We come now to Mishnah 9 in Maseches Eruvin, which deals with the law of pouring waste water into a courtyard on Shabbos. The Mishnah rules that "chatzeir shehi pechusah me'arba amos" - a courtyard that does not measure four amos by four amos, which is sixteen square amos - may not have waste water poured into it on Shabbos.
What is the concern underlying the prohibition:
The water may flow out of the courtyard into the public domain, and it will look as though the person deliberately propelled water by his own force into the public domain on Shabbos.
The person may in truth not want the waste water sitting in the courtyard itself, and as a result he will carry it out in his own hands into the public domain in order to empty it out, thereby desecrating Shabbos by carrying.
The solution - a pit that holds two se'ah:
This prohibition applies only if he has not made an 'ukah' for the courtyard - a pit dug to receive the waste water, something like a sewage pit - and provided that it is "machzekes sasayim" - able to hold two se'ah of water.
The reason for this measure: the Sages estimated that on average an ordinary person uses two se'ah of water each day, and therefore this quantity will generally suffice. This measure was fixed as a standard rule, even though in practice some people use more and some use less. Consequently, water that goes beyond this amount is not considered something done knowingly and willingly by the person, as though he wanted to push it out into the public domain.
"Min hanekev ulematah":
The two se'ah are measured from the hole through which the water exits and downward: the pit must be able to contain two se'ah before the water begins to flow out of it. It is not enough that there is a hole through which the water exits into the public domain once it reaches a certain height; if that height is not above the two se'ah mark, we have accomplished nothing. What is required, then, is a receptacle holding two se'ah beneath the point of exit.
The location of the pit:
The pit may be "bein mibachutz bein mibifnim" - one is permitted to make it inside the courtyard or even outside it. That is, even if it is adjacent to the courtyard but stands in the public domain, it is effective, so long as the water is poured directly into it without actually entering the public domain.
There is, however, a distinction between the two: a pit standing outside, in the public domain, requires a covering, so that it not be open in the public domain; whereas one standing inside the courtyard does not require a covering.
In summary: In this Mishnah we learned that in a courtyard that does not measure four amos by four amos it is forbidden to pour waste water on Shabbos, out of concern that it will flow into the public domain as though propelled by the person's own force, or that he will carry it out himself into the public domain. The permission depends on making a pit that holds two se'ah - the average daily use of water - with the measurement taken from the hole and downward, provided that the receptacle beneath the point of exit holds two se'ah. The pit is valid whether inside the courtyard or outside it, except that one standing in the public domain requires a covering, while one standing inside does not.