Ohalos, Chapter 8, Mishnah 4. This chapter sorts objects into categories according to two separate questions: does an object function as an ohel, spreading the tumah of a meis to whatever lies beneath it, and does it function as a chatzitzah, a barrier that stops tumah from rising through an opening into the space above? The two questions are independent, and the Mishnah has already given us objects that do both jobs (they bring the tumah and they block it), and objects that bring the tumah without blocking it. Now the Mishnah presents the reverse combination.
"Eilu chotztzin velo mevi'in": these are the items that serve as a barrier against tumah but do not carry tumah. Four are listed: "maseches perushah, vechavilei hamitah, vehamispelos, vehasariga shebachalonos."
Maseches Perushah: The Warp Threads Spread Out
A word about how cloth is produced will make this case clear. Every woven material rests on two sets of strands. One set, the eirev, runs horizontally from one edge to the other; the second, the shesi, runs vertically along the length of the piece. Weaving means drawing one group through the other, alternately above and below, over and over, until a solid fabric emerges. Examine a sleeve or a collar from up close and the pattern is plain to the naked eye.
The scene is the same one used throughout this chapter: a two storey house, with a hole one tefach square in the floor that divides the upper level from the lower, and a meis lying in the room underneath. Here the hole is covered by shesi strands, drawn taut and waiting for the weaving to begin. Why does the Mishnah choose the lengthwise strands in particular? Because of the way the loom is arranged: they are strung tightly beside one another, so that between no two of them is there a space measuring a full tefach across. Since tumah cannot travel through a gap narrower than a tefach, this array of strands keeps the tumah from reaching the storey above.
Now let us ask the second question. Suppose these same shesi threads, set and ready in the loom but before the weaving has actually begun, are stretched over a meis and over keilim. Do they form an ohel that would draw the tumah across and render those keilim tamei? They do not. Threads running in only one direction are not yet called an ohel. Everything is in place, the loom is ready to go, but nothing has been woven, and until it is woven it is not a covering.
The Tosfos Yom Tov brings a proof for this. The Torah calls the yeri'os of the Mishkan an ohel, and those yeri'os were woven fabric, threads running in both directions, not threads running in one direction alone.
The upshot is that these taut strands laid across the hole accomplish two things at once. As a chatzitzah they keep the upper room and everything in it tahor, and since they are no ohel at all, keilim resting downstairs alongside the meis are not rendered tamei through them.
Chavilei Hamitah: The Ropes of the Bed
The second item on the list also gives us both advantages at once: it does not spread tumah and it does block tumah. Picture a bunk bed with the mattress removed (do not remove it without permission). You will see strips of metal running crosswise, over and under. In the times of the Mishnah the same job was done with ropes, woven in both directions, and the mattress was laid on top of them.
Between those ropes there are small gaps, none of them a full tefach by a tefach, and for that reason the network of ropes blocks tumah from passing through. But if there is tumah below it together with keilim, it does not carry the tumah to those keilim, because in practical terms there is open air between the ropes, and such a surface is not an ohel.
Hamispelos: The Netted Baskets
The third item is hamispelos, baskets fashioned out of woven rope, something like netting. Their usual use was for carrying out zevel, refuse and manure.
Hasariga Shebachalonos: The Window Grilles
The fourth and final item is hasariga shebachalonos. These too are a form of mesh, set into windows as a screen, constructed either from narrow wooden laths or from slender bands of iron, serving much like a gate that keeps a person from crawling through. Picture a coiled fence. Spread such a screen above a meis while a keli lies beneath it, and the tumah is not carried onto the keli, since the screen is riddled with open spaces. All the same it works as a chatzitzah and stops the tumah from rising, because no pesach a full tefach wide remains open, and without a tefach the tumah has no route upward.
See how the two halves of the ruling dovetail in each of these four cases. Because open space runs through the item, it never becomes an ohel and never transmits tumah. Because that space nowhere reaches a full tefach in each direction, the item nonetheless functions as a barrier and holds the tumah back.
A Difficulty Raised by the Meforshim
The meforshim raise a difficulty here. There is an established rule that any item which is mekabel tumah is disqualified from serving as a chatzitzah against tumah. Yet a mishnah in Maseches Keilim rules that warp strands of this sort do receive tumah. If so, how can they block anything at all?
The Tiferes Yisroel suggests that our mishnah is speaking of shesi threads positioned at the stage just before they are actually mounted in the loom, resting on the part where they will be fed in but not yet truly set into it, and at that stage they are not yet mekabel tumah. Others explain that they are attached to the ground, part of a fixed installation, and therefore not mekabel tumah.
Precisely the same objection applies to the woven cords of a bed. Maseches Keilim rules that such cords are susceptible to tumah, which ought to disqualify them from acting as a barrier, and yet our mishnah credits them with blocking. The Tiferes Yisroel and the Mishnah Achrona answer that the cords described here are not tied into the bedstead at all; they have simply been stretched across the gap joining the two storeys.
The mispelos raise the identical question, which sends us back yet again to Maseches Keilim, to that famous twenty fourth chapter that runs through a long list of utensils in sets of three. The pattern there is that a given article may in one form receive tumas midras (an avi hatumah), in another form receive only tumas meis (which, depending on the circumstances, is not always an avi hatumah), and in a third form be tahor mikulam, susceptible to neither. One of the mishnayos there treats these very rope baskets used to cart off zevel: one variety catches tumas midras, since people would sit or recline in it, a second catches tumas meis alone, and a third is tahor mikulam. Our mishnah, then, must be describing the type that receives no tumah whatsoever, since only such a basket could block.
The difficulty is that the passage there seems to indicate that zevel baskets were susceptible in every case, and that it was other sorts of baskets that escaped tumah. The meforshim therefore labor to account for the ones in our mishnah being beyond the reach of tumah: one approach is that a piece of the basket has broken off, so that as a damaged vessel it is tahor; another is that it was fixed into the structure of the house, making it effectively a portion of the wall itself.
Four different objects, then, a spread of warp threads, the ropes of a bed, a netted basket and a window grille, all sharing one profile: gaps too small to let tumah rise, yet gaps enough that they never become a covering that carries tumah to what lies beneath.