Reference for Shabbat 69:7
אמר רב יהודה אמר שמואל ביה"ש דר' יהודה כהנים טובלין בו למאן אילימא לר' יהודה ספקא הוא אלא ביה"ש דר' יהודה לר' יוסי כהנים טובלין בו
'R. Nehemiah said: For as long as it takes a man to walk half a <i>mil</i> from sunset.' R. Hanina said: One who wishes to know R. Nehemiah's period should leave the sun on the top of the Carmel,<span class="x" onmousemove="('comment',' I.e., when the sun is going down and its dying rays illumine the top of the mountain. ');"><sup>12</sup></span> descend, dip in the sea, and reascend, and this is R. Nehemiah's period. R. Hiyya said: One who wishes to see Miriam's well should ascend to the top of the Carmel and gaze, when he will observe a kind of sieve in the sea, and that is Miriam's well. Rab said: A moveable well is clean,<span class="x" onmousemove="('comment',' Its waters cannot become unclean and it is fit for ritual purification (tebillah). ');"><sup>13</sup></span>