Continuing in Chapter 6 of Tractate Bava Metzia, Mishnah 1, there is another matter concerning renting animals for work: the load the animal carries. When discussing this, two dimensions must be taken into account:
The overall weight - added weight places a greater burden on the animal due to the heaviness.
The volume - even if the weight has not changed, a larger and wider load makes it difficult for the animal to move and can even injure it.
Therefore, a renter who deviates from the agreement and loads more weight than agreed upon, or a greater volume than agreed upon, may be held liable if damage is caused to the working animal he rented. This is the subject of the rest of the Mishnah before us.
Deviating in volume only:
The first case in the Mishnah: "Hasocher es hachamor lehavi alav chitim" - the renter agreed to transport a certain amount of wheat, meaning by weight, let us say hundred-kilo sacks on the donkey's back. In practice, "vehevi alav se'orim" - instead of wheat, he transported barley. Barley is less dense than wheat, and therefore those same hundred kilos of barley take up a larger volume, stick out to the sides, and are harder to carry. Since handling this load is more difficult and the risk of injury is greater, and he deviated from the agreement regarding volume - "chayav": if the animal is injured, the renter will bear responsibility.
In the opposite case, if he loaded less than the agreed weight even though he did so with barley, he is exempt. That is, if he had to transport wheat sacks weighing a hundred kilos and instead transported barley sacks weighing ninety-five kilos, even though the volume is slightly larger, since the weight is less, he is exempt.
The second case in the Mishnah is fundamentally identical to the first: if it was agreed to transport "tevuah" - grain, and there is no practical difference whether it involves wheat or barley, "vehevi alav teven" - instead of dense grain he transported straw, which is not dense at all. For that exact same weight, the volume of straw is infinitely larger, and therefore "chayav", "mipnei shehanefach kasheh lamasaoi" - added volume (meaning the physical space the load takes up) makes carrying the load increasingly difficult, and once the renter deviates from the limit, he is liable. Up to this point, the Mishnah dealt with a case where there is no added weight, but only added volume.
Deviating in weight:
What is the law when adding to the weight? Here, the parties agree on a fixed volume. The standard volume that a donkey carries is a lethech. This measurement appears not only in the Mishnah, such as at the beginning of Tractate Sheviis, but even in biblical verses, and it is equal to half a kor, which is fifteen se'ah. Assuming that a kav, which also appears in the Mishnah, is a two-liter bottle, a se'ah is a six-pack of two-liter bottles - twelve liters, and a lethech is fifteen such six-packs, meaning one hundred and eighty liters. This is essentially the volume of a bathtub, and this is the standard amount a donkey carries.
This was the agreed-upon amount: the renter said he would transport a volume of a lethech of wheat, and in practice he transported a volume of a lethech of barley. Since barley is less dense than wheat, that same volume weighs less - instead of a hundred kilos, for example, only ninety-five kilos. Since the volume is identical and the weight is less, he is exempt. However, if he increased the volume, meaning a lethech was agreed upon and he transported more than one lethech, he is liable, as he deviated from the volume limit.
From here the Mishnah seeks to clarify: for someone who deviates from the weight limit, what is the threshold for liability?
The renter assumed he was taking a lethech of wheat, and a lethech of wheat weighs a hundred kilos. There was a little wheat left over in the final amount, and he squeezed it in, and so instead of a hundred kilos the load reached a hundred and one, a hundred and two, or a hundred and three kilos - a slight deviation. At what stage does this addition literally break the camel's back, and from what deviation in the agreed weight does the renter bear responsibility?
Sumchus said in the name of Rabbi Meir that a deviation in the following amounts is sufficient:
"Begamal" - for a camel, one se'ah. Since a lesech is fifteen se'ah, this is an addition of one-fifteenth, about 6.7%. The camel is stronger, and therefore its amount is larger.
"Bachamor" - for a donkey, only three kavin, which are half a se'ah, meaning one-thirtieth of the lesech, about 3.3%, since donkeys are smaller.
The halachah therefore establishes that a deviation of one-thirtieth from the agreed amount is sufficient: if the agreement was for sacks of wheat weighing one hundred kilos and one hundred and four kilos were loaded - more than the permitted amount - the renter already bears responsibility, and will be liable if the animal is injured due to the load.
In summary: In this Mishnah we learned that the renter's liability for damage to an animal is measured in two dimensions - weight and volume. A deviation in volume alone creates liability, as in the case of wheat that was swapped for barley and grain that was swapped for straw, "mipnei shehanefach kasheh lemassa'ui" - because the volume is difficult for the load; whereas a reduction in weight for the same volume exempts him. When dealing with a deviation in weight within an agreed volume of a lesech, Sumchus established in the name of Rabbi Meir the amount for liability: a se'ah for a camel and three kavin for a donkey.