"DeAzil Rotev Mirtach Leh LeCheres VeHadar Cheres Mirtach Leh LeRotev"
I have a difficulty to understand what is the exact definition of Tataa Gavar and Ilaa Gavar. We see that cold object is warmed by the hotter one and vise versa no matter which is lower and which is upper. (Is it a Machloket BiMtsiut?)
Also how can the (lower) Cheres make the (upper) Rotev hotter as in reality its colder or (in time) equal to but never hotter than the Rotev. Besides, the Gemara rules in this point that the upper one overpowers.
Thanks in advance.
(a) The NODA B'YEHUDAH (Mahadura Tinyana 28) discusses your question concerning the Metzi'us of whether the temperature of the top item or the bottom item overrides the other. He first says that the Machlokes might be concerning only items which are Min b'Mino (identical food items, one of which is Mutar and one of which is Asur), and thus it is not possible to determine (by giving it to a gentile to taste) which one transfers its taste to the other. He concludes that the Amora'im held that it is not possible to determine, experimentally, which one overrides the other on a consistent basis, and therefore Rav and Shmuel issued their Halachic rulings to follow in every case.
It should be noted that the question of Tasa'a Gavar or Ila'a Gavar does not mean that the item on top or bottom is being totally cooked with the other item. Rather, it means that there is enough time (a few seconds) when the heat of one item enables a transfer of Ta'am (taste), before both are cooled off.
(b) You are asking that according to the Havah Amina of the Gemara, how can the Cheres, which is heated by the Rotev, become hotter than the Rotev in order to cook it.
The answer, it seems, is that the Cheres is not literally cooking the Rotev. Rather, it means that the hot Rotev becomes absorbed in the Cheres (like the transfer of Ta'am), and *then* that Rotev becomes heated by the Cheres (which is heated by the remaining Rotev), and then that Rotev -- heated by the Cheres -- goes back into the Korban.
However, the Gemara's wording implies that the Cheres actually cooks the Rotev as a result of the heat that it obtains from the Rotev. It could be, then, that the material the Cheres is made of retains its heat longer than the liquid Rotev. Thus, when the Cheres becomes heated by the Rotev, the Rotev cools off quicker than the Cheres, and then the Cheres heats up the Rotev.