Eiruvin 5:3-4

Eiruvin 5:3

Let’s say that three cities are arranged in a triangle. If there are 141 1/3 cubits (about 212 feet) between the outer two, the middle one combines them so that people may walk from city to city on Shabbos.

Eiruvin 5:4

The Shabbos boundary is only measured using a rope of fifty cubits in length (about 75 feet). The surveyor may only hold the rope at heart level. If the surveyor’s path is blocked by a valley or a fence, he doesn’t tilt the rope to measure it; rather, he keeps the rope level and measures parallel to the obstacle. He does the same in the case of a hill unless doing so requires him to measure outside the limits of the Shabbos boundary. Rabbi Dostai the son of Rabbi Yannai explained in the name of Rabbi Meir what they do when the hill is too big to measure around: they consider it “pierced” and measure the ground under it using a rope of four cubits (about six feet).