Mikvaos, Chapter 5, Mishnah 2. This chapter deals with the laws of a mayan, a natural spring, and how it compares with a mikveh. A mayan enjoys two advantages: it is metaher bezochlin, it purifies even while the water is moving and flowing, and it needs no minimum measure at all. A mikveh, by contrast, purifies only be'eshboren, when the water is gathered and standing, and it requires forty se'ah. A mikveh also carries a disqualification that a mayan does not have: mayim sheuvim, drawn water, water that was collected inside a vessel.
The situation before us is one in which someone diverted a spring so that its stream ran across the exterior walls of utensils. The flow touched nothing but the outer sides and never entered the hollow of the keli, so no receptacle played any role. At first glance, then, sheuvim ought not to be a concern here, since at no point did a utensil take the water in. What, then, is the halachic standing of such water? That is the question the Mishnah answers.
The Case of the Mishnah
The Mishnah begins with the word "he'eviru," he conducted the spring water along, "al gabei keilim," across utensils, meaning across their outer walls and not across any inner cavity, and it adds "al gabei safsal," across a bench as well, once more a surface with no cavity at all, for neither on this utensil nor on this bench was there any hollow in which water could collect.
To this Rebbi Yehudah responds: "harei hu kemoshehayah." In his view the water keeps precisely the status it held before it was diverted. No taint of sheuvim has attached itself to it, and it therefore continues to be kasher bezochlin, effective for purification even as it runs, and it is in no way regarded as drawn water.
A Question From the Tosfos Yom Tov
The Tosfos Yom Tov asks a question here. There exists another disqualification altogether, hava'ah: when water arrives at the mikveh by way of an object that is susceptible to tumah, a davar hamekabel tumah, the water is thereby ruined for the mikveh. Why, then, does water conducted across utensils not fall under that rule and become invalid?
Several answers are offered to this question. The Tosfos Yom Tov himself explains that our Mishnah is speaking of a situation in which the water would in any case have arrived at that same spot even had it not been led over the vessels, and therefore that disqualification does not apply.
Rebbi Yosi's Opinion
Rebbi Yehudah said that nothing about this water has changed. The Mishnah then presents the other view: "Rebbi Yosi omer, harei hu kemikveh." To Rebbi Yosi the water has indeed lost ground. Its privileged status as a mayan is gone, though the level of a mikveh remains to it. Practically, this means it purifies only be'eshboren, with the water still and not while in motion, and forty se'ah are now required.
Not to Immerse While Standing on the Bench
A final condition rounds off the Mishnah: "uvilvad shelo yitbol," provided that he does not immerse himself, "al gabei hasafsal," while positioned on the bench. The Chachamim instituted this safeguard so that nobody perform tevilah while perched upon the outer surface of a utensil. Because tevilah within a keli is pasul, they extended a prohibition even to a case where one merely stands upon a utensil's exterior, upon a bench, or upon anything similar.
Our Mishnah has thus given us two approaches to spring water that was rerouted over the backs of vessels: according to Rebbi Yehudah nothing has changed and it remains a full mayan, while according to Rebbi Yosi it is now no better than a mikveh, needing forty se'ah and still water, together with the rabbinic caution not to immerse while standing upon a vessel.