Baroscope

noun

1. An instrument used to indicate changes in the pressure of the atmosphere without measuring its absolute weight. See weather-glass and storm-glass.
2. A piece of physical apparatus used to demonstrate the upward pressure of the air.
3. Any instrument showing the changes in the weight of the atmosphere; also, less appropriately, any instrument that indicates or foreshadows changes of the weather, as a deep vial of liquid holding in suspension some substance which rises and falls with atmospheric changes.
4. barometer