Fifteen cubits upward— That is, fifteen cubits, or twenty-two feet and a half, upward, or above the highest mountains. It is plain, as words can make it, from this and the foregoing and the subsequent verses, that the deluge was universal, and not, according to the opinion of some, confined to a certain tract of country only.
One of our most celebrated philosophers observes, that the rain of forty days and forty nights will be found to be a very small part of the cause of such a deluge as Moses describes. For supposing it to rain all over the globe as much in each day, as it is now found to do in one of the most rainy counties in England in the whole year, viz. about forty inches of water a day, forty such days could cover the whole earth with but about twenty-two fathom water, which would only drown the low-lands next the sea: but the greater part would escape. Therefore, he says, we may reasonably conclude, that by the opening of the windows of heaven, is meant an extraordinary fall of waters from the heavens, not as rain, but in one great body; as if the firmament, supposed by Moses to sustain a supra-aerial sea, had been broken up, and, at the same time, the ocean did flow in upon the land to cover all with water. See Phil. Trans. abridged, vol. 6: part 2: p. 1. Perhaps ordinary continued rain for forty days and nights would be found adequate to the effect, if this philosopher took in, as he does in the conclusion, the rupture of the great deep, and the union of its waters with those from above.
The reader will find great satisfaction by consulting Saurin's eighth and ninth Dissertations.