Zavim, Chapter 4, Mishnah 7, the last mishnah of the fourth chapter. Throughout this chapter we have been weighing how much of a zav's weight has to rest on an object, or how much movement he has to cause, for that object to become tamei. Our mishnah closes the discussion with a series of practical scenarios: a bed, a riding animal, and two kinds of presses.
Four cloaks under the legs of a bed
The mishnah begins: "Hayah yoshev al gabei hamitah" - a zav has seated himself on top of a bed - and beneath the bed's four supports, one under each, lie four cloaks, "ve'arba tleyos", laid out "tachas arba raglei hamitah". The verdict is "tmei'os": every one of them contracts tumah. The mishnah supplies the ground for this, "mipnei she'eino yachol", since the bed is incapable, "la'amod al shalosh", of holding itself up on only three supports: take one away and the whole thing collapses. It follows that each of the four supports genuinely holds up a portion of the zav, so each cloak underneath is regarded as having borne him.
"Rabbi Shimon metaher." In Rabbi Shimon's opinion the cloaks all stay tahor. His argument is that not one of the four supports bears the larger part of the load, so no individual cloak can be said to have carried the zav in any real sense. The accepted ruling rejects this position: all four cloaks are tamei.
Four cloaks under the legs of an animal
"Hayah rocheiv al gabei behemah" - the case now shifts to a zav mounted on an animal that is standing in place, with a cloak, "ve'arba tleyos", spread beneath each of its feet, "tachas arba raglei behemah". Here the ruling is "tehoros", the cloaks remain clean, "mipnei shehi yecholah", because this creature, unlike a bed, is quite capable, "la'amod al shalosh", of holding itself up on three feet. Each single foot is therefore contributing nothing more than help to the other three. Support of that secondary sort, an aid rather than a necessity, is not reckoned as bearing the zav, and the cloaks below keep their taharah.
The mishnah continues: "Haysah talis achas" - suppose one and the same cloak is spread beneath the two forelegs, "tachas shtei yadayim", or beneath the two hind legs, "tachas shtei raglayim", or beneath one foreleg together with one hind leg, "tachas yad varegel". The ruling is "tmei'ah". The animal could not hold itself up on the two remaining feet, so the pair standing on this cloak is in fact supporting the main part of the zav's weight, and that suffices to make it tamei.
"Rabbi Yosei omeir": Rabbi Yosei distinguishes between species. Of a horse he says "hasus metamei veraglav", it conveys tumah by way of its hind feet, so a cloak lying under the hind foot of a horse ridden by a zav becomes tamei. Of a donkey, "vehachamor beyadav", the tumah passes through the forefeet, so with a zav on a donkey it is the cloak under a foreleg that is affected. The basis is the posture of each animal: "shemish'enas hasus al raglav", a horse throws its weight backward onto the hind feet, "umish'enas hachamor al yadav", while a donkey presses forward onto its front feet. Rabbi Yosei's view, however, is not the accepted ruling.
The beam of the olive press
The mishnah turns to an olive press: if a zav sat down "al koras beis habad", on its pressing beam, then "keilim sheba'eikel, tmei'in", the vessels in the eikel become tamei. Rashi takes the eikel to be the woven basket that holds the crushed olives under the beam, from which the last of the oil is forced out. The Rambam pictures something else: a ring like a low wall set around the olives to hold them together on the press bed while the beam presses down upon them. On either reading the logic is the same: the zav puts the whole of his weight onto the beam, and that weight is transmitted onward to whatever lies pressed beneath it.
The washerman's press
"Al hamachbeish shel koveis": now the zav sits on a laundryman's press, the pair of boards that squeeze clothing flat. Here the ruling is "keilim shetachtav, tehorin", the garments held inside stay tahor, since his seat is only at the rim of the apparatus and his full weight never comes down upon the clothing below. "Rabbi Nechemyah metamei" - Rabbi Nechemyah declares them tamei.
Rabbi Nechemyah's reasoning is that even a person perched at the rim, in the course of working the press, does bring his weight to bear upon it. We do not rule in accordance with Rabbi Nechemyah: in the overwhelming run of cases a person does not load his entire weight onto that spot, and the clothing inside therefore stays tahor.
With this the fourth chapter of Zavim comes to a close.