The barometer in its simplest form consists of a long inverted vacuum tube, sealed at the upper end. The lower end dips into a cup of mercury. A graduated scale on the side of the tube measures the rise and fall of the mercury. Such an instrument is often used to determine the height of mountains and other high places,

The air becomes thinner or rarified, the higher one goes and the pressure becomes less and less on the mercury in the open cup, so that the mercury in the long tube is made to fall. If the distance the mercury will fall for every 100 ft. of altitude is known, the height of a mountain may easily be ascertained by noticing the height of the mercury column first at the bottom of the mountain and then at the top.