Before us is Mishnah 5, the final mishnah in Maseches Shabbos. The mishnah lists a series of activities that are permitted on Shabbos, and it opens with matters of vows.
Annulling vows and releasing vows on Shabbos:
"Mefirin nedarim b'Shabbos" - a husband or a father may annul the vow of his wife or his daughter, but this annulment is possible only on the day he hears the vow. Therefore, if he learned of the vow on Shabbos, he has time only until Shabbos ends, and for that reason annulment was permitted on Shabbos itself.
"V'nish'alin l'nedarim she'hen l'tzorech haShabbos" - releasing a vow before a beis din is possible at any time and is not limited to a particular day, and therefore one turns to it on Shabbos only for the sake of Shabbos itself, that is, for something the person needs on Shabbos. The principle that emerges from here: an act performed in beis din is not performed on Shabbos unless there is a pressing need.
Stopping up a window and measuring:
"Pokekin es hama'or" - the windows had no glass panes, and when they wanted to close a window they would stuff an object into it, such as a vessel, which is not fixed in place in any permanent way. This is permitted on Shabbos.
"U'modedin es hamatlis" - although measuring is forbidden on Shabbos, one may measure a piece of cloth when there is a concern that it became tamei. A cloth smaller than three fingerbreadths by three fingerbreadths is not susceptible to tumah, and therefore even if it touched a dead mouse it does not become tamei, and consequently food that it afterward touched remains tahor as well. Since this makes a practical difference on Shabbos itself, one may measure the cloth to determine whether it is of a size that is susceptible to tumah.
"V'es hamikvah" - similarly, one may measure a mikvah in order to determine whether it is valid.
The incident in the mishnah:
The mishnah brings an incident from which several things are learned. Two houses stood on either side of an alleyway (a narrow passage), and their windows faced one another. Above the passage, in the area between the two windows, a large tub was resting. Had there been tumah in one of the houses - such as a corpse or part of a corpse - the tumah might have passed through the window, underneath that vessel, into the house on the other side of the passage. However, there was an opening in the vessel above, and it was not clear whether this opening allowed the tumah to escape through it rather than continue on to the second house.
At the outset, because they knew that a death was about to occur across the passage, they stopped up the window with an earthenware vessel - which is not susceptible to tumah from its outer side - in order to prevent the tumah from passing through. Now they wanted to know whether the vessel could be removed or not.
In the words of the mishnah: "Ma'aseh bimei aviv shel Rabbi Tzadok u'vimei Abba Shaul ben Batnis" - first they sealed the window with an earthenware jug, which is not susceptible to tumah from the outside, and in this way blocked the entry of tumah into that house. Afterward they tied a small vessel (a makidah) to a reed - a reed that is considered animal food and is not susceptible to tumah - in order to check with it whether the tub had an opening of a handbreadth.
The purpose of the check was to determine whether that tub had an opening of the required size, through which the tumah would, as it were, escape and not pass into the house across the passage, and consequently to know whether the earthenware vessel with which they had stopped up the window and blocked the tumah could be removed.
"U'midivreihen lamadnu she'pokekin u'modedin v'koshrin b'Shabbos":
Pokekin - one may stop up a window with a vessel, as was stated at the beginning of the mishnah.
Modedin - one may measure, since this measuring is done for the sake of a mitzvah: to determine whether the tumah spread or not.
Koshrin - one may tie on Shabbos, just as they tied the makidah to the reed, provided that it is not a knot that raises a concern of the prohibition of tying on Shabbos.
All of these actions were permitted so that these determinations could be made on Shabbos.
Thank you very much for joining us for Maseches Shabbos. We look forward to learning Maseches Eruvin together.