_Aer_ is the first element in many compound words of Greek origin
meaning air, the air, atmosphere; in this connection it is combined
with _motor_, defined as a machine which transforms the energy of
water, steam, or electricity into mechanical energy—in this instance,
is meant the changing of _the power of moving air or wind_ into
mechanical energy.
_Wind is air put in motion._ There are two ways in which the motion
of the air may arise. It may be considered as an absolute motion of
the air, rarefied by heat and condensed by cold; or it may be only an
apparent motion, caused by the superior velocity of the earth in its
daily revolution.
When any portion of the atmosphere is heated it becomes rarefied, its
specific gravity is diminished, and it consequently rises. The adjacent
portions immediately rush into its place to restore the equilibrium.
This motion produces a current which rushes into the rarefied spot from
all directions. This is what we call wind.
_Meteorology_ is the science which treats of the atmosphere and its
phenomena, particularly of its variations of heat and cold, _of its
winds_, etc.
This is the great division of science to which one has to turn when
searching for the first principles relating to the operation of
_aermotor pumps_. The vast volumes of air which flow “hither and yon”
are controlled by physical laws which act as accurately and unceasingly
as those which control and hold in check the seemingly solid substance
of the earth itself.
NOTE.—The portions north of the rarefied spot produce a north wind,
those to the south produce a south wind, while those to the east and
west in like manner, form currents moving in opposite directions.
At the rare spot, agitated as it is by winds from all directions,
turbulent and boisterous weather, whirlwinds, hurricanes, rain,
thunder and lightning, prevail. This kind of weather occurs most
frequently in the torrid zone, where the heat is greatest. The air,
being more rarefied there than in any other part of the globe, is
lighter, and, consequently, ascends; that about the polar regions is
continually flowing from the poles towards the equator, to restore
the equilibrium; while the air rising from the equator flows in an
upper current towards the poles, so that the polar regions may not be
exhausted.
To sum up all observations, it can be said with truth that the
_sole force_ immediately concerned in causing the movements of the
atmosphere, _is gravitation_.
So far as the prevailing winds are concerned it has been shown that
where pressure is high, that is to say, _where there is a surplus of
air, out of such a region winds blow in all directions_; and, on the
other hand where pressure is low, or where there is a deficiency of
air, _towards such a region_, winds blow from all directions in an
in-moving special course.
This outflow of air currents from a region of air pressure upon a
region of low pressure is reducible to a single principle, as already
stated, viz., the principle of gravitation.
A regular east wind prevails about the equator, caused in part by the
rarefaction of the air produced by the sun in his daily course from
east to west. This wind, combining with that from the poles, causes a
constant north-east wind for about thirty degrees north of the equator,
and a south-east wind at the same distance south of the equator.
From what has now been said, it appears that there is a circulation
in the atmosphere; the air in the lower strata flowing from the poles
towards the equator, and in the upper strata flowing back from the
equator towards the poles. It may be remarked, that the periodical
winds are more regular at sea than on the land; and the reason of this
is, that the land reflects into the atmosphere a much greater quantity
of the sun’s rays than the water, therefore that part of the atmosphere
which is over the land is more heated and rarefied than that which
is over the sea. This occasions the wind to set in upon the land, as
we find it regularly does on the coast of Guinea and other countries
in the torrid zone. There are certain winds, called trade-winds,
the theory of which may be easily explained on the principle of
rarefaction, affected, as it is, by the relative position of the
different parts of the earth with the sun at different seasons of the
year, and at various parts of the day.
A knowledge of the laws by which these winds are controlled is of
importance to the mariner. When the place of the sun with respect to
the different positions of the earth at the different seasons of the
year is understood, it will be seen that they all depend upon the same
principle. The reason that the wind generally subsides at the going
down of the sun is, that the rarefaction of the air, in the particular
spot which produces the wind, diminishes as the sun declines, and,
consequently, the force of the wind abates.
From its importance in practical meteorology _Buys Ballot’s law_ may be
stated in these two convenient forms. (1) Stand with your back to the
wind, and the center of the depression or the place where the barometer
is lowest will be to your left in the northern hemisphere, and to your
right in the southern hemisphere. This is the rule for sailors by which
they are guided to steer with reference to storms. (2) Stand with
the high barometer to your right and the low barometer to your left,
and the wind will blow on your back, these positions in the southern
hemisphere being reversed. It is in this form that the prevailing wind
of any part of the globe may be worked out from the charts.