A Hg barometer is a device for measuring atmospheric pressure.

The Hq barometer is prepared by filling a glass tube completely with mercury and inverting it into a reservoir of mercury. The mercury level will fall to a certain height. The height is determined by the weight of the atmosphere pushing on the surface of the mercury in the reservoir.

The weight of the mercury in the glass tube equals the weight of the atmosphere. The height of the mercury is equated to atmospheric pressure. In units of millimeters the height is normally 760 mmHg. In this measurement the height is 743 mmHg.

At the top of a mountain (high altitudes) the weight of the atmosphere is less because at higher altitudes there is less atmosphere above. So the height of the column of mercury is less, the atmospheric pressure is lower.