Ohalos, Chapter 8, Mishnah 5. Throughout this chapter the Mishnah has been sorting coverings and overhangs into levels. Certain ones are substantial enough to be reckoned an ohel in the full sense, so that they carry the tumah of a meis over to whatever lies on the far side of them, and they also form a barrier that prevents tumah from continuing upward. Others qualify as an ohel for only one of those two functions. Here we reach the last and lowest group of all: "Eilu lo mevi'in velo choitzetzin", the following do not transmit tumah to what is under them, and neither do they act as a chatzitza to stop tumah.
Seed plants and vegetables still growing in the ground
The list begins with two items, "hazero'im vehayerakos": seed plants and vegetables. Concerning zero'im, the Tiferes Yisroel writes that the reference is to plants grown for the seeds themselves rather than for their foliage, something on the order of sunflower seeds. And the vegetables spoken of here are ones described as "hamchubarim lakarka", still rooted and growing in the soil rather than picked.
To this the Mishnah attaches a qualification, "chutz min hayerakos shemanu", excluding the specific vegetables that were counted in the opening mishnah of our chapter. Those were the irus, the kisin, yarkos chamor and delas yevanis, and they appeared there among the things that both carry tumah and serve as a chatzitza. Their distinguishing feature is that their leaves are exceptionally wide and they survive the entire year round. Because they endure, and because a single leaf can reach the dimensions of a tefach by a tefach, they are viewed as kvu'im, permanently set in place, and weighty enough to be called an ohel.
The produce of our mishnah has neither advantage. Its leaves fall away once summer arrives, so there is nothing lasting about it. A second approach to the contrast: the growths of the earlier mishnah stand firm and do not move, while these vegetables are pushed around by any run-of-the-mill breeze, and whatever is not settled in one spot is no ohel.
The point of "attached to the ground"
Why did the Mishnah go out of its way to speak of produce that is still joined to the earth? The Bartenura answers with a mimah nafshach, a consideration that closes off both possibilities. Take the case of produce already harvested. Perhaps it came into contact with one of the shiva mashkin: then it is fit to become tamei, and nothing that is itself susceptible to tumah can ever function as a chatzitza against tumah. Or perhaps no liquid ever touched it: then it falls under the heading of ochlin tehorin, food that stays tahor. On either branch, no ruling would have been called for. It follows that the Mishnah's subject is produce still planted in the ground, and the teaching is that even in that state it fails to become an ohel, either because the leaves do not last or because the plant is too slight and moves too readily.
Hail, snow, frost, ice and salt
Next in the list: "vechipas habarad", along with "vehasheleg" and "vehakfor, vehaglid vehamelach". The word kipa means a dome, and that is also why a yarmulke carries this name, its shape resembling a miniature dome. Accordingly, kipas habarad would describe hail heaped up into a dome, barad being the same hail we know from makas barad. A second interpretation takes kipa to mean a chunk or a broken-off piece of hail. The Bartenura finds the root of the term in a pasuk that is part of our daily davening, the pasuk which describes how Hakadosh Baruch Hu "mashlich karcho chefitim", sending down His ice, kavyachol, in small crumbs; fragments of ice of that sort are termed kefitim, or kipa. (The same pasuk opens "Hanosein sheleg katzemer".)
Sheleg is snow. For kfor, the Bartenura speaks of water that had been flowing and then hardened from the cold; other meforshim translate it as frost or as dew that froze. Glid means rivers that have iced over, and it shares its root with glida, ice cream, which is likewise a frozen food. Melach is salt. The common thread running through all five is impermanence: sooner or later each of them melts or dissolves, and whatever will not hold the form it presently has cannot be an ohel, not to bring tumah down to what lies beneath it and not to hold tumah back.
Whatever is on the move
The Mishnah proceeds: "vechein hadoleig mimakom lemakom, vehakofeitz mimakom lemakom." One way of reading this is that the first refers to an animal springing across from one spot to another and the second to a human being doing the same. Others take both words as descriptions of a person, the distinction being that doleig is hopping, where one foot remains planted while the other is lifted, whereas kofeitz means that neither foot is touching the ground.
Then "veha'of haporei'ach", a bird on the wing. When a bird passes above a meis it is not an ohel, so it neither carries the tumah along with it nor stands in the way to contain it. The single reason applies to every case in this group: the object never stops moving, and an ohel has to be anchored in one location. Earlier in the chapter we were taught about a bird at rest, one that has settled and is going nowhere, and such a bird does both convey tumah and function as a chatzitza.
Also "vetalis hamenafnefes", a talis flapping in the air. Picture yourself outside on a blustery day, watching a talis balloon upward so that for an instant a canopy takes shape above. It will not bring tumah and it will not stop tumah, since the wind keeps throwing it about and it is not kavua bimkoma, held in a fixed position.
So too "usfinah shehi shata", a ship afloat "al pnei hamayim": no ohel in either direction, because it is forever drifting and never comes to rest anywhere. The meforshim spell out the precise scenario: the tumah is situated below the boat, and the boat does not serve as an ohel to transfer that tumah to keilim lying under it further along. Where a meis and keilim are both inside the vessel, though, the keilim do contract tumah, since as far as its own interior space is concerned the boat is unquestionably an ohel. Its failure is only as a roof over what is underneath it.
Once they are held fast
The Mishnah then adds, "koshar es hasfinah", if a person ties the vessel "bedavar sheyachol la'amod", with something sturdy enough to hold it, for instance a substantial chain, so that it truly remains stationary and stops rocking, it has become an ohel hamiskayeim, a covering that stays put. Were a meis and keilim to be lying below it, it would now pass the tumah along to those keilim.
Alternatively, "kavash osa even", one presses down "al gabei hatalis" with a stone. The image is of a rock pinning one edge of the talis while the wind raises the opposite edge, producing a canopy that holds its shape, and a talis in that state does transmit tumah. According to others, stones are laid on both edges so that the garment cannot swing back and forth, and it is the center that the wind lifts; that raised middle is the ohel that brings the tumah.
Rebi Yosi and the house on a ship
The Mishnah ends with a disagreement: "Rebi Yosi omeir, habayis shebasfinah", a house built on a ship, "eino mevi es hatumah." Consider the scene: a dwelling constructed on the deck of a boat, with a meis and keilim underneath the boat. In Rebi Yosi's view the boat's own disqualification is not the end of it; even the house resting upon the boat fails to bring the tumah of the meis down to the keilim lying below the vessel.
The Bartenura suggests a further way to understand him. A house put up on a ship stands exposed to the powerful winds of open water and therefore cannot hold out against a ru'ach metzuya, a wind of the ordinary sort. On that account, even when the meis and the keilim are together inside that very house, with nothing at all situated under the boat, the tumah still fails to reach the keilim, because a structure that cannot survive a normal wind does not earn the name ohel. The halacha, however, is not in accordance with Rebi Yosi.
A related halacha appears in Maseches Sukkah. There as well we learned that a sukkah put up aboard a ship needs to be able to hold out against a ru'ach metzuya, and there a leniency emerges: it is enough that the sukkah could withstand the average wind found on land, and it is kosher even if it could not last against the gales at sea.
Review
Let us collect the mishnah. These are the items that neither spread tumah nor serve as a chatzitza against it. Seed plants and vegetables while still rooted in the soil, with the exception of those enumerated in the chapter's first mishnah, whose leaves survive the year round (or, in the alternative explanation, which stand firm and do not move) and which for that reason are counted an ohel. Salt as well, together with frozen rivers, frost, snow and chunks of hail, all of which are destined to melt and therefore neither carry tumah nor hold it back. Likewise a person crossing from one place to the next, whether hopping or springing, a bird in flight, a talis whipping in the wind, and a ship riding on the water: not one of them brings tumah and not one of them blocks it, since none of them stays fixed anywhere. With the ship we clarified that the discussion revolves around a meis and keilim positioned beneath the hull, not around what is inside it. Yet as soon as the ship is moored, or the talis is pinned by a stone so that it keeps its shape, it does transmit the tumah, and from the Gemara in Maseches Shabbos we learn that it will also serve as a chatzitza.