Ohalos, Chapter 17, Mishnah 2. This chapter deals with the beis hapras, the field in which a grave was plowed over. Once the plow has passed through a kever, we must reckon with the possibility that it dragged along crushed bone, and therefore the field is treated as tamei for a stretch of one hundred amos. Our Mishnah teaches two things: what happens when something interrupts the plowing in the middle, and whether a fresh start inside a beis hapras begins the count all over again.
When the plow is jolted, the beis hapras ends
Picture the scene. A man is walking behind his plow, the ox is pulling, and suddenly the whole apparatus lurches and rattles. The ox turns its head as if to ask what happened. What happened is that the plow struck something. That is the case of the Mishnah: "Haya choresh veheti'ach besela o vegader", he was plowing and the plow got caught on a rock or on a stone fence. The Mishnah adds another case: "o sheni'ersam hamachresha", or the plowman himself gave the plow a good shake to knock the earth off it.
Either way, the hundred amos are cut short. The Mishnah rules that the ground carries beis hapras status only "ad sham", only as far as the place where the jolt occurred, and not a step beyond. The logic is straightforward: any fragment of bone riding along on the blade was shaken free precisely there, so from that point onward there is nothing that could still be traveling with the plow.
The Bartenura points out the chiddush in the second case. A deliberate shake by hand is hardly the same as the violent jolt of crashing into a boulder. One might have thought that such a mild shaking would not be relied upon to have dislodged the bone. The Mishnah tells us otherwise: it is good enough. We assume the bone came off, and the beis hapras stops at that spot.
The Bartenura's own wording is worth noticing. He writes that this holds even when the plowman is still far short of the hundred amos, even if he has only gone five or six amos from the kever. The meforshim are medayek from this precise language: five or six amos, yes, but within four amos of the kever the shaking will not help at all. Why not? Because the four amos surrounding a kever have the status of the kever itself, and a jolt of the plow does not remove that.
Does one beis hapras generate another?
The Mishnah now records a dispute. Rebbi Elazar rules that a beis hapras is itself "oseh beis hapras": a field that already bears this status can generate a further such field beyond it.
Here is how he reasons. A man sets his plow down at a place that already falls inside the hundred amos measured out from the kever, and he begins plowing from there. Our entire concern throughout those hundred amos is that a bone may be lying anywhere along them, which means that the very spot where he is now beginning may itself hold a bone. Once he plows forward from there, that bone may be traveling along with him. A fresh count of a hundred amos must therefore be measured from this second starting place, even if that place stood close to the outer limit of the original hundred. Rebbi Elazar's principle, then, is that wherever a plow might have caught up a bone, we must reckon with that bone being carried another hundred amos onward.
Rebbi Yehoshua disagrees. In his view we are never concerned that a bone traveled more than one hundred amos from the original kever. But he does not reject the idea of a second beis hapras entirely; he says it depends on the case: sometimes yes and sometimes no.
"Keitzad?", what is a practical example? A full ma'ana, a furrow's length, is one hundred amos. "Charash chatzi ma'ana", he plowed through the grave and continued for half a ma'ana, fifty amos, and stopped. That stretch is now a beis hapras. He goes off for his lunch break and his daf yomi, and then "vechazar vecharash chetzyah", he comes back and plows the other half. Since his new starting point is still inside the first hundred amos, everything he plows now is included in the beis hapras. "Vechein hatzdadin", and the same applies to the sides: if he plowed fifty amos eastward, then turned and plowed fifty amos southward, "harei zeh oseh beis hapras", that too becomes a beis hapras. In all of these cases the reason is the same: in practice he is plowing outward from a point that is less than a hundred amos from the kever, so he may have picked up a bone there and be carrying it further.
Here, though, is exactly where Rebbi Yehoshua parts ways with Rebbi Elazar. "Charash melo ma'ana", he plowed the full hundred amos, and then did not stop there but continued: "chazar vecharash heimena ulachutz", he went on plowing beyond it. In that case "eino oseh beis hapras", no new beis hapras is created. One might have protested: perhaps the first stretch of plowing carried a bone all the way to the far edge, and now he is pushing it further still. Rebbi Yehoshua answers that it makes no difference. He simply is not choshesh that a bone will be found more than one hundred amos from the kever.
Summary and halachah
To review the Mishnah: a man plowed through a kever, and then, even before completing the hundred amos, his plow was jolted, whether by striking a rock or a fence or by his own shaking of it. At that point the beis hapras comes to an end, because we assume that whatever was on the plow has fallen off. The Bartenura adds that this applies even a mere five or six amos out from the kever, though not within the four amos of the kever, which are like the kever itself.
And in the case of a man who plowed fifty amos, broke off, and later resumed: Rebbi Elazar holds that since the second stretch began inside the original hundred, a full new hundred must be measured from that point, for a bone could have been lying there, and every fresh stretch of plowing is capable of moving it another hundred amos onward. Rebbi Yehoshua holds that a bone is never suspected of having traveled past a hundred amos from the kever where the plowing began, so the beis hapras reaches only to the end of that hundred, and the ground past it is tahor, even though the plowing resumed before those hundred amos had been completed. The Rambam rules in accordance with Rebbi Yehoshua.