are still moving around, then it is not full. However, if dirt has been pressed down into it, then it is considered full and water that flows through the spout will not disqualify the mikveh.
Section three: Although water might gather in such a spout, whose ends are narrow and middle is wide, the middle was not designed to hold water, just to let it pass through. Its water will not disqualify the mikveh.