Matthew Poole and continuators on Genesis 1:7

7. And God made the Firmament , and divided the Waters which were under the Firmament, from the Waters which were above the Firmament , and it was so .

The Firmament here is, Either, 1. The Starry Heaven; so called, not from its solidity, but from its fixed, durable, and in a sort incorruptible, and unchangeable nature. Or, 2. The Air; called here, The expansion, or extension; because it is extended far and wide, even from the Earth to the third Heaven; called also, The Firmament, because it is fixed in its proper place, from whence it cannot be moved, unless by force.

The Waters under the Firmament are Seas, Rivers, Lakes, Fountains and other Waters in the bowels of the Earth. The Waters above the Firmament, or above the Heavens, as they are called, Psal. 148. 4. are either, 1. A Collection or Sea of Waters placed by God above all the visible Heavens, and there reserved for ends known to himself. Or rather, 2. The Waters in the Clouds; for the Clouds are called Waters, Psal. 18. 11. and 104. 3. and are said to be in Heaven, 2 Sam. 21. 10. Matth. 24. 30. and the production thereof is mentioned as an eminent work of Gods Creation, Iob 35. 5. and 36. 29. Psal. 147. 8. Prov. 8. 28. which therefore it is not credible, that Moses in his History of the Creation would omit, which he doth, if they be not here meant; and these are rightly said to be above the Firmament, i. e. The Air, because they are above a considerable part of it.

As God commanded and ordered it, so it was done and setled.