4 “And God said, let there be a firmament in the midst of the waters, and let it divide the waters from the waters. And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament.” Before laying hold of the meaning of Scripture let us try to meet objections from other quarters. We are asked how, if the firmament is a spherical body, as it appears to the eye, its convex circumference can contain the water which flows and circulates in higher regions? What shall we answer? One thing only: because the interior of a body presents a perfect concavity it does not necessarily follow that its exterior surface is spherical and smoothly rounded. Look at the stone vaults of baths, and the structure of buildings of cave form; the dome, which forms the interior, does not prevent the roof from having ordinarily a flat surface. Let these unfortunate men cease, then, from tormenting us and themselves about the impossibility of our retaining water in the higher regions.
Now we must say something about the nature of the firmament, and why it received the order to hold the middle place between the waters. Scripture constantly makes use of the word firmament to express extraordinary strength. “The Lord my firmament and refuge.” “I have strengthened the pillars of it.” “Praise him in the firmament of his power.” The heathen writers thus call a strong body one which is compact and full, to distinguish it from the mathematical body. A mathematical body is a body which exists only in the three dimensions, breadth, depth, and height. A firm body, on the contrary, adds resistance to the dimensions. It is the custom of Scripture to call firmament all that is strong and unyielding. It even uses the word to denote the condensation of the air: He, it says, who strengthens the thunder. Scripture means by the strengthening of the thunder, the strength and resistance of the wind, which, enclosed in the hollows of the clouds, produces the noise of thunder when it breaks through with violence. Here then, according to me, is a firm substance, capable of retaining the fluid and unstable element water; and as, according to the common acceptation, it appears that the firmament owes its origin to water, we must not believe that it resembles frozen water or any other matter produced by the filtration of water; as, for example, rock crystal, which is said to owe its metamorphosis to excessive congelation, or the transparent stone which forms in mines. This pellucid stone, if one finds it in its natural perfection, without cracks inside, or the least spot of corruption, almost rivals the air in clearness. We cannot compare the firmament to one of these substances. To hold such an opinion about celestial bodies would be childish and foolish; and although everything may be in everything, fire in earth, air in water, and of the other elements the one in the other; although none of those which come under our senses are pure and without mixture, either with the element which serves as a medium for it, or with that which is contrary to it; I, nevertheless, dare not affirm that the firmament was formed of one of these simple substances, or of a mixture of them, for I am taught by Scripture not to allow my imagination to wander too far afield. But do not let us forget to remark that, after these divine words “let there be a firmament,” it is not said “and the firmament was made” but, “and God made the firmament, and divided the waters.” Hear, O you deaf! See, O you blind!— who, then, is deaf? He who does not hear this startling voice of the Holy Spirit. Who is blind? He who does not see such clear proofs of the Only begotten. “Let there be a firmament.” It is the voice of the primary and principal Cause. “And God made the firmament.” Here is a witness to the active and creative power of God.
Source: Nine Homilies of Hexaemeron (New Advent)