R’ Eliezer ben Yaakov says: If a [drainage] conduit is covered for four cubits in the public domain, we may pour water into it on the Sabbath. But the Sages say: Even if [it runs along] a roof or courtyard [for] a hundred cubits, one may not pour into the opening of the conduit; but he pours from roof to roof and the water descends into the conduit. A courtyard and a colonnade combine to [total] four cubits.