12מִקְוֶה שֶׁיֵּשׁ בּוֹ אַרְבָּעִים סְאָה מְכֻוָּנוֹת וְיָרְדוּ שְׁנַיִם וְטָבְלוּ זֶה אַחַר זֶה אַף עַל פִּי שֶׁרַגְלָיו שֶׁל רִאשׁוֹן נוֹגְעוֹת בַּמַּיִם הָרִאשׁוֹן טָהוֹר וְהַשֵּׁנִי בְּטֻמְאָתוֹ שֶׁהֲרֵי חָסְרוּ הַמַּיִם מֵאַרְבָּעִים סְאָה. הִטְבִּיל בּוֹ תְּחִלָּה סָגוֹס עָבֶה וְכַיּוֹצֵא בּוֹ וְהִגְבִּיהוֹ אִם הָיְתָה מִקְצָת הַסָּגוֹס נוֹגַעַת בַּמַּיִם הֲרֵי הַטּוֹבֵל בָּאַחֲרוֹנָה טָהוֹר שֶׁהֲרֵי הַמַּיִם כֻּלָּן מְעֹרָבִין. הִטְבִּיל בּוֹ אֶת הַמִּטָּה וְכַיּוֹצֵא בָּהּ אַף עַל פִּי שֶׁדָּחַק רַגְלֵי הַמִּטָּה בְּטִיט עָבֶה שֶׁבְּקַרְקַע הַמִּקְוֶה עַד שֶׁצָּפוּ הַמַּיִם עָלֶיהָ לְמַעְלָה הֲרֵי זוֹ טְהוֹרָה שֶׁהֲרֵי לֹא נִטְבְּעוּ בַּטִּיט עַד שֶׁטָּבְלוּ בַּמַּיִם תְּחִלָּה. הִטְבִּיל בּוֹ יוֹרָה גְּדוֹלָה הֲרֵי זוֹ טְמֵאָה כְּשֶׁהָיְתָה מִפְּנֵי שֶׁהַמַּיִם נִתָּזִין וְיוֹצְאִין חוּץ לַמִּקְוֶה וְנִמְצָא חָסֵר מֵאַרְבָּעִים. כֵּיצַד יַעֲשֶׂה. מוֹרִידָהּ דֶּרֶךְ פִּיהָ וְהוֹפְכָהּ בַּמִּקְוֶה וּמַטְבִּילָהּ וּמַעֲלֶה אוֹתָהּ דֶּרֶךְ שׁוּלֶיהָ כְּדֵי שֶׁלֹּא יִהְיוּ הַמַּיִם שֶׁבְּתוֹכָהּ שְׁאוּבִים וְיַחְזְרוּ לַמִּקְוֶה וְיִפְסְלוּהוּ:
When a mikveh contains exactly 40 se'ah of water and two people descended and immersed, one after the other, the first is pure and the second remains impure. The rationale is that some of the water contained in the original 40 se'ah was removed. This stringency applies even in the feet of the first person were still in the water when the second immerses. If one immersed a thick sponge or the like in such a mikveh and lifted it up, as long as part of the sponge is touching the water, one who immerses afterwards is pure. The rationale is that all of the water is considered as intermingled. When one immerses a bed or the like, even though he pushed its legs into the thick mud on the bottom of the mikveh until the water covers it from above, it is pure. The rationale is that it does not become sunk in the water until it becomes immersed in the water first. When one immerses a large pot in a mikveh with a limited amount of water, it is impure, as it was beforehand, because the water will splash out of the mikveh. Thus the mikveh will be less than 40 se'ah. What should he do? He should lower it via its opening, turn it over in the mikveh, immerse it, and lift it up by its base, so that the water inside will not become "drawn," and return to the mikveh and disqualify it.