Reference for Eruvin 116:9

ובלבד שלא יצא חוץ לתחום: מאי טעמא אמר רב כהנא גזירה שמא יאמרו מדת תחומין באה לכאן:

one need only estimate its base and proceed with his measuring. PROVIDED HE DOES NOT GO BEYOND THE SABBATH LIMIT.<span class="x" onmousemove="('comment',' Cf. relevant notes in our Mishnah, and diagram ibid.');">¹⁸</span> What is the reason? - R'Kahana replied: This was ordained as a preventive measure against the possible assumption that the Sabbath limit reached to that point.<span class="x" onmousemove="('comment',' Beyond the permitted limit. In the absence of the preventive measure people might desecrate the Sabbath by walking as far as that point, believing it to be within the Sabbath limit of their town.');">¹⁹</span>

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