A barometer is an instrument for measuring the weight of pressure of the atmosphere, and was invented at Florence by Torricelli, pupil of Galileo, in 1644.

Ordinarily, it is a glass tube 3 ft. long, filled with mercury, and inverted into a vessel also containing mercury, this causing the liquid in the tube to descend a few inches, leaving a vacuum at the top.

The pressure of all points in the same horizontal plane of a liquid being equal, the surface of the mercury, after the inversion of the tube, cannot remain in one plane as when the atmosphere is pressing equally, but must rise when the air gets heavier and fall when the air gets lighter.

Gay-Lussac's barometer is siphon-shaped, with two scales graduating in opposite directions to a zero point; Bunter's is a slight improvement on this.