Ohalos, Chapter 4, Mishnah 3. This chapter follows a large closet, a migdal, as it stands in various positions relative to a house, and asks how tumah inside it, or inside the house, travels. Our Mishnah takes up a case where the migdal is not inside the house and not outside it, but planted squarely in the entrance itself, with its own door facing out toward the street.
"Haya omeid betoch hapessach veniftach lachutz": the migdal is positioned inside the entranceway itself, and the door of the cabinet opens toward the outside. Imagine approaching such a home. At the entrance you meet a cabinet occupying the space, protruding slightly toward you, its door facing you. It would be perfectly natural to take hold of that door assuming you are opening the house, and then discover that you have opened a closet.
Tumah in the cabinet, tumah in the house
"Tumah besocho, habayis tahor": should the tumah be located within the migdal, the house is unaffected and stays tahor. In the version of our Mishnah that stood before the Bartenura, the opposite case is recorded as well: "tumah babayis, mah shebesocho tahor", tumah situated in the dwelling leaves the cabinet's contents tahor.
The Mishnah's explanation is a principle: "shederech hatumah latzeis", tumah moves along the path by which it departs, "ve'ein darkah likaneis", and it will not take a path that obliges it to enter somewhere. Tumah resting in the migdal already has its way out, namely the cabinet's own door, which opens to the street. Tumah resting in the dwelling likewise has its way out: the entranceway, or at least whatever portion of it the cabinet does not block. Neither of them has any cause to force itself into the other space. Consequently the migdal conveys no tumah to the dwelling, and the dwelling conveys no tumah to whatever is stored in the migdal.
The Bartenura supplies two conditions that allow this to work. To begin with, the cavity of the cabinet needs a posei'ach tefach, a cubic tefach of open air. That measurement is what grants it the standing of an ohel in its own right, a space that retains its tumah within. Were the interior narrower than a tefach, nothing would hold the tumah back and it would spill out into the dwelling. Secondly, when we speak of tumah located in the dwelling, our assumption is that it has an alternative route available: either the cabinet fails to fill the entrance completely, so that open space remains around it, or the house is equipped with a second entrance. On that assumption there is no reason for tumah to make its way into the cabinet, notwithstanding that the cabinet occupies the entrance.
A second version of the text
A different girsa exists, and it reverses the second clause. That reading has: "tumah babayis, mah shebesocho tamei", tumah located in the dwelling renders tamei whatever is kept in the cabinet. And the justification offered is that identical formulation: "shederech hatumah latzeis", tumah follows the route of its departure, "ve'ein darkah likaneis."
According to this reading, the cabinet blocks the entrance entirely, leaving no gap at all, and the house has no second doorway. The tumah in the house must get out, and the only route out is through the migdal. Since tumah travels along its route of exit, it passes through the cabinet, and whatever is stored there becomes tamei. In the other direction, though, nothing changes: tumah inside the migdal still does not reach the house, because the cabinet's opening faces the street and the tumah leaves that way.
So the picture is this. Everyone agrees that we are dealing with a migdal standing in the doorway with its opening facing out, and everyone agrees that tumah inside the migdal leaves no effect on the house. Regarding tumah in the house, it depends on the text. One version holds that the contents of the cabinet stay tahor, on the assumption that the tumah has another way out, whether a gap alongside the cabinet or a second entrance. The other version teaches that where the cabinet seals the entrance completely and there is no alternative exit, the tumah does travel through the cabinet on its way out, and what is inside becomes tamei.
The wheel of the migdal
The Mishnah continues with what is practically a separate teaching, though it is still part of Mishnah gimmel. These cabinets were built to be moved. Think of a wheelbarrow: there is a wheel, the muchni, and you tilt the cabinet back onto it and roll it where you need it. The wheel sits at the back of the cabinet, which means that when the cabinet stands in the doorway facing outward, the wheel is on the inside, within the house. And this wheel is large and hollow, and our case is that there is tumah inside the hollow.
"Hayasa muchni shelo meshuchah le'achorav": the wheel of the migdal is drawn back behind it, set back somewhat from the body of the cabinet, but not more than three etzba'os. Hold the picture clearly: the cabinet stands in the doorway with its opening toward the street, exactly as before, and its wheel is inside the house, slightly behind it, within three fingerbreadths.
"Tumah sham keneged hakorais", the tumah sits there in the wheel, at a spot lying directly beneath the ceiling beam of the dwelling, and even so: "habayis tahor." The wheel stands near enough to the cabinet that halachah treats it as one of the cabinet's components, so tumah lodged in it carries the din of tumah lodged inside the migdal. And since the migdal's door faces the street, that tumah never enters the dwelling. We follow its route from the wheel through the cabinet and out to the street, and in truth we need not even trace such a route, since the wheel was reckoned a component of the cabinet from the outset. That is what leaves the house tahor.
Two elements of this clause deserve a word of clarification. First, what is meant by tumah in the wheel? The wheel is hollow, and the tumah is lying in that hollow. Second, what does "keneged hakorais" contribute? That a plumb line raised straight upward from the tumah would strike the dwelling's ceiling. Even in such a position we do not view the tumah as situated beneath the roof of the dwelling at all; its place is within the cabinet, whose opening turns toward the street.
Three conditions
"Bameh devarim amurim": the Mishnah now sets out the requirements for all of this.
"Bizman sheyeish sham posei'ach tefach": the wheel's cavity must hold a cubic tefach, a tefach in length, a tefach in width and a tefach in height. In order that tumah lying in the hollow be treated as tumah lying in the cabinet, the hollow must qualify as an ohel, a space with standing of its own rather than a mere extension of the dwelling. Lacking that tefach, it is no independent entity, and what we have is tumah retzutzah, tumah under pressure, which bursts vertically upward, piercing whatever lies above it and continuing on. Once that tefach of airspace is present, the tumah stays where it is.
This yesod has a familiar practical application, brought here purely as an illustration and not as a psak. A weighty question arises concerning kohanim who fly on an airplane that is transporting an aron. Among the approaches rabbonim have explored is constructing the aron so that it contains a posei'ach tefach, in which case, according to certain shittos, the tumah is bottled up inside the aron; absent such a space, it would ascend straight upward.
"Ve'eina yotzei": the wheel may not be detachable. It has to be fixed permanently to the cabinet. Were it able to slide out and back in at will, the muchni could not be counted as belonging to the migdal at all.
"Vehamigdal ba bemidah": the migdal must be of the substantial dimensions taken for granted throughout, with a capacity of forty se'ah, which places it outside the category of vessels that receive tumah. Forty se'ah is a liquid measure; in dry produce the equivalent comes to two kor. A cabinet of such capacity is not mekabel tumah, and it is exactly that immunity which enables it to function as a chatzitzah, a partition that halts tumah. A smaller one, the Bartenura explains, is simply a kli, it does receive tumah, and a vessel that itself takes on tumah has no power to block it. The Bartenura notes that this holds for the entire perek: every migdal spoken of here is one that is ba bemidah, holding forty se'ah of liquid.
In review
Our Mishnah stationed a migdal within the entrance, its door turned toward the street. Tumah inside the cabinet leaves the dwelling tahor, since tumah exits and does not enter. As for tumah found in the dwelling itself, the answer varies with the girsa. Where the cabinet stops up the entrance entirely and there is neither a gap alongside it nor another door, the dwelling's tumah exits by way of the cabinet and renders its contents tamei. But if space is left beside the cabinet, or the house has a second entrance, the tumah departs there and whatever is in the cabinet remains tahor.
The Mishnah then introduced the wheel, set behind the cabinet on the house's side, no farther back than three etzba'os. Although the wheel is hollow, and although a plumb line drawn from the tumah would place it beneath the dwelling's ceiling beam, the house is nonetheless tahor, for the wheel is reckoned as part of the cabinet, so that its tumah carries the din of tumah inside a cabinet whose opening faces outward, and such tumah does not defile the dwelling. Three requirements underlie all of this: the wheel's cavity must contain a posei'ach tefach, a cubic tefach; the wheel must be fastened permanently, not built to be inserted and taken out; and the cabinet must be ba bemidah, of a size that leaves it not susceptible to tumah, which is precisely what allows it to act as a barrier against tumah.