Eruvin Chapter 8, Mishnah 10. In the previous Mishnah we learned how a person may pour out his waste water into a courtyard on Shabbos: if the courtyard is smaller than four cubits by four cubits, it is permitted only if there is a pit or hollow there that holds two se'ah - the usual amount of water a person uses over the course of Shabbos. The pit has to be readily available, for otherwise it will look as though he is pouring water into a public domain. So what is required is either a courtyard of four cubits by four cubits in which the water will be absorbed, or alternatively a pit that holds two se'ah.
The View of Rabbi Eliezer ben Yaakov:
Our Mishnah presents a different version of this law: "Rabbi Eliezer ben Yaakov omer" - Rabbi Eliezer ben Yaakov says that when there is a drainage channel, a sort of trough, leading out of the courtyard, and it is arched over and covered for four cubits in the public domain - meaning four cubits by four cubits, which is the measure stated in the previous Mishnah in which water is absorbed - "shofech letocho mayim beShabbos" - he may pour water into it on Shabbos, and we assume the water is absorbed within that area of four by four.
Even though the pouring is done directly into the channel that passes through the public domain, since it is covered for a span of four by four, it is permitted, because it is comparable to pouring into the courtyard itself. This is not a public domain, since the place is covered. That is the view of Rabbi Eliezer ben Yaakov.
The View of the Sages:
"VaChachamim omrim: afilu gag o chatzer me'ah amah" - And the Sages say: even a roof or a courtyard a hundred cubits long, with the channel running along its entire length, a distance over which the water would naturally be absorbed. True, now and then water might make its way out into the public domain, but it does not look like a deliberate act, since normally it is absorbed. Even so - "lo yishpoch al pi habiv" - he may not pour directly into the mouth of the channel, for one who pours directly into it, and in the end the water flows out into the public domain, will appear as though his intention was to pour into the public domain, and the length of the channel makes no difference.
But "shofech migag legag" - he pours from roof to roof - and most commentators have the reading "aval shofcho lagag" (or "el hagag") - a person may pour on Shabbos onto the roof or into the courtyard, that is, outside the mouth of the channel. As long as he does not pour directly into the channel, but rather pours into the courtyard "umayim yordin laviv" - and the water runs down into the channel on its own - it is permitted even according to the Sages.
The Courtyard and the Portico:
The final section of the Mishnah comes to clarify something stated in the previous Mishnah. There we learned that a courtyard of four cubits by four cubits may have waste water poured into it, on the assumption that it will be absorbed within that area. And what is included in that area? On this the Mishnah concludes: "HaChatzer veha'achsadrah mitztarfin le'arba amos" - the courtyard and the portico combine to make up the four cubits. A portico is a roofed area open at its sides and supported by pillars, and it combines with the courtyard for the calculation of the four cubits by four cubits. Once the area of the courtyard and the area of the portico are combined and the required measure is complete, one may pour the water there on Shabbos.
In summary: This Mishnah discusses the law of a drainage channel leading from the courtyard out into the public domain. According to Rabbi Eliezer ben Yaakov, a channel that is arched over for four cubits in the public domain may have water poured into it on Shabbos, since it is covered and is not treated as a public domain. The Sages disagree: even with a roof or courtyard of a hundred cubits, one may not pour into the mouth of the channel itself; rather, he pours onto the roof or into the courtyard and the water runs down into the channel on its own. And in closing we learned that the courtyard and the portico combine with one another for the measure of four cubits by four cubits that permits pouring waste water into a courtyard on Shabbos.