Eiruvin 56 - 62
In measuring the distance of 2,000 amos which constitute the techum of a city - the distance which one may walk beyond the city limits without the need of an eruv - there are specific requirements for both the nature of the measuring instrument and its length.
The instrument must be a rope made of flax and its length must be fifty amos - no more and no less. As an explanation for the size of the rope, the gemara tells us that if it is shorter it is so flexible that it can easily be stretched by the people holding it at opposite ends and this increases the distance. If it is longer than fifty amos its weight is likely to cause some sagging in the middle and the distance is shortened.
But why not use a metal chain for measuring which would eliminate both the danger of stretching and that of sagging?
The answer is that our Sages saw in the words of a passage in the book of Zecharia (2:5) an indication that measuring in connection with a city must be done with a rope. The prophet Zecharia describes his vision of a man with a rope in his hand who informed him that he was measuring Jerusalem to determine its breadth and length. This was a Heavenly message that the return of the Jews from Babylonian exile to Jerusalem which took place in Zecharia's day would result only in a limited settlement in the city.
Then came an angel, Zecharia continues, to inform him that there will be another redemption and return to Jerusalem which will render measuring the city obsolete. At that time "Jerusalem shall be inhabited like unwalled towns because of the multitude of men and cattle in it."
Will this population explosion which leaves Jerusalem without protective walls endanger the security of the city's inhabitants?
"I, says Hashem, will be to her like a wall of fire all around her and will be the glory in her midst."
Balance of Forces
An interesting problem of violent conflict between two Jewish communities in Eretz Yisrael came before the leader of the generation, Rabbi Yehuda Hanasi (Rebbi). The inhabitants of Geder were a rather rough bunch, and when they became a little inebriated on Shabbos they would attack the residents of the neighboring city Chamsan who came to visit their town.
Rebbi's solution to this problem of violence was to forbid the residents of Chamsan to enter Geder on Shabbos. Since he did not wish to create a complete rift between these two neighboring communities he did permit the residents of Geder to enter Chamsan on Shabbos.
But does this eliminate the danger of violence, asks the gemara, if the spirited visitors from Geder are let loose on their neighbors in Chamsan?
"A dog removed from his habitat will not bark for seven years" is the folk saying applied to explain Rebbi's strategy. Although the fellows from Geder may be aggressive on their own turf, there was little likelihood that they would make trouble away from home.
"What about the residents of Chamsan?" asks the gemara. Won't they take advantage of this weakness of their visiting neighbors to attack them in retribution for the pain they suffered at their hands on earlier visits?
The answer given is a brilliant application of the principle of coexistence based on a balance of forces. The edge which the Gederites had over the Chamsanites in terms of aggressiveness was offset by their being on foreign ground, so that each side was deterred from any action against the other.