Reference for Zavim 4:1
רַבִּי יְהוֹשֻׁעַ אוֹמֵר, נִדָּה שֶׁיָּשְׁבָה עִם הַטְּהוֹרָה בְּמִטָּה, כִּפָּה שֶׁבְּרֹאשָׁהּ טָמֵא מִדְרָס. יָשְׁבָה בִסְפִינָה, כֵּלִים שֶׁבְּרֹאשׁ הַנֵּס שֶׁבַּסְּפִינָה טְמֵאִין מִדְרָס. נוֹטֶלֶת עֲרֵבָה מְלֵאָה בְגָדִים, בִּזְמַן שֶׁמַּשָּׂאָן כָּבֵד, טְמֵאִין. בִּזְמַן שֶׁמַּשָּׂאָן קַל, טְהוֹרִין. זָב שֶׁהִקִּישׁ עַל כְּצוֹצְרָא, וְנָפַל כִּכָּר שֶׁל תְּרוּמָה, טָהוֹר:
Rabbi Yehoshua says, [If] a <i>niddah</i> [a menstruant woman] sat with a clean woman on a bed, [even] the cap on her [the clean woman's] head are unclean due to [uncleanliness called] <i>midras</i> [treading contact]. [If] she [the menstruant woman] sat in a boat, [even] the vessels that are on top of the mast are unclean due to <i>midras</i>. [If] she took a tub of clothes, if their weight was heavy, they are unclean, [If] their weight was light, they are clean. [If] a <i>zav </i> knocks against a balcony, and [as a result] a loaf of <i>terumah</i> falls, it remains clean.