This is a Wallace & Tiernan FA-139-240 altimeter setting barometer, typically used to measure the altimeter setting of aircraft at airfields.  This particular instrument was designed for an elevation of 200 feet above mean sea level, with an adjustment screw to modify that +/-100 feet.  I've opened it up and tweaked it to display sea level pressure at my 720 foot elevation.  The case is 8.5 inches in diameter and is quite heavy at 4.9 pounds.  It works very well.
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Go to Wallace & Tiernan catalog page.

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