Part 96 (1/2)
975. _Why is a Barometer called also a ”Weather Gla.s.s”?_
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?_
Because it shows that as the air cannot support the full weight of the column of mercury, the atmosphere must be thin with watery vapours.
979. _Why does the Rise of the Barometer denote the Approach of Fine Weather?_
Because the external air, becoming dense, and free from highly elastic vapours, presses with increased force upon the mercury upon which the weight floats; that weight, therefore, sinks in the short tube as the mercury rises in the long one, and in sinking, turns the hand to Change, Fair, &e.
980. _When does the Barometer stand highest?_
When there is a duration of frost, or when north-easterly winds prevail.
981. _Why does the Barometer stand highest at these Times?_
Because the atmosphere is exceedingly dry and dense, and fully balances the weight of the column of mercury.
982. _When does the Barometer stand lowest?_
When a thaw follows a long frost, or when south-west winds prevail.
983. _Why does the Barometer stand lowest at these Times?_
Because much moisture exists in the air, by which it is rendered less dense and heavy. [1]
[Footnote 1: From ”The Reason Why--General Science, containing 1,400 Reasons for things generally believed but imperfectly understood.”
London: Houlston and Sons.]
984. Cheap Fuel