Zavim, Chapter Four, Mishnah Two
1) If he knocked against a strong beam, a window-frame, water-pipe, a shelf, even though it was fixed with ropes, or an oven, a flour container, the lower mill-stone, the base of a hand-mill, or the se’ah measure of an olive-grinder, [the loaf remains clean].
2) Rabbi Yose adds: also [if he knocks] against the beam of the bath-keeper, it remains clean.
Section one: In all of these cases there was a loaf of terumah bread sitting on one of these large items. A zav knocked against one of them and the loaf fell down. Since all of these items are set in their place we can attribute the loaf falling down to some force other than the zav. As we learned in yesterday’s mishnah, we can assume that the ground shifted.
Section two: Rabbi Yose adds that the same halakhah applies to the “beam of a bath-keeper.” This is the beam upon which it was customary for the bath-keeper to sit.