Zavim, Chapter Four, Mishnah One
1) Rabbi Joshua said: if a menstruant sat in a bed with a clean woman, [even] the cap on her head contracts midras uncleanness.
a) And if she sat in a boat, the vessels on the top of the mast [also] contract midras uncleanness.
2) If there was [on the boat] a tub full of clothes:
a) If their weight was heavy, they become unclean,
b) But if their weight was light, they remain clean.
3) If a zav knocked against a balcony and thereby caused a loaf of terumah to fall down, it remains clean.
Section one: A niddah defiles in the same way that a zav and a zavah do. According to the rabbis in 3:1, when a niddah (or a zav) sits in the same bed (or boat or carriage) as does a pure person she defiles the other person’s clothes because she might have stepped on them. Rabbi Joshua says that even the cap on the other person’s head is defiled, even though the niddah definitely didn’t step on the other woman’s head. Similarly, if she sits in a boat, even the vessels on the top of the mast are unclean, even though she clearly didn’t step on them. Rabbi Joshua rules more strictly than do the other sages.
Section two: This section is also part of Rabbi Joshua’s opinion. I have explained the section according to Albeck. If the clothes were heavy, then there is a possibility that they pushed the boat down on one side and therefore lifted up the niddah who was on the other side. As we learned in 2:4, if a zav (or zavah or niddah) is “hung” by something, the other thing becomes impure.
However, if the tub of clothes is light, they remain pure because they wouldn’t have caused the boat to shift.
Note that Rabbi Judah does not automatically make the clothes impure, as he did in section one. Albeck explains that Rabbi Judah rules strictly only with regard to single pieces of clothing. When it comes to a tub of clothing, he rules more in line with the other sages.
Section three: Although it seems that the zav caused the loaf of terumah to fall and thereby defiled it, the rabbis don’t think that the zav really could have caused the stone balcony to shift. Rather, it must have been some movement in the ground that caused the loaf to fall. Therefore, it remains pure.