975. 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.
976. Why docs the Hand of the Weather Dial change its Position when the Column of Mercury rises or falls?
Because a weight which floats upon the open surface of the mercury is attached to a string, having a nearly equal weight at the other extremity; the string is laid over a revolving pivot, to which the hand is fixed, and the friction of the string turns the hand as the mercury rises or falls.
977. Why does Tapping the Face of the Barometer sometimes cause the Hand to Move?
Because the weight on the surface of the mercury frequently leans against the side of the tube, and does not move freely. And, also, the mercury clings to the sides of the tube by capillary attraction; therefore, tapping on the face of the barometer sets the weight free, and overcomes the attraction which impedes the rise or fall of the mercury.
978. Why does the Fall of the Barometer denote the Approach of Rain?