The reason why : $b a careful collection of many hundreds of reasons for things which, though generally believed, are imperfectly understood. A book of condensed scientific knowledge for the million — John Stuart Mill — John Shaqi
The reason why : $b a careful collection of many hundreds of reasons for things which, though generally believed, are imperfectly understood. A book of condensed scientific knowledge for the million
John Stuart Mill · en
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._
[Verse: "Fair weather cometh out of the north: with God is terrible
majesty."--JOB XXXVII.]
[Illustration: Fig. 20.--BAROMETER.]
[Illustration: Fig. 21.--TUBE OF BAROMETER, WHEEL, AND PULLEY.]
695. _Why does 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_.
[Verse: "Thou visitest the earth, and waterest it: thou greatly
enrichest it with the river of God, which is full of water: thou
preparest them corn, when thou hast so provided for it."--PSALM LXV.]
696. _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 sides 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_.
Fig. 21 illustrates the mechanism at the back of the barometer. A
is a glass tube; between A and E there exists a _vacuum_, caused
by the weight of the mercury pressing downwards. This space
being a vacuum, makes the barometrical column more sensitive, as
there is no internal force to resist or modify the effects of
the external pressure. E represents the height of the column of
mercury; C the open end of the tube; F the weight resting on the
surface of the mercury; P the pivot over which the string passes,
and upon which the hand turns; W the weight which forms the pulley
with the weight F.
697. _Which is the heavier, dry or vaporised air?_
Dry air is _heavier_ than air impregnated with vapours.
698. _Why is dry air heavier than moist air?_
Because of the _extreme tenuity of watery vapours_, the density of
which is _less than that of atmospheric air_.
699. _Why does the fall of the barometer denote the approach of rain?_