1) "TUM'AS MERKAV"
OPINIONS: The Gemara says that even though a Moshav -- an object upon which a Zav sat (or reclined or leaned) -- is Metamei a person's clothing through any kind of contact (Maga), a Merkav is Metamei a person's clothing only when it is carried (Masa).
What is a "Merkav" of a Zav?
(a) RASHI (DH Tafus, and in Vayikra 16:9) explains that a "Merkav" is the handle at the front of a saddle that one holds while he rides. Since he does not actually sit on that part of the saddle, its Tum'ah is less severe than the Tum'ah of Moshav.
Rashi maintains that a Gezeiras ha'Kasuv teaches that even though the handle is connected to, and part of, the saddle, when one touches it, his clothing does not become Tamei. The handle is Metamei clothing only when it is carried. The saddle, in contrast, is Metamei clothing even when touched.
(b) TOSFOS (DH Iy d'Yasiv) and other Rishonim explain that while a Merkav is similar to a Moshav, the difference between them stems from the way a person sits on the object. If the normal way to sit on a particular object is with one's feet together, then it is Metamei with Tum'as Moshav. However, if the normal way to sit on an object is only with one leg on each side of it, then the object is considered a Merkav. Its Tum'ah is less severe because it is used as a seat only in an unsettled manner. The entire saddle is Metamei as a Merkav if it is not common to sit on it "side-saddle."
According to this explanation, when the Beraisa says that an "Ukaf" is a Moshav and a "Tafus" is a Merkav, it does not refer to the handle attached to a saddle. It refers to two different types of saddles. One saddle is an "Ukaf," upon which riders usually sit side-saddle, and the other is a "Tafus," a saddle that riders straddle with their legs.
(The RITVA points out a difficulty in the choice of wording in the Tosefta that is the source for the Gemara here. The Tosefta states that "its Tafus" (i.e., the Tafus of a saddle) is Merkav. This seems to support Rashi's opinion that the Tafus is a part of the saddle. The RITVA explains that, according to Tosfos, the Tosefta does not refer to the handle of a saddle, but to the front part of a saddle which can be detached and used separately. When all parts of the saddle are attached, it can be Metamei as a Moshav. When parts are used while detached, they can be Metamei only as a Merkav.)
2) USING WATER AND SALT FOR AN ERUV
OPINIONS: Rebbi Elazar and Rebbi Yosi bar Chanina argue about a statement in a Beraisa. The Beraisa says that the teaching of the Mishnah (26b) applies only to water alone or to salt alone, but not to water and salt together. According to one opinion, this refers to the statement of the Mishnah concerning an Eruv made with water or salt. When the Mishnah states that water and salt may not be used to make an Eruv, it means water by itself or salt by itself. Water and salt together, however, may be used as an Eruv.
According to the other opinion, the Beraisa refers to the statement of the Mishnah concerning water or salt bought with money of Ma'aser Sheni. When the Mishnah states that water and salt may not be bought with money of Ma'aser Sheni, it means water by itself or salt by itself. Water and salt together, however, may be bought with money of Ma'aser Sheni.
The Gemara concludes that, according to the second opinion, the Beraisa only permits using money of Ma'aser Sheni to buy water and salt that has been mixed with oil. Even though the money is also being used for the water and salt in the mixture, it is permitted because of the principle of "Havla'ah."
Does the first opinion, that the Beraisa's statement refers to an Eruv made with a mixture of water and salt, also require that oil be mixed with the saltwater, or may saltwater without oil be used to make an Eruv?
(a) TOSFOS (27a, DH Aval) and the RITVA assert that oil must be mixed with the saltwater in order to use it for an Eruv. (It is not necessary to mix with it a quantity of oil that would last for two meals, as long as the saltwater itself is enough to last for two meals. Saltwater alone, though, may not be used for an Eruv.)
(b) The RIF, RAMBAM (Hilchos Eruvin 1:8), and the ROSH maintain that oil is not necessary. Since people normally dip foods into saltwater during a meal, saltwater alone suffices for an Eruv. In contrast, when one redeems the money of Ma'aser Sheni, he may not buy saltwater alone (without oil mixed with it), because saltwater is not a "Pri" (produce).
(According to this explanation, when the Gemara says that the Amora who maintains that saltwater may be used for an Eruv also maintains that it may not be used for redeeming the money of Ma'aser Sheni, and the Amora who says that saltwater may be used for Ma'aser Sheni agrees that it may be used for an Eruv, it refers to two different types of saltwater. When discussing Eruv, the Gemara refers to a plain mixture of salt and water, with no oil. When discussing Ma'aser, it refers to salt and water with oil.)