Talmud for Eruvin 115:16

אמר רב יהודה אמר שמואל לא שנו אלא שאין חוט המשקולת יורד כנגדו

it is pierced in an upward, as well as in a downward direction.<span class="x" onmousemove="('comment',' The method of piercing is described infra 58b.');">²⁹</span> If it<span class="x" onmousemove="('comment',' The measuring line. iht');">³⁰</span> reached a wall we do not say: 'Let the wall be bored through';<span class="x" onmousemove="('comment',' Sc. that poles towering above it shall be held up on both its sides and the rope stretched from one to the other (Tosaf. s.v. a.l.) .');">³¹</span> its thickness rather is estimated and the measuring continues.<span class="x" onmousemove="('comment',' Tosef. 'Er. IV.');">³²</span> Have we not, however, learnt: HE SPANS IT AND RESUMES HIS MEASURING?<span class="x" onmousemove="('comment',' Why then is a mere estimate allowed in this case?');">³³</span> - There<span class="x" onmousemove="('comment',' In our MISHNAH:');">³⁴</span> it is a case of one that can be conveniently used<span class="x" onmousemove="('comment',' One for instance that rises gently to a height of ten handbreadths in all area of four cubits. Hence it must either be spanned or pierced.');">³⁵</span> but here it is a case of one that cannot conveniently be used.<span class="x" onmousemove="('comment',' A wall, for instance, that rose sharply in a perpendicular direction. As its sides are of no use for walking purposes they may be disregarded and only the estimated thickness of the wall need be included in the measurements.');">³⁶</span> Rab Judah citing Samuel stated: This<span class="x" onmousemove="('comment',' That the method of piercing is admissible.');">³⁷</span> was learned only in the case where a plumb line<span class="x" onmousemove="('comment',' Suspended from the edge of the glen and reaching the bed.');">³⁸</span> does not descend in a straight line<span class="x" onmousemove="('comment',' Lit., 'corresponding to it'. This is defined infra 58b.');">³⁹</span>

Jerusalem Talmud Eruvin