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 Re