QUESTION: The Gemara describes a Revi'is (which is a measure of liquid volume) in terms of measurements of cubic Etzba'os, or thumb-breadths (which is a measure of length). Rav Shmuel says that a Revi'is is equal to the volume contained within a box that is 2 Etzba'os long, 2 Etzba'os wide, and 1 5/6 Etzba'os high (2 X 2 X 1.833, or 7.33 cubic Etzba'os).

However, Rav Chisda in Pesachim (109a) teaches that a Revi'is is equal to the volume contained within a box that is 2 Etzba'os long, 2 Etzba'os wide, and 2.7 Etzba'os high (2 X 2 X 2.7, or 10.8 cubic Etzba'os; see Insights to Pesachim 109:1). How can these two opinions be reconciled?

ANSWERS: TOSFOS in Pesachim (109a, DH Revi'is) suggests two solutions, both of which are based on the understanding of REBBI ELAZAR HA'KALIR ("ha'Payat"). In the Piyut of Parshas Shekalim, Rebbi Elazar ha'Kalir describes the size of the Revi'is as mentioned in the Yerushalmi, or 2 X 2 X 1 5/6 ("2 minus a sixth"), and he adds that it is "2 X 2 around." That is, the Yerushalmi refers to the volume that is contained in a cylinder. The Bavli, in contrast, refers to the volume that is contained in a cube. How, though, are the two measurements, the volume contained in a cylinder and the volume contained in a cube, equal to each other?

(a) In his first explanation, Tosfos calculates the measurements in two steps. First, Tosfos explains that when the Gemara here describes the length and width as 2 X 2, it refers to a circle that is inscribed within a 2 X 2 square. The Gemara in Eruvin (78b) gives the ratio for the difference between the volume of a cylinder and the volume of a cube as 3:4; that is, a cylinder inscribed within a cube holds 3/4 of the volume that the cube holds.

Second, Tosfos points out that the measurements used in the Yerushalmi are the measures of Tzipori, while the measures used in the Bavli are Midbariyos measures. The measures of Tzipori are larger than the Midbariyos measures by two "Shetusim," or 6/5 X 6/5, which is 36/25 (1 11/25, or 1.44). Based on the fact that the measures of Tzipori are 1.44 times larger than the Midbariyos measures, Tosfos calculates the volume of a cylinder that holds a Revi'is from the volume of a cube as follows: if the cubic volume is 2 X 2 X ~2.7, or 10.8, then the cylindrical volume equals 3/4 X [2(2 X 36/25)(1 5/6 X 36/25)], or 11.4 Etzba'os. Although this amount is slightly larger than the 10.8 cubic Etzba'os of the Bavli, the Yerushalmi gives an inexact measure as a stringency.

(b) In his second explanation, Tosfos says that the Yerushalmi refers to a cylinder that is circumscribed around a cube that has a length and width of 2 X 2 Etzba'os. According to this approach, both the Bavli and Yerushalmi are using Midbariyos measures, but the cylinder that the Yerushalmi discusses is larger than the cube inside of it. To compensate for the increased volume, the height of the cylinder is reduced, as follows.

The Gemara in Eruvin (76b) demonstrates that the difference between the area of a circle and the area of a square inscribed within it (or the difference between the volume of a cylinder and the volume of a cube inside the cylinder) is a ratio of 3:2. The volume of the cylinder is one and a half times greater than the volume of the cube.

This formula enables us to determine how large a cube must be in order for its volume to equal the volume of a given cylinder. If the measurements of the cube inscribed within the cylinder are 2 X 2 X 1 5/6 Etzba'os (the volume of which equals 7.33), then the cylinder around that cube has a volume of 7.33 X 3/2, or 11 Etzba'os. Although this is not exactly the same as the Bavli's measurement of a cube of 2 X 2 X 2.7 (the volume of which equals 10.8), the Yerushalmi gives an inexact measure as a stringency.

OPINIONS: The Mishnah discusses the manner in which the Terumas ha'Lishkah is performed. Two weeks before Pesach, the first group of Shekalim are separated from the Lishkah. After this first Terumah is taken, the remaining coins are covered with mats. When the second Terumah is performed before Shavuos, the remaining coins again are covered with mats. After the third Terumah, however, the remaining coins are not covered with mats.

The Mishnah adds that the reason why the coins are covered after the first and second Terumos is in order to prevent Terumah from being taken a second time from the same coins.

Which coins are covered after the first two Terumos? What does the Mishnah mean when it says that the coins are covered in order to avoid taking Terumah from something from which Terumah was already taken?

(a) Most Rishonim explain that the coins that are covered are the Sheyarei ha'Lishkah, the coins that remain in the Lishkah after the Terumah has been taken from there. The reason the leftover coins are covered is because Terumah may be taken only once from each set of Shekalim that arrive at the Lishkah. After the first Terumah has been taken, new coins arrive and the second Terumah is taken from the new coins. Therefore, a mat is placed on the leftover coins in order to distinguish the old coins from the new coins.

(b) The RAMBAM (Hilchos Shekalim 2:5) has an entirely different understanding of the Mishnah. Although most Rishonim explain that the Shekalim are removed from the Lishkah three times a year and placed into three small Kupos of three Se'ah each (Mishnah 8a), the Rambam explains the procedure differently.

The text of the Rambam's Gemara did not include the seven words that appear in our edition of the Gemara (end of 8a), "Taman Taninan... Kupos." Accordingly, the Rambam understands that the question of the Gemara there, "What is the size of the Kupos," refers to Kupos used for the Terumas ha'Lishkah which are not mentioned explicitly in the Mishnah there. What Kupos are these? The Rambam explains that when the first Terumas ha'Lishkah is taken in Nisan, all of the coins that will be used for the entire year are separated and placed into three very large Kupos. Each of these Kupos holds a volume of nine Se'ah of Shekalim (3 X 3 Se'ah). When the Mishnah states that the Shekalim are separated three times a year, it means that three small Kupos, that each hold three Se'ah, are used to remove coins from the three large Kupos, three times a year.

Similarly, when the Gemara earlier (beginning of 2b) says, "All of the Shekalim would come at once," the Rambam explains that this means that the coins of the Terumas ha'Lishkah are separated from the Lishkah at only one time. The coins that were separated are then removed from the large Kupos that holds them at three times during the year. This second separation is done in order to publicize the Terumas ha'Lishkah.

When the Mishnah here says that the coins are covered, it does not refer to the leftover coins in the Lishkah. Rather, it refers to the coins of the Terumas ha'Lishkah which were separated into the three large Kupos.

The reason why the coins in the large Kupos are covered is in order "not to take [coins] from something from which Terumah has already been taken." What does this mean according to the Rambam, who says that the coins in the Kupos are supposed to be taken?

The Rambam explains that the Mishnah means that it is considered a "privilege" for a Kupah to be the first to have its coins used for the Korbanos, before coins are taken from the other two Kupos. Therefore, in order not to show preference to any particular Kupah, at each of the three times a year that coins are taken from the Kupos to buy Korbanos, the first group of coins is taken from a different Kupah than the one from which they were taken first during the previous period of the year. In order to remember from which Kupah the coins should be taken first at each period, the Kupah from which the coins were taken first at the last period are covered, as well as the Kupah that follows that one. Only the third Kupah is left uncovered, as an indication that the next time coins are separated, the coins should be taken first from this Kupah. This is what the Mishnah means when it says that the reason the Kupos are covered is in order "not to take [coins] from something from which Terumah has already been taken." That is, the Kupos are covered in order to prevent starting from the same Kupah from which the Terumah was started the previous time.

The words of the Gemara (9b) are consistent with this explanation. The Gemara says that the reason why the members of the house of Raban Gamliel would make every effort to ensure that their coins would get into the Kupah was because it was a "Nachas Ru'ach" to them for the Korban to be brought from their coins first. According to all of the other Rishonim, the word "first" is extraneous; the Gemara should have said that they wanted the Korban to be brought from their coins, as opposed to their coins being left in the Sheyarei ha'Lishkah. The word "first" is meaningless in this context. According to the Rambam's explanation, however, the word "first" is very appropriate. The house of Raban Gamliel wanted to get their coins into the first Kupah in order for the Korban to be brought from their coins first.