692. 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 the 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.
693. Why is a barometer called also a "weather-glass"?
Because changes in the weather are generally preceded by alterations in the atmospheric pressure. But we cannot perceive those changes as they gradually occur; the alteration in the height of the column of mercury, therefore, enables us to know that atmospheric changes are taking place, and, by observation, we are enabled to determine certain rules by which the state of the weather may be foretold with considerable probability.
694. Why are barometers constructed with circular dials, and an index to denote changes?
Because that is a convenient mechanical arrangement, by which the alterations of the relative pressures of the air and the mercury are more clearly denoted than by an inspection of the mercury itself.
"Fair weather cometh out of the north: with God is terrible majesty."—Job xxxvii.