Enquire within upon everything by Robert Kemp Philp

974. _Why does a Barometer indicate the Pressure of the Atmosphere?_

Because it consists of a tube containing quicksilver, closed at one end and open at the other, so that the pressure of air upon the open end balances the weight of the column of mercury (quicksilver); and when the pressure of the air upon the open surface of the mercury increases or decreases, the mercury rises or falls in response thereto.