baroscope

from The Century Dictionary.

• noun An instrument used to indicate changes in the pressure of the atmosphere without measuring its absolute weight. See weather-glass and storm-glass.
• noun A piece of physical apparatus used to demonstrate the upward pressure of the air.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun barometer