Zavim, Chapter 4, Mishnah 5. The rule underlying this mishnah is one we must have clearly in mind before we begin: for an object to become an av hatumah through tumas mishkav umoshav (midras), it must actually bear the zav's weight, and in fact the greater part of it. Mere contact with a zav does something less: it makes the object a rishon letumah. Our mishnah applies this principle to two very different pictures, a pile of garments and a pair of balance pans.
Here is how the mishnah's first case is stated: "Eser talayos zo al gav zo, yashan al gabei ha'elyonah, kulan tmeiyos." A stack of ten garments is arranged in layers, and a zav lies down to sleep on the uppermost garment of the pile. The ruling: all ten are tamei. Why should the bottom garment be affected when the zav never touched it? Because pressure travels. The zav's body weight is not absorbed by the single garment under him; it continues downward through layer after layer, and each layer in turn is holding him up. Every one of the ten therefore counts as an item that bore his weight, and every one becomes an av hatumah through midras. Nor should the word "yashan" mislead us into thinking that sleep is the operative factor. Sitting produces the identical result, since the halachah turns on nothing but whether the object supports the weight of the zav.
From a pile of garments the mishnah moves to an entirely different arrangement: "Hazav bechaf moznayim umishkav umoshav kenegdo." Picture a two-pan scale. In one pan sits the zav himself; facing him, in the other pan, lie objects designated for lying or sitting upon. The outcome here is not fixed, since a balance can tip either way, and the halachah follows whichever pan proves the heavier.
If the zav's pan sinks down, the objects opposite him stay tahor as far as midras is concerned. His weight has come down on his own pan, not on them; they have not carried him at all, and without carrying him there is no midras. They do not escape untouched, however: because they were lifted and held up by the zav, they take on the status of a rishon letumah, exactly as we said at the outset regarding contact with a zav.
Thus the ruling: "Chara hazav, tehorin. Char'u hein, tmeiyin." Should the zav's pan be the one that drops, the items facing him remain tahor; should their pan drop, they are tamei. The second half of that ruling reverses everything. When the load of mishkav and moshav outweighs the zav and sinks, his own pan is lifted off the ground, and what is keeping him aloft is precisely that load. He is being held in the air by them. An object that supports the weight of a zav in that fashion receives tumas midras.
Rebbi Shimon then qualifies the second half of the ruling: "Beyachid, tamei; bimrubin, tahor, she'ein echad nosei es rubo." One object by itself becomes tamei; a quantity of them together stay tahor, for not one among them is bearing the majority of his weight.
His point is a natural extension of the basic rule. When there is one mishkav in the pan opposite the zav and that pan sinks, that single item alone is holding up the zav's weight, the greater part of him rests upon it, and it becomes tamei. But if the pan is filled with many such items, several mishkavos together, and the loaded pan is the one that descends, then although the group as a whole bears the zav, each individual item carries only its share. Since no single one of them carries most of him, Rebbi Shimon holds that they all remain tahor from midras.
It should be noted that the halachah does not follow Rebbi Shimon's ruling at the close of the mishnah.
What emerges from this mishnah is a consistent picture. Weight pressing downward through a pile of cloaks reaches every layer, so every layer is defiled. Weight in a balance defiles only the side that is genuinely holding the zav up, and even then, in Rebbi Shimon's view, only when a single object is doing the holding.