J. Casartelli & Son Humidity Scale

Antique Humidity Percentage Scale by J. Casartelli & Son, Manchester. Used to calculate the relative humidity when wet and dry bulb temperatures are known. Drum inside box rotates to display the RH. Brass and wood construction. English, circa 1870's.

Measures approx 7" long by 1.75" wide by 1.5" high. Missing knob at bottom, otherwise fine condition.