Genesis 1:6

And God said, let there be a firmament in the midst of
the waters
On which the Spirit of God was sitting and moving, ( Genesis 1:2 ) part of which were formed into clouds, and drawn up into heaven by the force of the body of fire and light already produced; and the other part left on the earth, not yet gathered into one place, as afterwards: between these God ordered a "firmament to be", or an "expanse" ^F22; something stretched out and spread like a curtain, tent, or canopy: and to this all those passages of Scripture refer, which speak of the stretching out of the heavens, as this firmament or expanse is afterwards called; see ( Psalms 104:2 ) ( Isaiah 40:22 ) ( 42:5 ) and by it is meant the air, as it is rendered by the Targum on ( Psalms 19:1 ) we call it the "firmament" from the ^F23 word which the Greek interpreter uses, because it is firm, lasting, and durable: and it has the name of an expanse from its wide extent, it reaching from the earth to the third heaven; the lower and thicker parts of it form the atmosphere in which we breathe; the higher and thinner parts of it, the air in which fowls fly, and the ether or sky in which the sun, moon, and stars are placed; for all these are said to be in the firmament or expanse, ( Genesis 1:17 Genesis 1:20 ) . These are the stories in the heavens the Scriptures speak of, ( Amos 9:6 ) and the air is divided by philosophers into higher, middle, and lower regions: and so the Targum of Jonathan places this firmament or expanse between the extremities of the heaven, and the waters of the ocean. The word in the Syriac language has the sense of binding and compressing ^F24; and so it is used in the Syriac version of ( Luke 6:38 ) and may denote the power of the air when formed in compressing the chaos, and dividing and separating the parts of it; and which it now has in compressing the earth, and the several parts that are in it, and by its compression preserves them and retains them in their proper places ^F25:

and let it divide the waters from the waters;
the waters under it from those above it, as it is explained in the next verse; of which more there.