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).