Keilim, Chapter 18, Mishnah 2. This mishnah carries forward the discussion of the wagon and the measure of forty se'ah. The governing principle is that a vessel large enough to hold forty se'ah of liquid is not susceptible to tum'ah, because once it is filled it can no longer be picked up and carried; forty se'ah of liquid is simply too heavy. Here the Mishnah applies that principle, and several others, to the parts of the wagon: its wheel and its canopy. Some wagon wheels were fastened permanently to the body of the wagon, and some were made to be slipped off and put back on. That single difference turns out to decide four separate halachot.
The removable wheel
The Mishnah begins: "Muchni shelah bizman shehi nishmetet", the wheel of the wagon when it is detachable. In that case four rulings follow.
1. "Ein chibur lah", there is no connection between them. As far as tum'ah is concerned, wagon and wheel are counted as two independent kelim rather than as one unit. Consequently, if the wagon is small enough to be susceptible and it contracts tum'ah, the wheel remains tahor and is unaffected by it. The reverse holds as well: even where the wagon's capacity reaches forty se'ah and it therefore stands outside the laws of tum'ah, the wheel on its own is still liable to receive tum'ah should it touch a source of tum'ah. Each of them is its own separate object.
2. "Ve'einah nimdedet imah", its volume is not included in the wagon's measurement. The empty space inside the wheel does not join the tally of forty se'ah by which the wagon would be rendered immune to tum'ah.
3. "Ve'einah matzelet imah be'ohel hameit", it does not join the wagon in affording protection when the wagon spreads as a tent above a corpse. A wagon whose capacity is forty se'ah or more lies beyond the reach of tum'ah, and for that reason, when such a wagon is drawn above a corpse, it functions as a partition and safeguards whatever lies within it or rests on top of it. A wheel that comes off, however, is a keli in its own right and can receive tum'ah, so it is unable to serve as such a partition. Hence if a utensil resting in the wagon extends out the back and overhangs the wheel, the overhanging portion enjoys no protection, and that utensil remains susceptible to tum'ah.
4. "Ve'ein gorerin otah beShabbat", the wagon may not be dragged on Shabbat, "bizman sheyesh betochah ma'ot", in a case where coins are lying inside it. A keli that holds coins cannot be handled on Shabbat, since coins are muktzeh. And because a wheel that slips off has the status of a keli by itself, coins deposited in it place that wheel off limits for handling, and as a result the wagon may not be pulled either, for pulling it would drag the wheel along with it.
The permanently attached wheel
"Ve'im einah nishmetet", and if the wheel is not detachable, all four rulings are reversed.
1. "Chibur lah", it is connected to the wagon. Regarding tum'ah the two of them form a single object. So if the wagon's capacity falls short of forty se'ah and it receives tum'ah, the wheel is drawn into that tum'ah along with it; and where the wagon is not susceptible at all, the wheel too lies beyond the reach of tum'ah.
2. "Venimdedet imah", it is measured together with the wagon. When we calculate whether the wagon reaches the volume of forty se'ah that exempts it from tum'ah, the space of the wheel is included in the reckoning.
3. "Umatzelet imah be'ohel hameit", it protects together with the wagon when the two of them together spread as a tent above a corpse. A wagon able to hold forty se'ah of liquid, being itself immune to tum'ah, interposes when it travels above a source of tum'ah and shields what is inside it. A wheel that is fixed in place counts as part of that very wagon, and so it interposes as well. Accordingly, if a utensil sticks out beyond the rear of the wagon and hangs above the wheel, and the wheel then travels above a source of tum'ah, whatever is over the wheel is saved from tum'ah.
4. "Vegorerin otah beShabbat", the wagon may be dragged on Shabbat, and this holds "af al pi", even though, "sheyesh betochah ma'ot", there are coins inside it. A wheel that cannot be taken off is no keli at all, only a component of the wagon. The rule is that muktzeh forbids handling a vessel only when it rests on the essential portion of that vessel. Here the coins are sitting in the wheel, which is secondary and batel, nullified, to the wagon, so the wagon's main body may be moved even though the coins end up being transported in the process.
The canopy
"Hakamron shelah bizman shehu kavua, chibur lah", the covering of the wagon, so long as it is permanently affixed and does not come off, is connected to the wagon. The pair of them count as a single keli. Therefore, when the wagon is of a size that can receive tum'ah, contact with a source of tum'ah by the covering or by the wagon renders them both tamei. And when the wagon lies outside the reach of tum'ah, the covering stays beyond it as well.
"Venimdad imah", and it is measured together with the wagon. The airspace enclosed by the canopy is counted toward the forty se'ah, since it is part of the wagon itself.
"Ve'im eino kavua", but if the covering is not permanently affixed and can be lifted off, "eino chibur lah", there is no connection between it and the wagon. They are reckoned as two separate kelim: when tum'ah touches one of them, that one turns tamei while its companion stays tahor. "Ve'eino nimdad imah", nor does its space enter the wagon's measurement; the volume beneath the covering is excluded from the forty se'ah reckoning.
How the enclosed space is measured
Since a fixed canopy contributes its airspace to the forty se'ah, and since a canopy typically covers only part of the wagon rather than its entire length, the Mishnah must tell us how much of that space is counted. "Keitzad modedin otah? Rosh tor": we draw a diagonal line from the upper edge of the walls that the canopy does not cover up to the upper edge of the canopy. Everything beneath that diagonal is reckoned as part of the wagon's volume. Picture that triangle as though it were closed off with panes of glass; the space it encloses joins the measurement.
Rabbi Yehudah's ruling
The Mishnah closes with the view of Rabbi Yehudah, who maintains that even a wagon holding less than forty se'ah of liquid can sometimes stand outside the laws of tum'ah. In his words, "im einah yecholah la'amod", if the wagon lacks the ability to remain standing "bifnei atzmah", by itself, holding up only for as long as the covering is set in place, then "tehorah", it is tahor.