Ohalos, Chapter 12, Mishnah 3. This chapter is occupied with boards, ledges and similar flat objects that stretch out over a space and thereby form an ohel, a covering that can carry tumah from one place to another. The Mishnah here presents three cases: a board resting on an oven but protruding on only two sides, a tub sunk into the ground with a board laid over it, and that same tub with a ledge of a house projecting above it. In each case the question is the same: does the object standing in the middle serve as a chatzitzah, an interruption, or is everything under the covering treated as a single ohel?
A board that protrudes on only two sides
The Mishnah returns to the picture of a wooden board laid over an oven, but with a change in the board's dimensions. "Neser shenusan al pi tanur yoshan": a flat wooden board was set over the mouth of a tanur yoshan. The word yoshan literally means old, but the intent is not its age. It means that the oven was already used at least once, so it is a finished, fully functional keli. As in the preceding case, the board is sealed tightly over the opening of the oven.
Earlier the discussion involved a broad board, one that reached a tefach past the oven on every side. Our board is different: it is long but narrow. "Yotzei mizeh umizeh tefach": once it is placed across the oven's mouth it overhangs by a tefach at one end and by a tefach at the opposite end, east and west if you like. "Aval lo min hatzedadim": in the two remaining directions, north and south, nothing protrudes whatsoever. A person who comes up to the oven from the north has open air above him until he reaches the oven's own rim, and it is no different for one who comes up from the south. Overhang exists only at the two ends of the board.
The Mishnah now sets up the case. Tumah is found under one of the overhangs; let us place it at the eastern end. "Kelim shebetzad hasheini tehorim": the utensils resting beneath the western overhang stay tahor, because the tumah has nowhere to go. The oven blocks its path, since a clay oven closed off with a tight-fitting seal, mukaf tzamid pasil, arrests tumah rather than passing it along. Might the tumah make its way around the oven by way of the north or the south and reappear at the far overhang? It cannot, for in those two directions there is no covering at all: the board spreads over nothing except the two ends.
"Rabi Yose metamei." Rabi Yose disagrees and rules that the vessels on the far side do contract tumah. In his view this oven, sealed though it is, does not create a separation between the two ends of the board. The two overhangs are viewed as a single ohel, and once tumah is under one part of an ohel it is under all of it.
The betach with a board over it
"Habetach." What does betach mean? The Bartenura takes it to be a small tub, of the sort one bathes in, and he mentions that some texts spell it avtach with an alef. Whichever spelling one adopts, we are speaking of a tub-shaped vessel. Imagine a bathtub: long walls running down either flank, with a near end and a far end where the water goes in. So that the water should not lose its heat, a long board was placed on top. The board lies along the tub's length and reaches a tefach past one end and a tefach past the other, while over the two flanks it does not reach out at all. This tub, moreover, is set permanently into the earth, which means that it cannot become tamei.
The ruling is "eino meivi es hatumah": this covering does not transfer the tumah. Should tumah be lying at the near end of the tub under the board's overhang there, it leaves tahor the vessels at the far end under the overhang on that side, notwithstanding that a single board reaches across both. The tub in between forms a barrier and holds the tumah back. Nor can the tumah take a route around the flanks of the tub, since along the flanks there is no ohel: the board's overhang exists at the two ends alone.
But why should the tub serve as a barrier when, in Rabi Yose's opinion, an oven closed with a tzamid pasil does not? Everything depends on the tub's status as mechubar lekarka, fastened to the ground. Being joined to the earth makes for an unusually strong chatzitzah, placing such an object at an even greater distance from the domain of tumah than a keli whose protection consists only of a tight seal. A tub embedded in the ground therefore does stop tumah from crossing from one end of the board to the other, and on this Rabi Yose does not argue.
The betach with a zis above it
"Hoyah bo zis." Here we have the same tub, but with no board laid across it. Instead, a ledge or beam projects out of the wall of the house directly above the tub, again in order to shield the water and keep it warm. Note the practical difference: a board can simply be lifted off when a person wants to use the tub, but a ledge fixed to the house cannot be removed, so there must remain an opening through which a person can get into the tub. Picture it, then, as covering most of the tub but not all of it. This ledge is wide, and unlike the board of the previous case it does project a tefach beyond each of the two sides of the tub. The back of the tub stands against the wall of the house, and toward the front the ledge stops short so that the tub is accessible.
"Rabi Eliezer omeir eino meivi es hatumah." Tumah situated under the ledge along one flank of the tub leaves the vessels lying along the opposite flank tahor, though a single ledge covers them both, for the tub obviously divides the space into two.
"Rabi Yehoshua omeir ro'im es habetach ke'ilu eino": the tub is treated as though it did not exist. It stands there in fact, but halachah ignores it and it forms no interruption. "Vehaziz ha'elyon meivi es hatumah": the ledge overhead, being attached to the house, conveys the tumah across from the one flank to the other.
This calls for clarification. Where a board was laid over the tub, Rabi Yehoshua concedes that the tub keeps the tumah from moving across. Why then does that identical tub lose its power to divide once the covering is a ledge? Or ask it in reverse: what makes the ledge a stronger ohel than the board, and what weakens the tub as a chatzitzah in the face of a ledge? It comes down to which of the two is more kavua, more firmly established. True, the tub sits in the ground, but it does not form part of the building's structure; from time to time it is pulled out or dismantled. The ledge, by contrast, grows out of the wall and is a piece of the wall. So when a tub that is occasionally taken away stands under an ohel that belongs to the house itself, the ohel prevails over it and the tub does not hold tumah back. The earlier case was not comparable: there the tub can be taken away, and the board resting on it is lifted off with equal ease. The covering enjoyed no superiority over the tub, and therefore the tub in the middle ruptured the ohel that the board created.
The Rambam's understanding of betach
The Rambam, it should be noted, understands betach along completely different lines. In his reading no tub is involved at all: a betach is the sill of a window. When someone puts his head out of a window to glance this way or that, which is really not a proper thing to do, he supports himself on the sill; a sill is given a bit of bracing underneath exactly so that one may rest upon it while gazing out. For this reason, explains the Rambam, it carries the name betach, from the notion of security, since it keeps a person firmly in place while he leans.
Suppose now that a chatichah of a meis is lying beneath such a betach. The house is unaffected, and this is the Mishnah's meaning when it teaches that the betach does not transmit tumah: the tumah rests under the sill, and from that space there is no opening leading inside. So much would have been obvious even without being taught. The novelty lies in what follows.
Picture a window whose sill juts outward, the sort one rests one's arms on or lines with flowerpots, and picture in addition a ledge extending straight out from the building above that window, rather like an awning. The tumah is under the betach, that is, under the sill; remove the sill and the tumah would be positioned directly under the ledge that projects above the window. This is the machlokes Tanna'im: does the ledge above serve as an ohel over the tumah and bring it inside by way of the open window?
According to Rabi Eliezer the answer is no. Granted there is a ledge overhead, but the tumah as it actually lies is underneath the sill, and the sill checks its upward movement. What the higher ledge covers is the sill, not the tumah. Rabi Yehoshua takes the opposite view: the sill lacks the weight needed to hold the tumah down and keep it from ascending to the ledge over the window. Having reached that ledge, the tumah is under an ohel, and since the window is open the tumah passes into the house. Within the Rambam a number of explanations are advanced for why, in Rabi Yehoshua's opinion, the sill fails to serve as a barrier.
Review
The Mishnah began with a board laid over a used oven, protruding a tefach on two opposite sides but flush with the oven in front and in back. The Bartenura describes it as jutting out a tefach to the east and a tefach to the west, with the north and south sides level with the oven; it is easier for us to picture it as running from side to side with no overhang at the front or the back. If tumah is under one projection, the vessels under the opposite projection stay tahor: the oven is sealed tzamid pasil and blocks the tumah, and unlike the earlier case of a board protruding on all four sides, there is no ohel here by which the tumah could travel around. Rabi Yose disagrees and holds that even in this configuration the oven does not stop the tumah, since everything counts as one ohel.
We then studied the betach, which the Bartenura identifies as a tub embedded in the ground with a board resting on it. The board runs the length of the tub, overhanging at its two ends while covering nothing along its flanks. Tumah beneath the overhang at one end does not travel round to the other: the flanks have no ohel above them, and the tub, being mechubar lekarka, holds the tumah back. Now alter the picture: in place of the board there is a ledge coming out of the house, crossing the tub and reaching a tefach past each flank while leaving the two ends uncovered, the far end of the tub abutting the wall. With tumah under the ledge along one flank, Rabi Eliezer holds that the vessels along the opposite flank remain tahor, the betach having divided the space, whereas Rabi Yehoshua holds that the betach divides nothing and the ledge overhead carries the tumah across.
Lastly we presented the Rambam's approach: a betach is not a tub but the sill of a window, named for the security it provides a person leaning out so that he should not fall, chas veshalom. It protrudes a little, and tumah beneath it stays outside the house, for the ohel spread above the tumah gives no passage inward. Where a ledge also extends from the building above the window, the Tanna'im part ways: Rabi Eliezer maintains that the tumah stays put under the betach and the betach restrains it, while Rabi Yehoshua maintains that the zis overhead is the ohel that counts, to the point that the sill below it restrains nothing at all; the tumah climbs to the projection above the window, and the open window lets it into the house.