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Smith's Bible Dictionary
Firmament. In Scripture, the word denotes an expanse, a wide extent; for such is the signification of the Hebrew word. The original, therefore, does not convey the sense of solidity, but of stretching, extension; the great arch of expanse over our heads, in which are placed the atmosphere and the clouds, and in which the stars appear to be placed, and are really seen. - Webster.
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Smith, William, Dr. Entry for 'Firmament'. Smith's Bible Dictionary. https://www.studylight.org/dictionaries/eng/sbd/f/firmament.html. 1901.