Since the pressure of the air changes from time to time, the height of the mercury will change from day to day, and hour to hour. When the air pressure is heavy, the mercury will tend to be high; when the air pressure is low, the mercury will show a shorter column; and by reading the level of the mercury one can learn the pressure of the atmosphere. If a glass tube and dish of mercury are attached to a board and the dish of mercury is inclosed in a case for protection from moisture and dirt, and further if a scale of inches or centimeters is made on the upper portion of the board, we have a mercurial barometer.
If the barometer is taken to the mountain top, the column of mercury falls gradually during the ascent, showing that as one ascends, the pressure decreases in agreement with the statement. Observations similar to these were made by Torricelli as early as the sixteenth century. Taking a barometric reading consists in measuring the height of the mercury column.
FIG. - A simple barometer.