Reference for Mikvaot 6:8
מְטַהֲרִים אֶת הַמִּקְוָאוֹת, הָעֶלְיוֹן מִן הַתַּחְתּוֹן, וְהָרָחוֹק מִן הַקָּרוֹב. כֵּיצַד. מֵבִיא סִלּוֹן שֶׁל חֶרֶס אוֹ שֶׁל אֲבָר, וּמַנִּיחַ יָדוֹ תַּחְתָּיו, עַד שֶׁהוּא מִתְמַלֵּא מַיִם, וּמוֹשְׁכוֹ וּמַשִּׁיקוֹ, אֲפִלּוּ כַשַּׂעֲרָה, דַּיּוֹ. הָיָה בָעֶלְיוֹן אַרְבָּעִים סְאָה וּבַתַּחְתּוֹן אֵין כְּלוּם, מְמַלֵּא בַכָּתֵף וְנוֹתֵן לָעֶלְיוֹן, עַד שֶׁיֵּרְדוּ לַתַּחְתּוֹן אַרְבָּעִים סְאָה:
<i>Mikvaot</i> can be purified [i.e. made valid], a higher one by a lower one, and a distant one by a near one. How so? One brings a pipe of earthenware or lead [and puts one end in the upper <i>mikveh</i>], and puts his hand under [the other end of] it until it fills with water, and he brings it and touches it [to the lower and valid <i>mikveh</i>] even by a hair's width; that is sufficient [to render the upper <i>mikveh</i> valid]. If the upper [<i>mikveh</i>] contained forty <i>se'ah</i> [of valid waters], and the lower one had nothing, one can fill [water] by one's shoulder [i.e. drawn water which one carried over] and add to the upper one, until forty <i>se'ah</i> flow down into the lower one.