A barometer is a device for measuring atmospheric pressure.The average atmospheric pressure at sea level is 1 atmosphere, which is the pressure that will support a column of mercury 760mm (29.92 inches) high, i.e., about 1Kg/cm2 (14.71 1b./in2 ). This would correspond to the pressure exerted by a column of air about 5 miles (8 Km) high if its density were constant and equal to that at sea level.This explains why the barometric pressure on the top of a mountain is, say, 10,000 ft. (3000m) high, the column of air pressing down at the top is only about 3 miles (assuming constant density).

Actually the atmospheric pressure at any particular altitude is never constant, but varies by relatively small amounts about the average.These variations provide data for weather forecasts. When a rubber suction pad is pressed against a smooth wall, it will remain adhering to the wall. By pressing the pad down flat against the wall surface the air is expelled from the cavity under the pad and a vacuum is formed there.

The air pressure which originally acted in that cavity now no longer thrusts against the inside of the pad. When the latter is released, it will remain in the compressed position because the air pressure acts on the outside only and thus presses the edge of the pad down so firmly that no air will penetrate into the cavity, which therefore remains void of air.

The external air pressure thus keeps the suction pad firmly pressed against the wall. Difference of air pressure is also utilised when we drink a liquid through a straw. Air is sucked out of the straw , and the barometric air pressure, which acts upon the surface of the liquid in the glass, will force the liquid up through the straw.

If a long glass tube, which is sealed at one end and open at the other, is filled with mercury and is then stood upright, with the open end downwards, in a dish containing mercury, then so much mercury will flow out of the tube until a column of mercury not more than 760 mm in height above the mercury surface in the dish remains. The air pressure which acts upon the surface of the mercury in the dish is therefore able to hold up a 760 mm high mercury column.