THE WIND
Once was the remark pointedly made: "The wind bloweth where it listeth."
And that is nearly true still. The leading winds are under the calculation
of the meteorologist, but the others will not be bound by laws.
Yet there are instruments for measuring the velocity and force of the
wind, after it is on; but "whence it comes" is a different matter. A
gentle air moves at the rate of 7 miles an hour; a hurricane from 80 to
150 miles, pressing with 50 lbs. on the square foot exposed to its fury.
Some of the gusts of the Tay Bridge storm, in 1879, had a velocity of 150
miles an hour, with a pressure of 80 to 90 lbs. to the square foot.
Before steamers supplanted so many sailing vessels, seamen required to be
always on the alert as to the direction and strength of the wind, and the
likelihood of any sudden change; and they chronicled twelve different
strengths from "faint air" to a "storm."
In general, the wind may be considered to be the result of a change of
pressure and temperature in the atmosphere at the same level. The air of a
warmer region, being lighter, ascends, and gives place to a current of
wind from a colder region. These two currents--the higher and the
lower--will continue to blow until there is equilibrium.
The trade winds are regular and constant. These were much followed in the
days of old. A vast amount of air in the tropics gets heated and ascends,
being lighter, and travels to the colder north. A strong current rushes in
from the north to take its place. But the earth rotates round its axis
from west to east, and the combined motions make two slant wind
directions, which are called the "trade winds," because they were so
important in trade navigation.
Among the periodical winds are the "land and sea breezes." During the day,
the land on the sea coast is warmer than the sea; accordingly, the air
over the land becomes heated and ascends, the fine cool breeze from the
sea taking its place. Towards evening there is the equilibrium of
temperature which produces a temporary calm. Soon the earth chills, and
the sea is counterbalancingly warm--as sea-water is steadier as to
temperature than is land--the air over the sea becomes warmer, and
ascends, the current from the land rushing in to take its place. Hence
during the night the wind is reversed, until in the morning again the
equilibrium is restored and there is a calm, so far as these are
concerned. These are therefore called the "land and sea breezes." Of
course, it is within the tropics that these breezes are most marked. By
the assistance of other winds, a hurricane will there occasionally destroy
towns and bring about much damage and loss of life; but better that
hundreds should perish by a hurricane than thousands by the pestilence
which, but for the storm, would have done its dire work.
In countries where the differences of pressure are more marked than the
differences of temperature, in the surrounding regions the strength of
the wind thereby occasioned is far stronger than the land and sea breezes.
The variable winds are more conflicting. These depend on purely local
causes for a time, such as "the nature of the ground, covered with
vegetation or bare; the physical configuration of the surface, level or
mountainous; the vicinity of the sea or lakes, and the passage of storms."
Among these winds are the simoom and sirocco.
The _east_ winds, which one does not care about in the British Islands
during the spring time, are occasioned by the powerful northern current
which rushes south from the northern regions in Europe. Dr. Buchan points
out a very common mistake among even intelligent observers who shudder at
the hard east winds. It is generally held that these winds are damp. They
are unhealthy, but they are dry. It is quite true that many easterly winds
are peculiarly moist; all that precede storms are so far damp and rainy;
and it is owing to this circumstance that, on the east coast of Scotland,
the east winds are searching and carry most of the annual rainfall there.
But all of these moist easterly winds, however, soon turn to some westerly
point. The real east wind, so much feared by invalids, does not turn to
the west; it is exceeding dry. Curious is it that brain diseases, as well
as consumption, reach their height in Britain while east winds prevail.
Once in Edinburgh, during the early spring, I had rheumatic fever, and
during my convalescence my medical adviser, Dr. Menzies, would not let me
have a short drive until the wind changed to the west. The first thing I
anxiously watched in the morning was the flag on the Castle; and for
nearly two months it always waved from the east. How heart-depressing!
Creatures are we in the hands of nature's messengers. We so much depend
upon the weather for our happiness. Joyful are we when the honey-laden
zephyr waves the long grass in June, or when
"The gentle wind, a sweet and passionate wooer,
Kisses the blushing leaf."
Compared with this, how terrible is Shakespeare's allusion to the
appalling aspects of the storm:--
"I have seen tempests, when the scolding winds
Have rived the knotty oaks; and I have seen
The ambitious ocean swell, and rage and foam,
To be exalted with the threat'ning clouds;
But never till to-night, never till now,
Did I go through a tempest dropping fire."