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, Psalms 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, Psalms 18:11 Psalms 104:3, and are said to be in heaven, 2 Samuel 21:10 Matthew 24:30, and the production thereof is mentioned as an eminent work of God's creation, Job 35:5 Job 36:29 Psalms 147:8 Proverbs 8:28; which therefore it is not credible that Moses in his history of the creation would admit, 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 settled.