A glass case for a Fortin's barometer is a protective enclosure designed to house and safeguard this precise instrument used for measuring atmospheric pressure. The case is typically made of clear, high-quality glass, allowing for easy visibility of the barometer's scale and the mercury column inside. It serves to protect the sensitive components of the barometer, such as the mercury reservoir and the scale, from dust, contamination, and accidental damage while also preventing evaporation or spillage of mercury.