An optical instrument or device (now obsolete) for examining the thinness or rarity of air or other gases.
From Greek 'areos' (thin, rare) + 'skopein' (to look at, examine). A rare 18th-19th century scientific instrument term that became obsolete as more precise gas-measuring equipment was developed.
Scientists in the 1700s literally built instruments to 'look at' thinness—it seems quaint now, but they were developing the foundations of modern pneumatics and chemistry!
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