Midrash su Levitico 27:18

The firmament is similar to a lake and above the lake is a covering. From the from the lake heat condensation flows from the covering, and the condensation [as rain] descends to the midst of the salt waters [of the sea] and the rain does not mix itself [with the salt water]. Rabbi Jonah said "do not be amazed, as it is the case that the Jordan River passes through the Sea of Tiberias (the Sea of Galilee) and it does not mix itself with it; this is a miraculous thing to say! A man sifting wheat or chaff in a sieve, the grains have not descended two or three finger-breadths and they have mixed together, but these [raindrops] have traveled and traveled year after year and have not mixed themselves [with salt water]." Rabbi Yudan, son of Rabbi Shimon says "it is because he sends them down by a measured deduction, as it is said "for He draws away (yegara') the drops of water (Job 36:27)". But see how it says "and an abatement shall be made (venigra') from your assessment (Leviticus 27:18)" [showing that the financial use of this root gr' proves that God sends them down in a measured deduction]. The thickness of the earth is equal to the thickness of the firmament, as it is said "He sits above the circle (chug) of the earth (Isaiah 40:22)" [and] "He walked about the circle (chug) of the heavens (Job 22:14)". Since "circle" (chug) occurs in both verses, they are a gezeira shavah [proving the earth and firmament have the same thickness]. Rabbi Acha said in the name of Rabbi Chanina: "This is like a metal plate". Rabbi Joshua, the son of Rabbi Nehemiah, said: "They are about two or three finger-breadths". Rabbi Shimon the son of Pazzi said: "The upper waters are greater than the lower waters by about thirty xestes (pints). "Between the waters above the waters (la-mayim)" [the lamed's gematria is] thirty [thus proving his point that the waters above have thirty extra pints]. The rabbis said "they are half and half".

5) R. Elazar said: It is written here "Then he shall reckon" and elsewhere (Vayikra 27:18) "Then he shall reckon." Just as there, he gives the sowing of a chomer of barley for fifty silver shekels, so, here.

6) (Vayikra 27:18) ("And if after the Yovel year he shall consecrate his field, then the Cohein shall reckon for him the money remaining until the Yovel year, and it shall be deducted from your valuation.") "after the Yovel": (This implies) next to the Yovel (i.e., the first year after the Yovel). Whence do we derive (the same for a year) removed from the Yovel (i.e., the second year, the third year, etc.)? From "And (connoting inclusion) if after the Yovel year he shall consecrate his field." "his field": (See 4 above).