Ohalos, Chapter 9, Mishnah 10. Throughout this perek the Mishnah has been examining a caveres, a beehive, that sits in an entrance or within a house, asking how tumah moves when such a vessel obstructs the passage in whole or in part. Until this point the beehive was resting on its side, its opening facing either out toward the street or inward toward the house. Now a fresh arrangement is introduced: the vessel is set on its base, rising vertically in the entranceway.
"Hayesa omedes besoch hapessach": the caveres was standing within the doorway. The Tosfos Yom Tov clarifies that we are speaking of a caveres that occupies the doorway completely. There is no gap at the sides through which a person could squeeze in, and it is not possible to climb over it. The doorway is, in effect, sealed shut by the beehive, with no opening left through which anything could be pushed.
Ein Beina U'vein Hamashkof Posei'ach Tefach
The Mishnah goes on to specify that there is no "posei'ach tefach," no opening of a handbreadth, "beina u'vein hamashkof," between the vessel and the mashkof. The term mashkof has come up more than once in our mishnayos; it refers to the lintel, the crossbeam spanning the top of an entrance. We know the word from Parshas Bo (Shemos 12:22), where Klal Yisroel in Mitzrayim were instructed to take a bunch of hyssop, dip it into the blood of the korban Pesach, and "vehigatem el hamashkof," apply it to the beam above the entrance, along with the two doorposts. In our Mishnah as well, then, the mashkof is the upper frame of the entranceway.
Our picture, then, is a caveres standing upright in the doorway, rising so high that less than a tefach of open space remains between its rim and the lintel above it. The meforshim explain how this is arranged: either the doorway is unusually deep, or the caveres has a narrow mouth at the top. Either way, the lintel above must cover the entire mouth of the caveres, and the space between the mouth and the lintel is less than a tefach.
Tumah Inside the Caveres
"Tumah besochah, habayis tahor." Should there be tumah within the beehive, a kezayis from a meis for example, the house is unaffected and remains tahor. The Bartenura explains that the mouth of the vessel is treated as though it were closed off, with the lintel serving as its lid. Our discussion assumes a beehive with no perforations. An earlier point in the perek addressed a beehive that was pierced and stopped up with straw; this one is unpierced entirely. Had it been perforated, the tumah would escape from it into the house, a halachah we have already studied. But being whole, and reaching all the way up to the top of the entrance, the vessel serves as an ohel by itself, retaining the tumah within and barring it from reaching the house.
A question presents itself: did we not establish the rule of derech hatumah latzeis, that tumah must ultimately find an outlet, and that whatever lies along its route of escape contracts tumah? The resolution is that an outlet does exist here, only it does not run through the house. The tumah passes straight out of the entrance into the open air. In the earlier cases the vessel stood within the house itself, so that the tumah's path toward the entrance had to cross the house, and that crossing is what made the house tamei. Here the beehive already occupies the entrance, so the tumah exits directly outward without ever passing through the house.
Tumah in the House
The Mishnah now reverses the arrangement: "tumah babayis, mah shebetochah tamei." When the tumah lies in the house, the contents of the beehive contract tumah. The Mishnah supplies the reason: "she'derech hatumah latzeis," the nature of tumah is to leave, "ve'ein darkah lehikanes," and it is not its nature to enter. Tumah resting in the house has to find its way out, and its way out is by means of the entrance. Whatever occupies that route becomes tamei, and the beehive is planted right there in the passage, so that its contents lie squarely in the path of the departing tumah.
The Bartenura also brings a different girsa, which he actually presents first, according to which the contents of the vessel stay tahor: the tumah does leave by way of the entrance, yet it does not penetrate into the beehive. In the end, however, the Bartenura rules that the reading found in the majority of seforim is "mah shebetochah tamei," since tumah in the house has to depart by way of the entrance, which is exactly where the vessel is positioned.
Summary
This Mishnah discusses a beehive set on its base within an entrance, blocking it completely, with under a tefach of open space separating its rim from the lintel overhead. Tumah located within the vessel leaves the house tahor, because the tumah does have an outlet, and that outlet carries it directly outside rather than into the house. Tumah situated in the house, though, renders the house tamei and the contents of the beehive tamei as well, since in order to leave the house the tumah must make its way through the vessel occupying the entrance.