FORCING THE ART
The decade that inaugurated dynamic man-flight had closed without fully
demonstrating the capabilities of such aëroplanes as had been so far
developed. No considerable altitude record had as yet been achieved. No
very long cross-country flight had yet been attempted, though for many
months the New York _World_ had offered $10,000 for the first aërial
voyage from Albany to New York, and the London _Daily Mail_ had long
offered $50,000 for a flight from London to Manchester. The uses of the
aëroplane for scouting by land and sea had not been tested, much less
its probable value in aggressive warfare. Such experiments were for
the immediate future, as also the development of specialized types of
machines for racing, for climbing, for burden bearing, for distance,
for endurance, for landing on water, for rising from water, for
protection of passengers from severe weather. To air men and spectators
alike the future of the art promised to be quite as captivating as the
past.
The first startling achievements to usher in the new decade were the
great altitude flights. New world records followed in rapid succession
all through the year 1910, with marked persistence and wonderful
progress. Levels that had been regarded as the peculiar region of motor
balloons were passed one after another, until the aviators vanished
beyond the clouds, their limbs palsied with cold, and their aëroplane
wings whitened with frost. Though the greatest prizes were not offered
for this species of flight, and frequently none at all, it had an
abiding fascination for both the flyers and the public. At the same
time it proved to be as safe as it was theatrical and popular.
The starter in this exciting race for cloudland was Hubert Latham,
already the official holder of the world’s altitude record. At Bouy,
on January 7th, in presence of official witnesses, he rose in his
_Antoinette_ monoplane, describing a great upward spiral till his
barometer recorded 1,050 meters; then returned to earth with like ease
and precision, landing softly near his hangar, before his assistants,
transported with enthusiasm. He had touched the goal of Gallic
ambition, having driven his aëroplane to the height of one kilometer.
Latham’s tenure of the world’s altitude record quickly passed to
his doughty rival, Louis Paulhan. At Los Angeles, on the twelfth of
January, Paulhan, mounted on a Farman biplane, ascended 4,165 feet, as
against Latham’s record of 3,444 feet. This was a great step upward,
due not only to Paulhan’s prowess and dexterity, but also to the
science and constructive skill of the less spectacular gentlemen in the
designing room, workshop and laboratory.
Latham strove again for the world’s altitude record and gained it on
July 7th at the second Rheims tournament, by driving his _Antoinette_
to a height of 4,541 feet.[53] But again his victory was soon eclipsed;
for two days later, Walter Brookins at Atlantic City ascended 6,175
feet in a Wright biplane. An American was thus the first to fly above
one mile, as a Frenchman had been first to pass the 1-kilometer limit.
The 2-kilometer and 2-mile elevations were exceeded before the close of
the year, as shown by the following table, which also manifests a fair
distribution of honors among various nations and types of machines:
--------+------------+------------+---------------+------------
Feet | Aviator | Aëroplane | Place | Date
--------+------------+------------+---------------+------------
3,445 | Latham | Antoinette | Betheny Plain | January 7
4,165 | Paulhan | Farman | Los Angeles | January 12
4,541 | Latham | Antoinette | Rheims | July 7
6,175 | Brookins | Wright | Atlantic City | July 9
6,604 | Drexel | Blériot | Lanark, Sc. | August 11
8,271 | Morane | Blériot | Havre, France | September 3
8,406 | Chavez | Blériot | Issy | September 8
9,104 | Wijnmalen | Farman | Mourmelon | October 1
9,714 | Johnstone | Wright | Belmont Park | October 31
10,499 | Leganeaux | Blériot | Pau | December 9
11,474 | [54]Hoxsey | Wright | Los Angeles | December 26
--------+------------+------------+---------------+------------
Such lofty flights have proved a severe test of both the aëroplane and
the pilot. In the lighter atmosphere the engine must turn the propeller
at higher speed to secure the same thrust, and the aëroplane must sail
faster to support the same weight as at the lower levels. Thus more
power is required on high, though the explosive medium, being less
dense, is less capable of exerting power. The driver has, therefore, to
jockey his machine with assiduous care and alertness, at a time when he
is least fitted for exertion, owing to fatigue, cold, and it may be,
physical discomfort due to the great change of atmospheric pressure.
But still, both aëroplane and pilot are capable of ascending well above
any levels thus far attained.
After the triumphant altitude flights of 1910 the aëronautical
skeptics could no longer contend that the aëroplane was useless in
transportation and warfare, because of its inability to fly above
high land or the usual range of the guns of battleships and coast
fortifications. Most of the important mountain passes lie below 10,000
feet. The safe elevation for motor balloons menaced by terrene guns is
taken to be much less than two miles, and in military practice they
usually operate below the one-mile level. The aëroplanes, therefore,
may not only cross mountain ranges, but may also scrutinize, or
grievously molest, land forces, marine squadrons and perhaps even the
great gaseous cruisers of the atmosphere, which they can far outspeed,
and may even destroy.
The increase in speed of flight during 1910 was also quite remarkable.
The official record by which Mr. Curtiss won the Bennett Aviation
Contest at Rheims, in 1909, showed a speed of 47.04 miles an hour.
Still higher velocities, ranging from 50 to 60 miles an hour, were
reported later in that season from England and France. In 1910,
however, at the Rheims aviation meet, Morane, with a Blériot monoplane,
covered the 20-kilometer course in 12 minutes 45.2 seconds, or at an
average speed of 66.2 miles an hour, showing a gain of forty per cent
on Mr. Curtiss’s speed of the preceding year. Still better was achieved
at the international tournament held at Belmont Park in 1910. Le Blanc
in a 100-horse Blériot monoplane, especially designed for speed,
covered nineteen laps of the 5-kilometer course at an average rate of
61 miles an hour, and his fastest lap at the rate of 71.68 miles an
hour, thus exceeding Curtiss’s speed of the previous year by fifty per
cent. Other spurts during the latter part of 1910 were reported to have
attained nearly 80 miles an hour over a closed circuit, though perhaps
not a level one. The best results were achieved with machines having
high power engines, small surfaces and slight forward resistance.
The advance in long-distance flying in 1910 more than kept pace with
the progress in speed. The best achievement at the close of the
preceding year had been Farman’s flight of 144 miles at an average rate
of 35.06 miles an hour in a closed circuit. At the Rheims aviation meet
in 1910, Jan Olieslaegers, in a Blériot monoplane, driven by a Gnome
engine, covered 244 miles in a rectangular course, at an average speed
of 48.31 miles an hour. At Buc, on the 28th of October, an aviator of
three months’ practice, Maurice Tabuteau, in a Maurice Farman biplane,
driven by a Rénault engine, flew over a closed circuit, covering 288.8
miles at an average speed of 47.9 miles an hour. At Pau on December
21st, M. G. Leganeaux, in a Blériot monoplane, flew for the Michelin
Cup, covering 516 kilometers or 320.6 miles in six hours and one
minute, or at an average speed of 53¼ miles an hour—a splendid showing.
Finally, at Buc, on December 30th, Tabuteau, flying for the annual
Michelin prize, covered 362.66 miles in a Maurice Farman biplane with
an 8-cylinder 60-horse Rénault motor. The average speed in this very
long flight was 47.3 miles an hour, or practically the rate by which
Curtiss won the international contest of the preceding year. Of course
a considerably better showing of both distance and velocity could have
been made on a longer course.
The world records for cross-country flying and for endurance and load
illustrate both the increasing perfection of the machine and of the
pilot’s skill and confidence. At Los Angeles, on January 19th, Mr. and
Mrs. Paulhan, in a Farman biplane, flew together 21 miles overland from
the aviation field to Redondo and Hermosa Beach and return. On January
31st Van der Born made a world’s duration record with a passenger on
a Farman biplane, flying 1 hour 48 minutes 50 seconds. On March 5th,
Henri Farman, who had previously twice broken the world’s duration
record for a pilot with two passengers, set a new and astonishing
pace at Mourmelon, by carrying Mr. Hevardson and Madame Frank in
easy flight for 62.5 minutes on his new biplane. In France, on April
3d, Emile Dubonnet on his _Tellier_ monoplane flew from Juvisy to La
Ferte-Saint Aubin, a distance of 109 kilometers or 70 miles in 1 hour
and 50 minutes, thus winning the ten-thousand-franc prize offered
by _La Nature_ for the first straightaway flight of 100 kilometers
to be effected in less than two hours, over a previously indicated
course. This fine record voyage was achieved in a machine never before
thoroughly tried. At Chalons-sur-Marne, on April 8th, Daniel Kinet,
a Belgian, mounted with a passenger on a Farman biplane driven by
a 50-horse Gnome engine, broke the world’s record for duration and
distance for two persons by flying round a closed circuit 2 hours 19¼
minutes, covering a distance of 152 kilometers, or 94 miles. On April
17th, H. Farman, with a passenger in his biplane, voyaged from Etampes
to Orleans, 28 miles. Next day, Paulhan, mounting the same machine,
flew 108 miles, and the following day 42 miles. This tour established a
new cross-country record for total distance, for single stage distance
with one passenger, and for duration and single stage distance with two
passengers. During the same month Farman made a new record for four
passengers by carrying three gentlemen for 1 hour and 4 minutes on his
new biplane, spreading 47.6 feet. On June 9th, two French officers,
Lieutenant Fequant piloting and Captain Marconnet observing, flew
on a Farman biplane from Bouy to Vincennes, 145 kilometers, in two
hours and a half, thus breaking the world’s cross-country distance and
duration record for a pilot with a passenger. On June 13th, Charles K.
Hamilton, in a Curtiss biplane, flew from New York to Philadelphia, a
distance of 86 miles in 103 minutes, and returned the same day, thus
completing 172 miles in one day. This was an exhibition flight made for
_The New York Times_ and the _Philadelphia Ledger_, for a sum reported
to be $10,000. It was a sequel to Glenn H. Curtiss’s memorable flight
on June 5th, down the Hudson River from Albany to New York, for the New
York _World’s_ $10,000 prize. Hamilton’s average speed was 50 miles
an hour going and 51 miles returning. On August 29th, at Lille, Louis
Bréguet is reported to have carried with him on a biplane of his make,
five passengers, who, together with the gasoline, weighed 921 pounds.
It may be added that the Bréguet biplane of that date was advertised
and guaranteed to carry a cargo, or extra load, of 250 kilograms. It
thus appears that by 1910 the aëroplane had grown powerful enough for
an aërial cab service, and that it could carry sufficient explosive
gelatine to derange a battleship.
The contest for cross-country records continued unabated all that
memorable year. During the first three days of September, Jean
Bielovucic, a youth of twenty-one, mounted on a new type of Voisin
biplane, with but a few days’ practice, flew from Paris to Bordeaux,
covering 540 kilometers, or 336 miles, in four stages, comprising
altogether 6¼ hours on the wing. In spite of severe weather, at times,
he beat the regular express train and established a new world’s record
for cross-country straightaway distance flying with stops. On August
17th, Alfred Le Blanc, finished a six-stage tour round a hexagonal
circuit northeast of Paris, with the finish at Issy, near Paris,
covering a total distance of 785 kilometers, or 440 miles, in 12
hours 56.4 seconds effective time. On September 7th, Weyman flew with
a passenger from Paris to Clermont near the Puy de Dome, covering
205 miles in one day, while trying for the Michelin prize of 100,000
francs, for a flight to the Puy de Dome inside of six hours. On
December 18th, Thomas Sopwith, competing for the longest flight across
the Channel and into Belgium, on a British-built aëroplane, flew from
the Isle of Sheppy across the Channel, and landed at Beaumont, Belgium,
covering a distance of 174 miles in 3.5 hours. At Buc, on November
27th, Laurens, in a 60-horse R. E. P. monoplane, flew with his wife 53
miles at an average speed of nearly 50 miles an hour. On December 22d,
Lieutenant Cammerman, a French army officer, won the L. Weiller prize
by flying across country with a passenger, 147 miles in 4 hours and 2
minutes.
These are but a few of the records which serve to illustrate the
progress in cross-country flying during that year of strenuous and
world-wide popular demonstrations. But the bare numerical statement of
facts can give no conception of the delight and exultation aroused in
millions of souls who witnessed or learned of these marvelous human
achievements. They were the advancing triumph of a proud and fortunate
generation, happy in realizing one of the fondest dreams of the ages.
Often during one of these cross-country flights the aëroplane was
accompanied by a swift railway train whose passengers were delirious
with enthusiasm. The entire route was thronged with people assembled
from afar. It was a general holiday for all the fortunate cities and
villages along the way. Mills and factories blew their whistles and
forgot the serious business of life, homes were deserted, schools
were dismissed; the whole population for the time congregated in
the open; bearded mechanics in their aprons, bare-armed housewives
holding their children aloft, girls and boys with wondering eyes, all
shouting, waving banners, throwing up hats, and hailing with tumultuous
demonstration that strange and huge-winged creature gliding from
horizon to horizon with the steadiness, precision and directness of a
mighty projectile. But beyond stating the records of this season of
aërial wonders, only a passing notice can be given to some of the more
conspicuous events.
The most famous overland voyages of the season 1910 began with the
race for the London _Daily Mail_ prize of $50,000, offered by Lord
Northcliffe for the first person who should fly from London to
Manchester, 183 miles within twenty-four hours, with not more than
two stops. An Englishman, Claude Grahame-White, comparatively new in
the pilot’s art, was first to undertake that difficult and perilous
adventure. Starting from London, without competitor, on April 24th, he
flew in his Farman biplane, from London to Rugby, thence to Hademore,
about halfway to Manchester, landing at a quarter past nine o’clock
at night, after a four-hour trip, and hoping to reach Manchester next
day. But during the night his aëroplane, which was left in the open,
was damaged by the wind, thus necessitating repairs and a new start. On
April 27th, while he was strenuously mending and adjusting his biplane
for a new start, Louis Paulhan, who the day previously had arrived from
France with a Farman biplane to enter the contest, was also vigorously
setting up and adjusting his machine.
At half past five in the afternoon, Paulhan suddenly set out for
Manchester. Mr. White, who was much fatigued and expecting to start on
the morrow at dawn, after much-needed rest, learned toward six o’clock
that his rival was on the wing, and hurriedly sailed from London,
hoping by skill and good chance to overtake the flying Frenchman.
The race was now the most exciting event in the world. The first
flyers of France and England were competing for the greatest prize
yet offered in the history of aviation, competing in a most modern
and extraordinary race, attended with abundant danger and hardship.
The contestants were evenly matched in mechanism and capability, but
the Frenchman had gotten the march on the unwary Englishman. Paulhan
followed the Northwestern Railway, at times outracing the special
pilot train carrying his mechanics and supplies. At ten minutes after
eight o’clock, he landed at Lichfield, having covered 115 miles. Mr.
White had landed five minutes before eight near Roade, after flying
fifty-nine miles.
Next morning, Paulhan sailed away at a quarter past four. Mr. White,
hoping to overtake him, had started at dead of night and covered
twenty miles before Paulhan had started. It was a heroic effort, but
unavailing. At twelve minutes after five, Mr. White landed at Hademore,
having completed two thirds of the entire journey. Twenty-five minutes
later Paulhan landed on the outskirts of Manchester, greeted by a
thousand persons. He had covered the whole distance in 4.2 hours, and
had fulfilled all the essential conditions for winning the great prize.
The next world-famous aëroplane voyage was that of Glenn H. Curtiss for
the New York _World’s_ prize of $10,000 for the first aërial journey
from Albany to New York, allowing two stops. Aviators had been yearning
for this prize since the previous year, but had been too timidly shying
at the dangers of the route. After most careful preparations for this
voyage, Curtiss, bearing a letter from the Mayor of Albany to the
Mayor of New York, sailed away at seven o’clock on Sunday morning, May
29th, accompanied by a New York Central special train, bearing his wife
and a few friends and newspaper men. He landed an hour for supplies
and adjustment at Camelot, 41 miles down the river, and thence flew to
Spuyten Duyvil, at the northern extremity of New York, having completed
the required distance, 128 miles, in 2 hours and 32 minutes, or at the
rate of 50.52 miles per hour along the course. An hour later, he flew
down the river to New York Harbor and landed on Governor’s Island,
where he received a becoming ovation.
Perhaps the most exciting incident of the voyage to Mr. Curtiss was his
transit of the Storm King Mountain. As he was flying through the narrow
gap at this place he caught the down-rolling air on one side more
than on the other, and dropped very suddenly sidewise 30 or 40 feet.
By shifting his front control, he quickly gained headway and promptly
righted his machine.
Commenting on Mr. Curtiss’s average speed of 50 miles an hour and his
rugged course, _Aëronautics_ makes comparison between his voyage and
Paulhan’s great prize flight as follows:
“Paulhan took 4 hours 12 minutes elapsed time to cover 183
miles when he won the _London Mail’s_ $50,000 and made it in
two stages of 117 and 66 miles each. The 117 miles were covered
in 2.39, a rate of nearly 44 miles per hour. A night’s sleep
intervened and the remaining 66 miles were covered in 1.23, a
rate of nearly 48 miles per hour. The average for the above
was 44.37 miles per hour. Paulhan could have landed at almost
any time and started again, whereas Curtiss could not have
started if he had had to land in the water, and for the whole
distance there was scarcely a suitable space for landing on the
ground, as for nearly the entire way rocky, wooded hills with
precipitous sides line the river.”
The most audacious and marvelous aëronautic exploit of the year was the
flight of George Chavez across the Alps from Brig to Domodossola, in
his attempt to win the prize of 70,000 francs offered by the Italian
Aviation Society for the first aëroplane flight from Brig to Milan, a
distance of 75 miles. From the nine volunteers for this contest who
presented themselves to the committee in charge, five competitors
were selected, and these for several days made tentative efforts to
scale the lofty pass, but were baffled by the wind or fog. Finally at
one-thirty, on September 23d, the conditions being favorable, Chavez
rose, from Briegen-Berg, in his white-winged Blériot, spiraled upward
1,000 meters, circling around the vast amphitheater of the mountains,
and in nineteen minutes appeared in magnificent career well above the
Simplon Pass, probably 7,000 feet above the sea, whence he glided
grandly down the Italian slope, parrying the rude cross winds and
finally reaching Domodossola, where the enthusiasm was at its climax.
Here he expected to land on a level spot to replenish his supplies,
thence proceed over the easy remaining two thirds of his journey. But
though the perilous pass had been crossed so successfully, disaster
appeared in the valley when least expected. As the aëroplane was
gliding thirty feet high over the level tract chosen for landing, it
met a sudden gust, its wings collapsed, and it fell crashing to earth,
pinioning its brave pilot under the débris.
Poor Chavez suffered severe wounds about the face and head, had both
legs broken, and for some moments lay unconscious. But he was soon
revived by his friends and taken to a hospital, where he died four
days later. Thus ended the career of a brave and most promising youth
of twenty-three. He had taken his pilot’s license only in February,
1910, yet had established a new world’s record on September 8th, by
driving his Blériot to an elevation of 8,406 feet. He was of Peruvian
parentage and born in Paris.
The exact nature of the accident was never ascertained, but it was
surmised that the sudden starting of his engine preparatory to landing
overstressed some part of the structure already fatigued from hard
usage. However this be, the committee recognized that Chavez had with
excellent skill covered all the really difficult and dangerous part
of this journey. Accordingly they very generously waived the exact
letter of the rules, and awarded him one half the prize, though he had
completed but one third of the journey.
Quite as dangerous, spectacular and brilliant as the flight across
the Alps, though less arduous, was Hubert Latham’s aërial voyage over
Baltimore. On previous occasions cross-city flights had been made,
but never one of such length or one executed under such exacting
conditions. At various times aviators had flown above Paris, Rome,
Berlin, etc. On October 14th Mr. White had flown across Washington,
landing on a narrow street between the White House and War Department;
on October 15th Leganeaux had flown above Paris with a passenger; but
these were short flights over an uncharted course. Latham’s voyage
was unique; for he had to follow a long and a prescribed course over
the business section and closely built residence portion of the city.
This great exploit was an exhibition flight made on the invitation
of the _Baltimore Sun_ for a sum of $5,000. It was to be made at the
time of the Baltimore aviation tournament at Halethorpe, Md., and was
calculated to be seen by half a million people; for the whole city was
to be notified and would cease its usual activities to witness the rare
and hazardous demonstration.
The voyage was triumphant and glorious in every feature. Starting from
the aviation ground, seven miles south of Baltimore, about noon on
November 7th, Latham drove his beautiful _Antoinette_ about the field
in an ascending spiral, like some imperial bird taking its bearings;
then, chart in hand, deliberately sailed away over his elaborately
prescribed journey. This was a figure 8 course with its bottom at the
aviation field and its center at the _Sun_ Building in the heart of
Baltimore, the whole length being 22 miles. As the long-winged bird
in majestic poise, with the intrepid rider on its back, approached
in the distance, soaring 1,000 feet above the gleaming waters of the
Chesapeake, the great bell of the City Hall sounded a mighty peal, and
the whole populace responded in tumultuous chorus; whistles, bells and
a myriad voices mingling their heartiest welcome to the bravest of
aviators. With arrowlike speed and directness he rounded the center of
the course at the _Sun_ Building, then looped the vast northern half of
the city, flying a thousand to three thousand feet high, more easily
to parry the surging eddies of the northwest wind; rounded again the
center of his course and then returned to the aviation field, where he
landed with infinite coolness before the excited throng of applauding
spectators, whose acclaim was all too feeble to express their mingled
wonder, admiration and delight. The voyage lasted forty-two minutes and
fulfilled perfectly every minute requirement, including a short circle
and salutation before the home of Mr. Ross Winans, an invalid gentleman
who had solicited this unique favor, and rewarded it with a gift of
$500. It was the climax of the aviation week at Baltimore.
Among the many brilliant flights of that memorable year of strenuous
piloting will long be remembered the voyage of the Hon. C. S. Rolls to
Calais and return without landing, and that of Mr. Sopwith, already
recounted; the splendid flight of Mr. Clifford B. Harmon in his Farman
biplane from Mineola, Long Island, to a small rounded island before
his house on the Connecticut shore, for the trophy offered by _Country
Life_ to the first person who should fly across Long Island Sound;
Henri Farman’s flight of December 18th, for the Michelin long-distance
prize, covering 288 miles, and establishing a new endurance record of
8 hours 23 minutes; Mlle. Helene Dutrieu’s flight of December 21st,
for the Coupe Femina, covering 103¾ miles in 2 hours and 35 minutes
in a Farman biplane. Interesting, too, were the first attempt to fly
from Paris to Brussels with a passenger, when Mahieu and Manihé on
starting were brought to bay by a vicious dog which violently attacked
the propeller and was cut in two; and when Loridan and Fay landed on a
tree, from which they descended by a ladder. After this followed the
glorious voyage of Henri Wijnmalen, the youthful and many-sided Dutch
sport, for the prize of 150,000 francs offered by the Automobile Club
of France for the quickest aëroplane trip not exceeding 36 hours, with
a passenger from Paris to Brussels and return. This voyage of some
320 miles was valiantly accomplished by Wijnmalen and his companion
Dufour, in a day and a half, of 13.2 effective hours, and in weather
for the most part windy or tempestuous. Finally to the foregoing list
of splendid achievements must be added the glorious voyage of John
Moisant, who in August flew with a passenger, by compass, from Paris to
London, though he had never been over the route before and had only
just learned to use an aëroplane.
The International Aviation Tournament of 1910, held at Belmont Park,
Long Island, October 22d to 31st, was the most prominent and eventful
meet of the year, and the second of its kind in history, as the
meeting of the preceding year at Rheims was the first. The present
meet was conducted by the Aëro Corporation, Limited, of New York,
under the auspices and official sanction of the Aëro Club of America,
representing the Federation Aëronautique Internationale.
This tournament was the annual aërial Olympic contest of the world, and
should have been indicative not only of the aviator’s skill, but also
of the state of national progress in the science and art of aëroplane
construction. Unfortunately, however, for the prestige of the most
deserving nations, the rules of the International Aëronautic Federation
did not confine the contestants to the use of home-built machines, to
prevent the glory of winning the international contest from passing to
the nation which merely furnished the operator, a person who might be
an illiterate jockey, and representative of a country wholly devoid of
science. As luck decided, however, the highest honor in 1910 was won by
a first-class French machine driven by a first-class English aviator.
In some respects the raw material and working elements of this meet
were most satisfactory. The site is near the wealthiest and most
populous center in America. The grounds are spacious and level,
and provided with all the equipment of a great race course; the
transportation facilities by carriage and by rail from the heart of
New York are adequate to every requirement. The personnel of the
meet comprised the most experienced and most devoted members of the
Aëro Club of America, the oldest and strongest aëronautical body in
the western world, and the only one representing the International
Aëronautic Federation. It is true the season was late and the weather
would probably be cold and tempestuous; the management was burdened by
a costly license, whether just or unjust, imposed upon it as the price
of immunity from patent litigation; the remaining time, after the final
placement of the meet, was all too short for the myriad preparations
to be made. But whatever the obstacles, physical or financial, the
personnel was paramount, and naturally made the huge tournament a
glorious triumph. It was the cardinal sporting event of the year.
The status of aviation was well represented in both pilots and
machines. Twenty-seven aviators were entered on the program, many of
them world famous. Of these Alfred Le Blanc, Hubert Latham, Emile
Aubrun were the formidable champions of France in the contest for the
James Gordon Bennett aviation trophy; Claude Grahame-White, James
Radley, A. Ogilvie represented England; while Walter Brookins, J. A.
Drexel, Charles K. Hamilton were enlisted as defenders of the coveted
cup and of American prestige. All told, the aviators brought with them
nearly two-score machines, ranging in capacity from 30 to 100 horse
power. Of these about half were monoplanes and half biplanes, for the
most part of French and American manufacture.
The prizes and remuneration awarded to the contestants were on a
scale proportionate to their skill and number. All told the winnings
aggregated more than $60,000. Further appropriations were made to
cover the expenses of the aviators, and a further sum equal to about
forty per cent of the winnings was paid for immunity from prosecution
for possible infringement of an unlitigated patent. Considering the
immense expenditures for buildings, for policing and other incidentals
of the meet, it may be readily inferred that there was an ample
deficit, and that the air men as a whole were much better rewarded
than some of the sportsmen who gave so much time and labor to the
organization of the tournament.
A conspicuous feature of the meet was the display of hardiness and
skill of several of the aviators in facing the cold and tempestuous
weather. This was particularly characteristic of Latham in his
_Antoinette_ monoplane, and of Ralph Johnstone and Arch Hoxsey in
Wright biplanes. On October 27th Latham flew round the regular course
for an hour when it was nearly impossible to turn the pylons against
the fierce wind, while Johnstone and Hoxsey performed lofty altitude
flights in a powerful gale which carried them backward, sometimes at
the rate of 40 miles an hour. As a consequence they landed in the
open country, remained overnight and returned next day. Johnstone was
carried backward to Holtsville, 55 miles east of the aviation grounds,
and Hoxsey was blown to Brentwood, 25 miles away, both landing at dusk
in open fields, and both having attained great elevations: Hoxsey,
6,903 feet; Johnstone, 8,471 feet.
An interesting novelty of the aviation week, at least to Americans,
were the erratic Demoiselle monoplanes, invented by Santos-Dumont and
piloted by Garros and Audemars. These aëroplanes were notable as having
the pilot under the sustaining plane, and the engine above with its
direct mounted propeller. The lateral stability was enhanced by a low
placement of the center of mass, and by a slight dihedral inclination
of the wings. Furthermore, as there was not much leverage or surface in
the rear double rudder, the flight was more stable than steady, like
that of a propelled parachute. In fact, the little monoplanes pitched,
rocked, and fluttered about so like huge butterflies as to provoke
constant merriment. They gave a faint suggestion of how ludicrous
aëroplane clowns could be made by one who has genius for such things.
Barring the stormy voyages above mentioned, the most memorable events
of the tournament were the Gordon Bennett speed contest, the Statue of
Liberty race and Johnstone’s great altitude flight. Of the numerous
other performances little need be said, except that they contributed
to the general success of an elaborate and most interesting program.
They served the daily need of a costly tournament; they delighted vast
throngs of spectators whose admission fees helped to promote the aërial
sport; but they did not of themselves have more than local interest, or
constitute an advance in the records of first-class achievement.
The chief race of the meet, the James Gordon Bennett speed contest,
was scheduled for Saturday, October 29th. The prize of $5,000 and the
coveted cup were to be awarded to the pilot who should make the best
average speed in 20 laps over a 5-kilometer course, aggregating 100
kilometers, or 62.14 miles. The winner should have the distinguished
honor of taking to his own country the next annual contest for the
precious speed prize.
Grahame-White, England’s foremost aviator and strongest hope in
the contest, brought forth his untried 100-horse Blériot in the
calmest part of the day, and took wing a quarter before nine. He
flew with steady poise and swift, well-sustained speed, completing
the 100-kilometer distance in 1 hour 1 minute and 4.7 seconds, at an
average speed of 61 miles an hour.
Le Blanc, the most likely winner of all, sailed at nine o’clock. He was
mounted on a 100-horse Blériot with nearly flat wings, the swiftest
monoplane of French manufacture. He was the boldest, sturdiest and
most dexterous pilot in a nation of renowned aviators, the winner of
unnumbered trophies, the “Vainquer de l’Est.” He now flew at unwonted
speed, establishing new world records at every round of the course. It
seemed evident to the timers that only an accident to this impetuous
Frenchman could retrieve the glory of England and save that of America.
Suddenly the accident came. In the last lap, when victory seemed
assured, the gasoline failed; the monoplane shot downward, knocked off
a telegraph pole, and, with broken frame and engine, fell crashing
to earth, entangling the brave aviator. Le Blanc was cut and bruised
about the forehead, and was taken to the hospital to be bandaged,
not seriously injured but in a towering rage, suspecting that some
trickery had given him a shortage of fuel. He had lost the day, though
his average speed for the whole flight was 67 miles an hour as against
Grahame-White’s speed of 61 miles.
No well-tried machine was available to defend the American prestige.
Curtiss had constructed a new monoplane designed for speed, but though
he had brought the cup to America, he was not chosen as one of its
three defenders. The little Wright biplane of 61 horse power had
flown a few minutes with great velocity, and was looked to with some
confidence. Mounted by Walter Brookins, it set out with tremendous
speed, but had only well started when the cylinders began to miss fire.
Brookins turned toward the infield to land, struck the ground with
terrific shock and tumbled violently on the field beside his broken
machine. He, too, was taken to the hospital for treatment, but was not
seriously injured.
It was now granted that Grahame-White would be the ultimate winner.
Other aviators attempted to defeat him, but lacked either the necessary
speed or endurance. The cup was accordingly taken from the nations
that had done the most to develop the practical art of aëroplaning. Of
these two nations, the one most deserving of victory, by virtue of its
more careful preparation, was defeated by an extraordinary mishap, when
victory was at hand; the other failed perhaps for want of preparation
rather than from lack of manipulative or constructive skill.
Of the various highly coveted stakes the largest in monetary value was
known as the Thomas F. Ryan Statue of Liberty Prize. This was a cash
sum of $10,000, to be awarded to the properly qualified contestant who
should fly from the aviation ground to and around the Statue of Liberty
in New York Harbor, and return in the shortest time, the airline
distance being 16 miles each way. The prize was founded by Mr. Thomas
F. Ryan, whose son, Allan A. Ryan, was Chairman of the Committee on
Arrangements of the tournament, and who though suffering with pain and
ill-health, labored so indefatigably to insure the success of the event
so germain to the aëronautical prestige of his country.
The Statue of Liberty race occurred on Sunday afternoon, October 30th,
beginning just after three o’clock. Count De Lesseps in a 50-horse
Blériot monoplane led the race, followed three minutes later by
Grahame-White. They passed toward the southwest in perfect poise and
vanished beyond the horizon unchallenged by an American contestant;
for Moisant, the American champion, had shortly before injured his
racing monoplane, and the other American racing machines had been
damaged the week before, or had not yet been fully tested. But with
admirable enterprise, Moisant telephoned Le Blanc, in New York, who
was not racing because of the accident to his 100-horse Blériot the
day previously, and offered the Frenchman $10,000 for his 50-horse
Blériot monoplane. The sale was effected in time for the race that
day. But for all that the enterprise seemed futile; for as Moisant
was preparing to start, the others were returning, Grahame-White well
in the lead, having overtaken De Lesseps. As these two aviators were
receiving the applause of innumerable spectators and the felicitations
of their friends, audacious Moisant, the impetuous soldier of fortune,
and hero of the famous flight by compass from Paris to London, started
toward the declining sun, just after four o’clock. He was determined
to win by superior skill and daring. His prudent competitors had
followed a circuitous southern route interspersed with landing places;
but he flew like a maniac straight over the church spires and crowded
buildings of Brooklyn, guided to his goal by a compass, rounded the
Statue of Liberty at a great altitude and plunged homeward with all
possible speed and directness. The megaphone announced his progress,
which indicated some hope of victory so little expected and so much
desired by the vast throng that stood gazing toward the western sun.
In headlong career the swooping monoplane shot by the judges’ stand,
circled and softly landed on the field, triumphant by 43 seconds
over the 100-horse Blériot of Grahame-White. As the intrepid aviator
approached the vast and delighted throng of spectators to acknowledge
its noisy and tumultuous ovation, he was met by the chiefs of the
tournament, draped in an American flag, and paraded before the grand
stand, “which shook in its effort to do honor to the little air
conqueror.” Ultimately, however, the prize was awarded to Count
De Lesseps, because Moisant had failed to qualify properly, and
Grahame-White had fouled the initial pylon.
The final day of the tournament was made memorable by Johnstone’s
altitude flight. The best previous record was that of Wijnmalen to an
elevation of 9,104 feet, made at Mourmelon, France. Johnstone ascended
on a small Wright machine with powerful propellers adapted to rapid
climbing, determined not only to surpass Wijnmalen but to exceed,
if possible, the ten-thousand-foot level, and win the special prize
offered for such achievement. He actually rose to the great elevation
of 9,714 feet, but could not develop power enough to continue upward.
On his descent he fully exhausted his fuel at 3,000 feet, and thence
glided to earth, landing softly, 1 hour and 43 minutes from the time of
starting.
Thus the greatest tournament of the year terminated with fine new
laurels for the science and art of aviation; for the spectacular
pilots and for the unseen men behind them—the scientific men in the
laboratories, the designing rooms and the workshops. New standards had
been established in speed, in altitude, in prowess and daring. In these
elements, the spectators could hardly ask for a better exhibition. What
is it to the onlooker to have an aëroplane go higher than the cumuli,
since at that level a thousand feet makes no perceptible difference?
What more could he wish in dexterity of manipulation and audacity in
braving the elements? One thing more, doubtless, and that is, security
and precision of flight in stormy weather. When these improvements
shall have been effected much will have been added to both the sportive
interest and practical utility of the aëroplane.
The most businesslike and crucial flying contest of the year was the
famous “Circuit de l’Est,” organized by the Paris _Matin_. It was a
competitive voyage over an irregular hexagonal course, lying generally
northeast of Paris, and having its vertices at various cities to the
east and north of the national capital. The main prize offered by
the _Matin_ was one hundred thousand francs for the first air man to
complete the entire course, doing the first side of the hexagon on
August 7th, and the succeeding sides in regular order on successive
odd days of the month, the place and hour of starting each stage
being assigned in advance. Various subsidiary prizes aggregating
nearly a hundred thousand francs more, were available for meritorious
performances at the various stages and stopping-places along the
route. But there were also penalizations for those contestants who
failed to start on schedule time and observe the rules of the course.
[Illustration: FIG. 41.—MAP OF THE “CIRCUIT DE L’EST.”]
The race began at Issy, near Paris, on August 7th, with eight aviators
on the wing—Le Blanc, Aubrun, Leganeaux, Mamet, Lindpainter, Weyman. It
terminated August 17th, headed by Alfred Le Blanc on his Blériot, and
followed by Emile Aubrun on a Blériot, then by Weyman on a Farman, all
three driven by Gnome engines actuating Chauvière propellers. Le Blanc
completed the tour of six stages, covering an air-line distance of 488
miles, in 12 hours’ effective flying, or at the average rate of 40.6
miles per hour.
This long tour on schedule time over a rough and varied country in
face of fog, wind and rain, was a most severe trial of the prowess and
endurance of the brave pilots who had the hardiness and pertinacity
to complete the voyage. Needless to add that it created unbounded
enthusiasm among millions of people who witnessed the event, or read of
it, and that the clocklike precision of the “grand raid” inspired new
confidence in the practicability of the aëroplane.
A particularly impressive feature of the event was that many of its
participants, the aviators, government officers, and members of the
controlling committee, assembled at Issy and other posts of duty,
not by rail, but by aëroplane, sailing across country from many
directions and from great distances. This matter-of-fact procedure led
many persons to believe that the period of mere demonstrations had
approached its close, and that the epoch of practical utility was at
hand; that after marveling so much at the aëroplane, with mingled faith
and skepticism, people would next calmly turn it to practical use.
Though the progress in designing and constructing aëroplanes in 1910
did not keep pace with the wonderful advance in new records, still the
inventors and manufacturers continued industriously to perfect the
details of their best standard machines, and in a few instances to
make radical innovations. The perfection in details of construction
manifested itself in the public performance of aëroplanes, particularly
in their greater reliability and their increased capabilities. The
radical innovations were mainly experimental, and not generally
exhibited, though none the less important for all that. Chief of these
perhaps were the hydro-aëroplane developments of Fabre in France, and
of Mr. Glenn H. Curtiss in America, which enabled the aviator to launch
into the air directly from the water and to alight safely on the water,
thus virtually adding a new and very important domain to the empire of
dynamic flight.
Curtiss, in 1909, succeeded in landing his aëroplane safely on the
water of Lake Keuka, first with sheet iron cylindrical floats under
each wing, and a simple float well to the front of his protruding
chassis, then with a hydroplane surface to the front as being more
effective than the float. But when he attempted to glide up from the
lake with this arrangement, he could not entirely clear the surface,
though his aëroplane under the powerful thrust of her aërial screw,
very nearly lifted from the water. Then he planned to use hydroplane
floats, of hollow wing form, and of such size that they would buoy up
the machine when at rest, and during motion would skim over the water
like a skipping stone, till the biplane should acquire sufficient speed
to rise by the dynamic reaction of the air. In the successful execution
of this plan, however, he was anticipated by Fabre, who made the first
successful flight from the water, on March 28th, 1910, at Martigues,
France. But the Frenchman was not left to bear the palm alone. Early in
the year 1911, Mr. Curtiss rose and landed successfully on the water at
San Diego Bay, Cal., by means of a single float like a flatboat placed
centrally under his biplane, seconded by small auxiliary floats at the
wing ends. A full account of these valuable contributions to aviation
is given in Appendix V.
PLATE XXXI.
[Illustration: FABRE HYDRO-AËROPLANE.
_Photo E. Levick, N. Y._]
[Illustration: PAULHAN HYDRO-AËROPLANE.
_Photo E. Levick, N. Y._]
[Illustration: MOISANT METAL MONOPLANE.
(Courtesy A. J. Moisant.)]
As shown in Plate XXXI, Fabre’s hydro-aëroplane was substantially a
monoplane mounted on three richochet floats. It was propelled by a
screw at the rear, and controlled in flight by the usual three-torque
system, in this case consisting of horizontal rudders in front,
vertical rudders front and rear, and suitable mechanism for twisting
the wings. The floats were hollow to give them static buoyancy; they
were curved fore and aft like wings, to give them dynamic lift, both in
water and in air; they were elastically constructed with thin veneer
bottoms and flexibly attached to the framing, so as to endure the
severe buffeting, at high speeds, against the uneven water surface;
they were capable of landing the machine safely on a sandy beach or
meadow, as well as on the water. Indeed, a plan was conceived for
rising and alighting on land and water indifferently.
[Illustration: FIG. 42.—DIAGRAM OF CURTISS HYDRO-AËROPLANE.
(_Scientific American._)]
The first machine weighed in flight 950 pounds and spread 280 square
feet of surface, giving a loading of 3.4 pounds per square foot. It was
driven by a 50-horse Gnome engine actuating a Chauvière propeller 7.5
feet in diameter. In the trials of March 28th, the machine cleared the
water at a speed of 34 miles per hour, and flew about one-third of a
mile, at an elevation of two to three yards; then at the will of the
operator it alighted softly on the water.
The structural design of the Fabre monoplane was novel and unique, not
to say radical. The wing framing consisted of a single Fabre trussed
beam with ribs attached like the quills of a bird, over which was
stretched the light sailcloth cover, then laced to the beam. The girder
itself was formed of two ash planks eight inches wide by one-fourth
inch thick trussed together by flat steel plates zigzagging trelliswise
between them. As all parts of the beam cut the air edgewise it offered
very little resistance, while at the same time being very strong. The
ribs being attached only at one end allowed the sailcloth to be quickly
slipped on and off for washing and proper care.
The characteristic features of Fabre’s wing construction were adopted
by Paulhan in his novel and picturesque biplane shown in Plate
XXXI. Trussed beams were used for all parts requiring considerable
stiffness, the longitudinal ones being covered with fabric to reduce
the resistance. The wings whose solid ribs were fastened only at their
front ends were quite elastic, a quality conducive to stability, as
long taught by writers[55] on aviation. In addition to the front
rudder, there was at the rear a horizontal rudder with a vertical one
just before it. To reduce the air resistance further the pilot and
passenger were to sit tandem in a torpedo-shaped car with the 50-horse
Gnome engine and fuel tank back of them. Beneath the longitudinal
girders were two Farman skids flanked with the usual wheels,
elastically connected. The machine, besides flying well, was readily
demountable. The wings could be quickly removed, thus allowing the
biplane to enter a door fifteen feet wide. The entire machine could be
packed in a case 15½ feet long by 3¼ feet square, the whole case cubing
less than six solid yards. Hundreds of them, therefore, could be stowed
away in an ocean cruiser.
The flying quality of adequately designed flexible aëroplanes is well
illustrated by the swallowlike monoplane shown in Fig. 43. This airy
creation of the distinguished Austrian engineer, Igo Etrich, came into
public prominence in the spring of 1910, though it had been developing
privately for half a decade or more. On May 14th, near Vienna, it
carried pilot Illner 84 kilometers in 80 minutes, at an elevation of
300 meters, thus surpassing all previous Austrian records for distance,
duration and altitude. Its successor, _Etrich IV_, had wing tips still
more turned up, and possessed such stability that during the meet at
Johannisthal in October, Illner circled the pylons with his hands off
the warping levers. At times he wheeled round curves of only ten meters
radius, the whole machine tilted at an alarming angle, yet maintaining
its poise with the natural ease and grace of a soaring albatross.
The prominent feature of Etrich’s monoplane was the elastic
construction of its wings and tail. Across the rigid main bars of each
wing were fastened numerous ribs with bamboo terminals, thus making
the rear margin and tip of the wing flexible. Similarly the tail, or
horizontal rudder, was framed of bamboo. Hence the pilot, by use of
control wires, could flex both the wing margins and the tail up and
down at will, to steer the machine, or he could let go the controls and
allow the distorted surfaces to spring into their normal positions,
and the machine to pursue the even tenor of its way. Moreover, the
gusts and whirls in the air, on striking the elastic rear margins of
the tail and wings, exert a propulsive effort. Thus could be utilized
the wind’s energy of turbulence, as indicated by the present writer
in 1893, in a paper on “Windgusts and Their Relation to Flight,”
published in the _Proceedings of the International Conference on Aërial
Navigation_ of that year. In passing it may be remarked that many other
aëroplane designers, notably Bréguet, have emulated Mr. Etrich, though
unconsciously perhaps, in providing elastic ribs, hinges or pivots to
permit the rear parts of the wings and tails of their machines to yield
freely to intentional or unusual impulses, and then spring back to
their normal positions.
[Illustration: FIG. 43.—THE ETRICH MONOPLANE OF 1910.]
The carefully elaborated monoplane of Robert Esnault-Pélterie, which
had been steadily improving for eight years, had now attained great
perfection of finish, and merited prominence in actual flight. As shown
in Plate XXIX, it had a general resemblance to the _Antoinette_, though
differing throughout in its manifold details. The stream-line body was
of steel tubing, braced with wire, and tightly covered with smooth
fabric to reduce resistance. A five-cylinder R. E. P. motor in front
connected directly with the two-blade propeller. The pilot sat between
the wings with the passenger before him at the center of gravity, both
having control levers when desired for instruction. The wings could be
warped and the rudders, at the end of ample empennage planes, occupied
the extreme rear as shown. An elastically cushioned skid between the
two freely turning wheels served to absorb the shock of hard landing,
though usually not touching the ground. The R. E. P. monoplane of 1910
was a very graceful, swift and strong machine, of marked efficiency.
As always happens in the many-minded development of a complex
invention, the general exhibition and use of the aëroplane led toward
uniformity of design. This became particularly noticeable during the
world-wide demonstrations of 1909 and 1910. Whatever predilection the
inventor might have for his own devices, he would rather cast them
aside than lose at the tournament and in the market. Without a monopoly
of the flying art, he could ill afford to retain too affectionately
his own second-rate device in competition with a rival having a more
effective one. Accordingly there was a judicious and general adoption
of those devices which had proved best in practice, from whatever lowly
intellect they had emanated. Thus there was a marked tendency to the
general use of starting wheels, landing skids, large warping surfaces,
and, in racing machines, to the stream line concentration of the load,
and the severe elimination of resistance.
A few examples will illustrate this tendency to choose the most
practical devices from the world’s general stock. The Wright brothers,
who, following Maxim, had been ardent votaries of the forward
horizontal rudder, discarded this in 1910 for the elastic rear
horizontal rudder introduced by Etrich. At the same time they abandoned
the antiquated catapult introduced by Langley, and adopted the
combination of wheels and skids introduced by Farman. In their racing
machine they no longer placed the aviator beside his engine, presenting
a broad front to the wind, but, like Curtiss and foreign designers,
they placed the driver and power plant in line, to diminish the
atmospheric resistance. These manifold and timely improvements indicate
clearly the advantages to mankind of an “open door” in a crescent art.
But if the Wrights adopted the most successful devices of their
neighbors, these in turn were not slow to reciprocate that policy.
There was ample recognition of the merit of the combination of warping
sustainers and double rudder proposed by scientific men before the
advent of power aëroplanes, and so admirably employed by the Wrights
and Prof. Montgomery in their early coasting flights. The warping wing
was quite generally used on monoplanes in 1910; not to mention the
_ailerons_, which frequently were an adaptation of the same principle.
As further illustrations, it may be noted that Voisin brothers adopted
the Farman _ailerons_ and abandoned the cellular type of sustaining
surface introduced by Hargrave, finding the vertical surfaces strongly
frictional and unnecessary for lateral equilibrium, in presence of
the _ailerons_. They also abandoned the forward horizontal rudder,
seeing that it could very well be omitted. On the other hand, it must
be observed that the Farmans, Sommer and Curtiss still retained the
combined fore and aft rudder. Curtiss and Farman also tried their hands
at monoplane construction, though without abandoning the biplane. The
most famous monoplanists, however, held firmly to their first love.
In this they were emulated by many new designers, Nieuport, Hanriot,
Déperdussin, etc. These show a marked tendency to employ smoothly
covered hulls shaped after the fish or torpedo.
To drive the little aëroplanes so far developed, especially the
racers, there was a general preference for a single-screw propeller
mounted directly on the engine shaft, though doubtless for machines
weighing many tons a multiplicity of such propellers would be used.
Theoretically the advantage of twin screws was conceded, but in
practice they were employed by very few constructors. The Chauvière
wooden propeller was the favorite in France, and was approved by
the constructors of propellers elsewhere, at least in its general
features. The Voisin firm, indeed, still adhered to the metal
propeller, and occasionally some experimentalist employed the more
venerable French screw consisting of radial sticks covered with fabric.
But the great records in the sporting world were achieved with solid
wooden propellers.
A special chapter would be required to describe the various motors,
even cursorily. Their relative values, however, may be summarized in
the following brief words by Réné Gasnier, in the _Aërophile_ for
November, 1910:
“Last year we had but few light types; this year there is no
dearth of them, and at their head stands that admirable motor
_Gnome_, which has enabled aviators to accomplish all their
fine performances. At first many persons had no confidence in
the future of the rotatory motor. One must bow to the facts;
on considering the nature of this motor it is seen to be of
an admirable simplicity. It is evidently the typical aviation
motor, and an approach toward the veritable rotatory motor
which later will be the turbine. Numerous motors of four to
eight cylinders are very well spoken of, but none attain the
lightness of the _Gnome_. Among the air-cooled motors the
_Esnault-Pélterie_ is remarkable for the series of trials it
has endured, and among water-cooled motors we may cite the
splendid performance of the _Antoinette_—2,100 kilometers in
one week at the Bordeaux meeting. This would be quite a good
run even in an automobile. It is noticeable that the aëroplane
motor tends distinctly to differentiate itself from its senior,
the automobile motor, and assume a type absolutely adapted
to its special work. In addition to the greatest possible
lightness, a demand now arises for a slight consumption of
fuel, and a range of speed which is indispensable for landing.
It is dangerous to descend rapidly with the motor at full
speed; on the other hand, in cutting off the ignition to glide
down, one risks not being able to restart the motor, if need
be, while if the motor relax sufficiently the descent takes
place in perfect security. It suffices to speed up at the right
moment.”
The practical utility of aviation began now to be questioned. The
aëroplane had passed the primary epoch of experimental development
and was becoming a standard article of manufacture representing a
considerable industry. But what was it all worth? Aviators had flown
faster than the eagle, higher than the clouds, farther than the common
distance from metropolis to metropolis. Schools were licensing new
pilots from day to day. But what career had these before them, and what
essential function in the affairs of humanity could they perform? Some,
indeed, might fit themselves for aërial service in warfare, some for
the pleasant profession of amusing and entertaining mankind; but in the
serious business of life, what important rôle could the air men hope
to play? This was the pertinent inquiry, and it was largely a question
of the reliability and economy of the aëroplane. Improvement in these
two elements might therefore receive attentive consideration in the
immediate future.
The reliability of the aëroplane depends partly on its environment,
partly on its plan and structure, partly on the skill of its pilot.
The pilot’s skill had been admirably developed in the tournaments and
public exhibitions. The aërodynamic design conducive to stability and
steadiness, the structural design conducive to maximum strength and
resiliency, uniformly proportioned to the stress and work of each
part of the complex machine; and above all the design of the motor,
to ensure it against a thousand foibles—all these could be improved
by the patient methods of theoretical and experimental science.
The environment could, of course, be chosen. At first only the most
favorable regions need be attempted for regular transportation, regions
of level plain and farm land, or of lake and river surrounded by
country not too rough and precipitous.
The general cost of the aëroplane to mankind depends on its plan and
structure, on the methods of manufacture, on the material running
expense; but its particular cost to the passenger is determined largely
by the cupidity or business acumen of those who furnish the machine
and those who operate it. Naturally when the world first awoke in
the morning of practical sporting aviation, with a sudden and strong
relish for flying, the prices would be fabulous, not to say ridiculous.
During that hour no commercial transportation could be contemplated.
But without monopoly the prices must quickly abate; for neither the
manufacture nor manipulation of the aëroplane demand rare ability or
training. The cost of manufacture would promptly be diminished by means
of specialized tools and operatives, immediately upon the assurance
of large and continuous orders. The cost of pilotage would become
insignificant when a single chauffeur could take a dozen passengers on
one aëroplane.
So much for the human and external elements in the cost of aviation.
The inherent and material cost of the aëroplane could also be reduced,
though perhaps less readily. It was unlikely that the machine would
be built of much cheaper materials, or made much lighter per pound of
cargo. Nor were such improvements of so much importance since they
would affect only the first cost of the flyer. But an increase of
aërodynamic efficiency in the propeller and aëroplane proper, together
with increased thermodynamic efficiency in the motor, would materially
lower the current cost of transportation at any given speed. These
improvements would require careful research in the laboratory and
patient trial in the workshop and field. The refinement and perfection
of the aëroplane might therefore be looked for in those communities
where men have sufficient foresight, enterprise and liberality to endow
research, and to encourage the science and the art of aviation to
supplement each other.
PART III
AËRONAUTIC METEOROLOGY
CHAPTER XIII
GENERAL PROPERTIES OF FREE AIR
For aëronautic uses the atmosphere may be regarded as a mixture of two
substances, dry air and water. The first remains always in the gaseous
state; the second shifts erratically through all possible states. Rain
drops freeze or evaporate; sleet, snow, and hail evaporate or melt;
the aqueous vapor condenses or congeals. Thus the world is wrapped in
a dual sea, one part naturally serene, the other capricious, protean,
and turbulent. Dry air, indeed, is a composite of many gases of vast
concern in chemistry and biology; but in relation to aëronautics it
is practically a single permanent gas. This placid element and its
inconstant mate, so curiously mingled, constitute the medium whose flux
and vicissitudes the aërial sailor has duly to learn before he can
navigate with skill or safety.[56]
But these aërial oceans, the moist and dry, are of very different
depth. They commingle only in the lower levels of the atmosphere, whose
qualities vary accordingly, both physical and transportational. While
the dry air may reach up to more than a hundred miles, substantial
enough to singe a meteorite, the sea of aqueous vapor is bounded
practically by the shallow region of the visible clouds. Beyond the
feather-like cirri, which just overtop the loftiest mountain peaks,
there is scant, if any, moisture. Never rain, nor cloud is there, nor
tempest nor any great perturbation. Beyond the highest excursions of
the cirri, at an elevation of some ten miles, stretches the deep ocean
of eternal sunshine, of equable and nearly constant temperature. Into
that zone of perpetual serenity no tumult of the nether atmosphere
can penetrate; against the floor of the isothermal layer the cyclonic
currents spread and dissipate. The upper air has, of course, a
considerable drift, like a majestic river or stream of the sea, but
never turmoil or tempest disturbs its stately march.
In some respects, therefore, that lofty ocean is an ideal one for swift
transportation. But at present it is beyond the range of any navigable
craft of human invention. Occasionally, indeed, a gauzy balloon from
the hand of some inquisitive weather sage penetrates a little way into
the exalted deep next the cosmic void, bearing its delicate recorders
of heat and pressure; but it wanders alone in a silent and vast
solitude outcubing all the habitable space allotted to bird, beast
and fish; then at last sinks down to deliver the story of its strange
voyage in that lifeless outer sphere. Volcanic and celestial dust may
flourish there, tingeing the twilight with rosy flush, but no biologic
forms from the teeming underworld may find refuge or sustenance. It is
the unconquered domain of who knows what meteoric craft of the future,
sweeping the globe from continent to continent, with now unimaginable
celerity, grace and precision.
Incidentally and aside from its aëronautic interest, the composition
of the atmosphere may be presented in fuller detail, showing the
wide variations from level to level, and the manifold complexity of
the fluid we daily breathe, not to mention the myriads of motes and
germs inhabiting every inch of it. The gaseous components and their
distribution are well exhibited in the following table,[57] which
represents an average condition:
TABLE I
_Percentage Distribution of Gases in the Atmosphere_
Key: HT = Height in Kilometers.
A = Argon.
N = Nitrogen.
WV = Water Vapor.
O = Oxygen.
CD = Carbon Dioxide.
HY = Hydrogen.
HE = Helium.
TP = Total Pressure in Millimeters.
----+--------------------------------------------------+--------
| GASES. |
HT +------+-------+------+------+------+-------+------+ TP
| A | N | WV | O | CD | HY | HE |
----+------+-------+------+------+------+-------+------+--------
150 | | | | | | 99.73 | 0.27 | 0.0043
140 | | | | | | 99.70 | 0.30 | 0.0048
130 | | 0.02 | | | | 99.64 | 0.34 | 0.0054
120 | | 0.10 | | | | 99.52 | 0.38 | 0.0060
110 | | 0.40 | | 0.02 | | 99.16 | 0.42 | 0.0067
100 | | 1.63 | | 0.07 | | 97.84 | 0.46 | 0.0076
90 | | 6.57 | | 0.32 | | 92.62 | 0.49 | 0.0090
80 | | 22.70 | | 1.38 | | 75.47 | 0.45 | 0.0123
70 | 0.02 | 53.73 | | 4.0 | | 41.95 | 0.27 | 0.0248
60 | 0.04 | 78.16 | | 7.32 | | 14.33 | 0.15 | 0.0810
50 | 0.08 | 86.16 | |10.01 | | 3.72 | 0.03 | 0.466
40 | 0.16 | 86.51 | |12.45 | | 0.88 | | 1.65
30 | 0.22 | 84.48 | |15.10 | | 0.20 | | 8.04
20 | 0.55 | 81.34 | |18.05 | 0.01 | 0.05 | | 39.6
15 | 0.74 | 79.56 | |19.66 | 0.02 | 0.02 | | 88.2
11 | 0.94 | 78.02 | 0.01 |20.99 | 0.03 | 0.01 | |168
5 | 0.94 | 77.89 | 0.18 |20.95 | 0.03 | 0.01 | |405
0 | 0.93 | 77.08 | 1.20 |20.75 | 0.03 | 0.01 | |760
----+------+-------+------+------+------+-------+------+--------
Fixing attention first upon the gases other than water, it will be
at once observed from the table that these gases show a very uniform
mixture in the moist and turbulent region, while farther aloft the
lighter of them tend to predominate in relative proportion. This
uniformity of composition at the lower levels, which accords with
experience, is due to the constant circulation and turmoil in that
region. But for this constant agitation, the uniformity of mixture
could not last. If the atmosphere were perpetually at rest throughout,
or moving only in horizontal flow, each constituent gas would assume
the same status and distribution as if the others were absent. Each,
therefore, obeying Dalton’s law of diffusion, would form an atmosphere
of itself, independent of the others, and unaffected in density by
them. Such a condition is assumed for the higher levels. The percentage
distribution in the higher levels is calculated from the known
elasticity and density of the gases, assumed as resting in perpetual
calm at a constant temperature of .55° C. beyond eleven kilometers, or
above the highest ascent of man, and, furthermore, as having at the
earth’s surface 1.2 per cent moisture and a temperature of 11° C.
But only in the quiescent outersphere can that dynamic gradation be
established or perpetuated. Below this lofty region is the sea of
water vapor, mingled intimately with the dry air, and churned with
it, yet not sharing its uniformity of distribution. Why this rapid
diminution of moisture with elevation, as shown in the table? Because
throughout the moist region the temperature falls rapidly—about 6° C.
per kilometer ascent above the earth—thus chilling and precipitating
the vapor, whose pressural resistance to liquefaction diminishes with
waning temperature. The explanation is obvious; but why does it not
apply as well to the other elements of the atmosphere: why do not the
other gases present liquefy with falling temperature as well as the
water vapor, which is merely water in the gaseous state? The question
cannot be answered very profoundly, but an essential condition of
liquefaction of any gas can be stated in learned phraseology, after the
preliminary exposition of certain general properties of matter.
We may first set forth those general physical properties, then apply
them to answering the above question. Every known substance may exist
in either of three states, the solid, liquid or gaseous. For every
substance there is a critical temperature above which it can exist
only as a gas, and cannot be liquefied by any pressure, but below
which a suitable pressure will cause liquefaction. Below its critical
temperature a gas is called a vapor, above it a permanent gas. Now in
the free atmosphere some of the gases are never below their critical
temperatures and, therefore, cannot be liquefied by any pressure,
without special cooling; others are sometimes below their critical
temperatures and are then capable of liquefaction by sufficient
pressure, which however is not always found in free space, but can be
supplied by a compression pump; one other gas, that is water vapor,
is always below its critical temperature in the free atmosphere, and
therefore may always be turned into water by sufficient pressure at its
actual atmospheric temperature. Such sufficient pressure in the water
vapor actually occurs from time to time in all parts of the atmosphere
from the earth’s surface to the highest cirrus region, but more
frequently in the nimbus levels, a mile or two above the earth. Thus
at all parts of the lower atmosphere liquefaction of aqueous vapor is
sometimes observed, either as mist or rain, snow or ice particles, and
on the earth as dew or frost. In order to illustrate the above ideas
by numerical citation, the accompanying table is given, showing the
critical temperature and pressure of the chief gaseous constituents of
the atmosphere.
TABLE II
_Critical Temperature and Corresponding Pressure of Liquefaction for
the Chief Constituent Gases of the Atmosphere._
--------------+-------------+-------------
| Critical | Critical
Substance | Temperature | Pressure
| C. | Atmospheres.
--------------+-------------+-------------
Dry Air | -140 | 39
Nitrogen | -146 | 34
Oxygen | -118 | 50
Carbonic Acid | - 31 | 75
Argon | -120 | 51
Hydrogen | -242 | 20
Ammonia | 130 | 115
Water | +365 | 200
--------------+-------------+-------------
A glance at this table shows that for the pressures and temperatures
prevailing in our atmosphere most of the constituents are permanent
gases. The conspicuous exception is water which, when in the gaseous
state, always exists as a vapor, and never as a permanent gas, since
it never even approaches the critical temperature. Fortunately for all
life on earth the aqueous vapor condenses at very ordinary temperatures
and pressures, else there would be no rainfall for irrigation and
drinking. Fortunately also the other gases do not so precipitate, else
the world might be flooded with liquid nitrogen and oxygen, entailing
who knows what disastrous consequences.
After this digression on the composition of the atmosphere, we may
henceforth regard the aërial ocean as a mixture of two substances,
dry air and water; the first, a permanent gas; the second, a
variable element, existing at times in either the solid, liquid, or
vaporous state. For the sake of convenience we may first study the
dry atmosphere, then the moist. The dynamic properties of the dry
atmosphere may in large measure be deduced by an application of two
well-established laws of physics. These will be taken in order.
By careful investigation it has been proved that throughout a
considerable range of pressure and temperature the permanent gases
very approximately obey the following law; the volume of a permanent
gas varies directly as its absolute temperature and inversely as its
pressure. In other words the product of its pressure and volume equals
the absolute temperature multiplied by a numerical constant. This may
be expressed algebraically by the following formula:
_PV_ = _RT_ (1)
in which _P_ is the pressure and _V_ the volume of a given portion of
gas at the absolute temperature _T_, and _R_ is a numerical constant
for the gas in question.
The value of _R_ in the foregoing equation has been determined
experimentally for the component gases of the atmosphere, and for dry
air as a whole. For dry air, which, under such conditions as surround
the aëronaut, may be treated as a single uniform gas, the equation
applied to one kilogram gives R = _PoVo/To_ = 29.27, where _Po_, _Vo_,
_To_, are respectively the pressure, volume and temperature, in the
metric system, of the one kilogram of air under standard conditions;
i. e., _Po_ = 10,330 kilograms per square meter, being the normal
atmospheric pressure; _Vo_ = 1/1.293 cubic meter, being the volume of
one kilogram of dry air at normal pressure and freezing temperature;
_To_ = 273° C., being the absolute temperature of freezing. In passing,
be it said that the absolute temperature is that measured from the
absolute zero, which on the Centigrade scale is 273° below freezing, on
the Fahrenheit, 460.6° below freezing.
The second law referred to follows directly from the principle of the
permanence of mass. It is a general observation in physics that a given
portion of matter is of constant mass, however its pressure, volume,
temperature and other conditions may vary. In particular, the mass of a
given portion of matter always equals the product of its mean density
and volume, since density is defined as the amount of mass in the unit
volume. Expressing this physical law, or relation algebraically, gives
ρ_V_ = mass = ρ_o_, _Vo_, in which ρ, _V_, are the general symbols
for the density and volume of the given portion of matter under any
condition, while ρ_o_, _Vo_, are the specific values of ρ and _V_
observed for some one state and circumstance of the substance in
question. In particular, if the mass of air be unity, we may write:
ρ_V_ = 1 (2)
This relation, together with that expressed in equation (1), will
enable us to deduce many of the properties of dry air and of a dry
atmosphere.
First let us observe from equation (1) the effect, in turn, of keeping
constant one of the quantities _P_, _V_, _T_, while the other two vary.
The equation shows that if the temperature of a gas is kept constant
the volume is inversely proportional to the temperature. This is called
the law of Boyle and Mariotte from its two independent discoverers,
of whom Boyle seems to have been the first. As an example of Boyle’s
law, if any empty glass, or diving bell, be inverted over water,
then submerged deeper and deeper, the air within it will shrink with
increase of pressure, its volume becoming one half when the pressure is
doubled, one third when the pressure is trebled, etc. In particular, if
the pressure changes by one unit, the corresponding change of volume
is 1/_P_ part of that volume. For example, if a captive balloon is
anchored in air at constant temperature, while the barometric pressure
changes from 30.0 inches to 30.1 inches, the volume of the balloon
will contract 1/300 part of itself.
Again equation (1) shows that if the pressure of a gas is kept
constant, the volume is proportional to the absolute temperature. This
is the law of Charles and Gay Lussac, so called from its discoverers,
of whom Charles is thought to have been the first. As an example of
this law, if a captive thin rubber balloon is heated, or cooled, its
volume will vary directly as its absolute temperature. In particular,
if the temperature is changed one degree, the volume changes 1/_T_
part of itself. For example, if the temperature of a balloon in air of
constant barometric pressure is heated from 300° C. to 301° C., its
volume will expand 1/300 part of itself. Historically, be it said, this
law of Charles and the law of Boyle were discovered separately, then
combined, giving equation (1).
Still a third, though not independent relation may be read from
equation (1), thus: when the volume of a gas is kept constant, the
pressure is proportional to the absolute temperature. In particular, if
the temperature is changed one degree, the pressure varies accordingly
by 1/_T_ part of itself. For example, if an air tank or gas tank, in a
room at 500° F., changes one degree in temperature, its pressure will
change 1/500 part.
With minute detail these three conclusions from the general equation
(1) have been set forth and illustrated, because of their practical
importance. Other valuable results may be obtained by similar
reasoning. Thus equation (2) may be read; the volume of a unit mass of
any substance is the reciprocal of its density. Hence, if in the three
foregoing conclusions, the reciprocal of the density is everywhere
written for the volume, three new relations will be obtained which
are of frequent practical use. Two of them may be expressed in the
following important law; the density of a gas varies directly as its
pressure and inversely as its temperature. Useful applications of this
law in aëronautics suggest themselves at once.
By means of the various foregoing equations, the value of either one
of the four quantities _P_, _V_, _T_, ρ, representing respectively
the pressure, volume, absolute temperature, and the density, may be
obtained in terms of any two of the others. If then any two of the
quantities is observed, the others can be at once computed. If, for
example, the pressure and temperature of dry air be observed at any
point, its density can be computed from the formulæ, also its volume
per kilogram weight, and thence its volume for any other weight. It is
important therefore to be able to measure satisfactorily at least two
of the four quantities. In usual studies of the atmosphere the pressure
and temperature are observed directly. The method and instruments
employed for that purpose are too well known to require description
here.
In some speculations the pressure and temperature of the atmosphere
are assumed, and certain interesting conclusions drawn. For instance,
if the temperature is assumed constant throughout a dry atmosphere,
the fluid will obey Boyle’s law, and it can be easily shown that the
height of such a medium is the same whether it comprise much gas or
little.[58] Again assuming the temperature and pressure constant, the
height of the normal homogeneous atmosphere can be computed by dividing
the pressure per square unit by its weight per cubic unit. In this way
the height of the normal homogeneous atmosphere has been found to be
about five miles. But these are hypothetical cases, of purely theoretic
interest. In practice the temperature may, on the average, be assumed
to decrease 6° C. for each kilometer of ascent, and the pressures may
then be computed for various elevations by use of Boyle’s law, as done
for Table I.
This leads us to a study of the gaseous properties of moist air. By
moist air is meant a mixture of dry air and aqueous vapor in the form
of an invisible elastic gas. The definition does not comprise air
containing visible steam, or mist, or cloud, but clear moist air such
as one ordinarily breathes. The study of this mixture may be preceded
by a brief account of the gaseous properties of the vapor alone.
If water in sufficiently small quantity be introduced in a vacuum
bottle at any ordinary temperature, it will promptly evaporate,
forming an invisible gas known as aqueous vapor, filling the bottle
and exerting a uniform pressure on its walls, except for the minute
difference at top and bottom due to gravity. The vapor weighs 0.622 as
much as dry air having the same volume, temperature and pressure, or
quite accurately ⅝ as much. It obeys all the laws given above for
ordinary gases and dry air. But it has one singularity; at ordinary
atmospheric temperatures, it cannot be indefinitely compressed
without condensing to a liquid. In this respect it differs from the
chief components of the atmosphere, which at ordinary temperatures
can endure indefinite pressure without liquefaction. The ammonia and
carbon dioxide in the air can, it is true, be condensed by pressure at
their usual temperatures, but not by such pressures as occur in the
free atmosphere, thus still leaving aqueous vapor the one singular
constituent.
Reverting to the behavior of the water in the assumed vacuum
bottle at fixed temperature, it may be observed that the pressure
of the invisible vapor is directly proportional to the amount of
liquid evaporated. In other words, for any fixed temperature the
vapor pressure is directly proportional to its density. When this
density reaches a certain definite amount, dependent solely upon the
temperature, no further evaporation will occur, unless some of the
vapor condenses. The pressure of saturation for that temperature has
been reached, and any attempt to increase the pressure, by diminishing
the volume of the vapor, will cause liquefaction at constant
temperature.
If, however, the space is not saturated, the mass of vapor present may
be expressed as a percentage of the amount required for saturation at
that temperature. This percentage is called the relative humidity. Thus
if the relative humidity is seventy per cent, the actual mass of water
vapor present at the observed temperature is seventy per cent of the
maximum that can exist in the given space, at the given temperature. In
other words, the relative humidity is the ratio of the actual to the
possible humidity at a given temperature.
In like manner, for any given vapor pressure there is a definite
saturation temperature, known as the dew-point. If with constant
pressure the vapor is given various temperatures higher than the
dew-point, it will remain gaseous and invisible; but if it falls in
temperature to the dew-point, liquefaction occurs, and drops of water
appear on the inner wall of the vessel. Further cooling will entail
still further liquefaction and reduction of pressure; for the lower
the temperature the less the possible mass and pressure of saturation.
But for all temperatures, down to freezing and considerably below,
some vapor exists, and obeys the same laws as at higher temperatures.
When, however, saturation occurs below freezing, the vapor may be
precipitated as snow instead of water. This is a familiar phenomenon
in the free atmosphere.
The actual mass of water vapor present in a cubic unit of space
is sometimes called the absolute humidity. A formula giving the
absolute humidity _f_, in kilograms per cubic meter, for any observed
temperature _t_, and vapor pressure _e_, may be written as follows:
_f_ = 0.00106 _e_ / (1 + 0.00367 _t_)
in which _e_ is the vapor pressure in millimeters of mercury, and _t_
is the common Centigrade reading. As an illustration of the actual
values of the pressure, temperature and density of saturated water
vapor, for various conditions, the following table is presented:
TABLE III
_Temperature, Pressure and Density of Aqueous Vapor, in Metric
Measures._
-------------+--------------+-----------------
Temperature, | Pressure, | Density Kilos.
Centigrade. | Millimeters. | per cubic meter.
-------------+--------------+-----------------
-25 | 0.61 | .557
-20 | 0.94 | .892
-15 | 1.44 | 1.395
-10 | 2.15 | 2.154
- 5 | 3.16 | 3.244
0 | 4.57 | 4.835
+ 5 | 6.51 | 6.761
10 | 9.14 | 9.329
15 | 12.67 | 12.712
20 | 17.36 | 17.117
25 | 23.52 | 22.795
30 | 31.51 | 30.036
35 | 41.78 | 39.183
40 | 54.87 |
45 | 71.36 |
-------------+--------------+-----------------
Now by Dalton’s law, each gas or vapor in a mixture of several behaves
as if it were alone. Thus if the foregoing experiment be conducted in
a bottle containing various gases chemically inert to water, the same
mass of water will be evaporated, and exert the same uniform pressure,
in addition to those exerted by the gases. Now the density of each
gas or vapor present, will equal its mass divided by its volume, and
the density of the mixture will equal the total mass divided by the
volume. Furthermore, it is well known that aqueous vapor is less dense
than dry air at the same temperature and pressure. From this it is at
once evident that moist air, which is merely a mixture of dry air and
aqueous vapor, must be lighter than dry air at the same temperature and
pressure. This is true whether the two fluids compared be in closed
vessels or in the free atmosphere.
Accordingly in all precise dealing with the free air, whether involving
its buoyancy, its resistance, its energy or any other mass function,
its density as affected by the humidity must be taken into account.
This can be computed from the observed pressure, temperature and
relative humidity as revealed by well known instruments, the barometer,
thermometer and hygrometer. Thus from the observed temperature and
relative humidity, the mass of vapor present per cubic meter is read
from Table III, the reader, of course, multiplying the given tabulated
mass by the observed percentage of humidity. To this aqueous mass must
be added the mass of dry air present. Then the total mass per cubic
meter is the density.
Various formulæ are available for computing the density of moist
air from the readings of the three instruments mentioned above.
Also, tables have been worked out giving the density without further
calculation. Moreover, the density of free air may be directly
measured, accurately enough for most purposes, by means of a
densimeter. A simple formula for finding the density of moist air is
as follows:
ρ = 0.465(_b − e_)/_T_
in which _b_, _e_, are the pressures in millimeters mercury
respectively of the moist air and its vapor, as revealed by the
barometer and hygrometer.
In practice no great error will be made in assuming the relative
humidity to be fifty per cent. For the moisture content never exceeds
five per cent of the mass of the moist air, and hence in assuming a
fifty per cent relative humidity, when there is actually a maximum
or minimum humidity, the greatest possible error in estimating the
moisture content is 2.5 per cent of the mass of moist air. Now if 2.5
per cent of a mass of air be assumed to be aqueous vapor when all is
really dry air, or conversely if 2.5 per cent of the whole mass be
assumed as dry air when it is really aqueous vapor, an error of much
less than 2.5 per cent is made in estimating the true density. No error
at all would ensue if both air and vapor were of the same density; but
since one is ⅝ as heavy as the other, the possible error is ⅜ of
2.5 per cent, or 0.6 per cent. This is a negligible quantity in all
mechanical considerations, except where great accuracy is required.
When any gas changes density or volume it also changes temperature,
unless there be transfer of heat between it and its environment. When
change of volume occurs without such transfer of heat the expansion,
or contraction, is called “adiabatic;” when it occurs at constant
temperature, the expansion is called “isothermal,” the temperature
being kept uniform by suitable transfer of heat; when it occurs at
constant pressure it is called “isopiestic.” In either case work may
be done by the enlarging gas, if it press against a moving piston,
or yielding envelope of some kind; and conversely work may be spent
on the gas in compressing it either isothermally, adiabatically or
isopiestically.
If, for example, a balloon rises rapidly its contents will expand
adiabatically, pushing the envelope out in all directions against the
static pressure of the embracing atmosphere. Thus it will do work and
rapidly cool. But if it rapidly sinks, it will contract adiabatically
and grow warm, owing to the work done by the surrounding air in
compressing it. A like thing occurs when a great volume of air rises
or sinks quickly in the free atmosphere. In this case the change of
temperature is about 6° C. for each kilometer change of level, so long
as the air remains unsaturated. A familiar example of this effect in
Nature is manifested when an uprushing column of moist air chills, and
precipitates moisture, forming a cloud toward its top. Thus a lone
thundercloud in a clear sky may mark the upper part of such a column,
or upward vortex in the air. And contrarywise, a descending column may
absorb its visible moisture, causing it to become clear aqueous vapor,
and thus vanish from view.
CHAPTER XIV
GENERAL DISTRIBUTION OF HEAT AND PRESSURE
Having thus briefly examined the composition and certain gaseous
properties of free air, both dry and moist, we may now study the
atmosphere as a whole. We wish particularly to know of its distribution
of temperature and pressure; of its general and permanent circulation;
of its great periodic currents; of its vertical movements, and its
minor local winds with their pulsations of velocity and direction.
Fortunately much information is available, due both to governmental
and private research, though this was collected more for purposes of
meteorology than of aërial locomotion. Of late, however, attention has
been given to the aëronautic study of the atmosphere, which will, it is
hoped, prove valuable to the aërial navigator.
The movements of the atmosphere are due mainly to the sun’s heat and
to the rotation of the earth. The earth’s internal heat and the moon’s
attraction are other minor agencies, but these may be neglected by
comparison. The earth’s rotation also would be ineffectual in modifying
the aërial movements, except for the coöperation of the sun. Without
his influence the atmosphere, always stagnant, would simply rotate with
the globe, at constant angular velocity and uniformly graded density at
various levels. This evenness of density for any level is broken by the
solar radiation increasing the temperature and moisture, otherwise the
air would remain practically at a standstill.
Though the moisture by its lesser density causes some lightening of
the air at fixed temperature, this at most is hardly one per cent, as
already shown, and on the average is much less. Its effect, therefore,
is equivalent to less than that caused by a rise of temperature of
three degrees. But if precipitation occurs, an enormous amount of
stored sunshine, or latent heat, is liberated and applied to warming
the associated air. Thus each pound of vapor condensed may, by the
release of its thermal store, heat more than a ton of air one degree
in temperature, or more than half a ton of air two degrees, etc.
The actual number of pounds of air at constant pressure, raised one
degree Centigrade by the condensation of one pound of vapor at various
temperatures, is given in the following table:
TABLE IV
--------------------------------+-------+-------+-------
Temperature of condensation | 0° | 25° | 50°
Pounds of air heated one degree | 2550 | 2480 | 2407
--------------------------------+-------+-------+-------
The sun then is father of the wind. By uneven heating of the atmosphere
it disturbs the uniform density gradation that would otherwise exist.
Thus abnormal pressures are generated which disturb the repose of the
aërial sea, causing the fluid to flow from regions of excessive to
regions of defective pressure. Hence the study of insolation[59] and
temperature distribution is fundamental to the science of the winds.
Without detailed study, we may note the aggregate insolation received
by the earth, at various latitudes, and its general effect on
terrestrial temperature. The sun emits a nearly constant stream of
radiation, from year to year, which plays continuously upon the earth
as a whole, with an intensity which varies but slightly from month to
month, due to the slightly varying distances of the earth from the
sun. Owing to the sun’s seasonal wandering across the equator, the
insolation at any latitude varies considerably month by month, and
the polar regions receive much more light than if no such wandering
occurred. The total yearly insolation for every 5° of latitude is shown
in the following table from Hann, in which the unit is the amount that
the earth would receive in one day at the time of the equinox, if the
sun were at its mean distance from the earth:
TABLE V
_Annual Amounts of Insolation_
-----------+---------+------------
Latitude. | Thermal | Difference.
| Days. |
-----------+---------+------------
0° | 350.3 |
5° | 349.1 | 1.2
10° | 345.5 | 3.6
15° | 339.4 | 6.1
20° | 331.2 | 8.2
25° | 320.5 | 10.7
30° | 307.9 | 12.6
35° | 293.2 | 14.7
40° | 276.8 | 16.4
45° | 258.7 | 18.1
50° | 239.6 | 19.1
55° | 219.4 | 20.2
60° | 199.2 | 20.2
65° | 180.2 | 19.0
70° | 166.2 | 14.0
75° | 156.5 | 9.7
80° | 150.2 | 6.3
85° | 146.5 | 3.7
90° | 145.4 | 1.1
-----------+---------+------------
From this it appears that the equator receives nearly 2.5 times as much
heat yearly as the poles. Since, moreover, the equator enjoys nearly
constant insolation, while the polar regions suffer great variations of
heat, with the varying altitude of the sun, the equatorial atmosphere
is both much hotter and more equable than the poles, and high latitudes
generally. Thus at the equator the frost level stands constantly
at 18,000 feet, while in the middle latitudes it varies greatly in
height from season to season. If, for example, a circle be drawn to
represent the earth, and above it a line to indicate the mean altitude
of the frost level in July, the frost line starting at the equator
at an elevation of 18,000 feet will decline north and south, finally
touching the earth well toward the frigid zones. The levels for other
temperatures, above and below freezing, are similarly inclined downward
from the equator to north and south. Obviously these isothermal
levels vary with the varying season, and at any fixed time differ on
different longitudes. On the plane of any given latitude the frost
line varies much less in altitude, and so for the other isothermals.
This is particularly true at the poles and equator, and everywhere at
considerable altitude. If one voyaged around the earth at the equator
at an elevation of 5,000 feet, he should find the average temperature
about 65° F. In the temperate zone, following a line of latitude
at the same height, he should have a lower temperature, but still
comparatively equable. The average annual temperature of the earth’s
entire surface is about 60° F.
In practical meteorology the temperature is observed at many points
simultaneously over a wide stretch of the earth’s surface. These
are then plotted on a weather chart, and through all points of like
temperature are drawn lines known as isothermals. These lines not only
map the earth’s surface into regions of equal temperature, but they
also show the direction of fall or rise of temperature, and its space
rate of change. This rate is called the “temperature gradient,” and
when estimated straight across from isothermal to isothermal, that is
in the direction of liveliest change of temperature, it is the maximum
gradient. Such a map is very useful in forecasting the weather. It is
but a particular instance of the more general map conceived by the
physicist, exhibiting the thermal condition of the entire atmosphere by
means of a series of equal temperature surfaces one above the other.
Here, of course, the temperature gradient at any point is the space
rate of change of temperature in any direction, being zero along the
isothermal surface and greatest normal to it.
The vertical temperature gradient is of particular interest, since
it determines the condition of fluid equilibrium at any point in the
atmosphere when the level surfaces are isothermal. If, for example, a
balanced balloon or portion of air, on starting upward from any level,
cools faster than the environing stagnant air, it will become more
dense, and cease to ascend, in which case the atmospheric equilibrium
is stable. Again, if the ascending gas or air cools more slowly than
the surrounding medium, it will become less dense, and so continue
to ascend, in which case the atmospheric equilibrium at the point
is unstable. Thirdly, if the rate of cooling be identical for the
ascending gas and its surrounding medium, the equilibrium is neutral,
and the motion will be stopped by friction but unaffected by change
of buoyancy, since no such change can occur. Of these three states of
equilibrium, the stable is dominant above the cirrus level, while below
that level each state may be found, at various times, prevailing at
random in all parts of the world, but more generally the stable and
neutral states. When the unstable condition occurs at any locality and
any level, it is usually followed ere long by a commotion or upheaval
in the atmosphere, until the temperature gradient alters to the neutral
or stable.
Many observations have been made to determine the variation of
temperature along the verticle in various places and in different
seasons. From the temperature records obtained in 722 balloon
ascensions near Paris, France, the mean fall of temperature per 1000
feet up to 20,000 feet was found to be 2°.4 in winter, 2°.8 in spring,
2°.6 in summer, 2°.5 in autumn and 2°.6 for the year. Near Berlin 3°.1
for the year was found from 75 balloon ascensions, the rate being
nearly the same for the halves of the year. Fig. 44 gives the average
of 52 winter and 65 summer temperature gradients, taken at about 8 A.M.
by means of sounding balloons sent up at Munich, Strassburg, Trappe and
Uccle. It will be noted that in both summer and winter the temperature
falls rapidly with increase of elevation, up to ten or eleven
kilometers, but above twelve remains nearly constant for all altitudes.
The difference in temperature summer and winter is interesting, also in
its gradual diminution with altitude. Another striking feature is the
inversion of gradient shown at twelve kilometers elevation, where the
temperature ceases to diminish, and may even increase with altitude.
This region is known as the upper inversion level of the atmosphere, as
distinguished from other levels at or below three kilometers height,
known as lower inversions, where the temperature gradient is sometimes
reversed, though not so illustrated in the diagram.
Thus the atmosphere divides into three marked layers. The lower
layer, three kilometers deep, is the region of turbulence and storm,
the home of heavy rain clouds, lightning, wind gusts and irregular
temperatures. The middle layer, some seven kilometers thick, bounded
top and bottom by the upper and lower inversion levels, is a clear
region of steady-falling temperature, for the most part frigid—a region
of far reaching and rapid winds, sweeping eastwardly, except near the
equator, and bearing on their backs the frosty cirrus clouds. The upper
layer reaching from the cirri to the cosmic void, is always cloudless
and very frigid, with temperature nearly constant, or maybe slightly
increasing with elevation.
[Illustration: FIG. 44.—SUMMER AND WINTER AVERAGE VERTICAL TEMPERATURE
GRADIENTS.]
A striking peculiarity of these three regions is that the lower and
middle layers may freely intermingle with each other, but never with
the upper, or isothermal layer. Owing to its constant temperature, the
upper layer floats on its neighbor like oil on water.[60] If a mass of
dry air were forced up into it from below, with the natural cooling due
to adiabatic expansion, such mass would be denser than the surrounding
medium, and hence would promptly sink back to its initial position.
Thus whatever turmoil may vex the middle or lower region, it can at
most upheave the floor of the isothermal layer, leaving inviolate the
crystal depths of the empyrean.
We may now turn to the distribution of barometric pressure in
the atmosphere and the effect of its variation. In general, the
distribution is not very uniform, but it can be graphically pictured by
drawing a series of surfaces connecting all points of equal pressure.
These are called isobaric surfaces. In a stagnant uniformly heated
atmosphere, for example, these surfaces would lie one above the other
parallel to the ocean face; but where turmoil exists, and irregular
temperature distribution, the isobaric surfaces are bent into hills
and hollows of varied form. These surfaces not only map the aërial sea
into regions of equal pressure, but they also show the direction of
fall or rise of pressure, and its space rate of change. This rate is
called the “pressure gradient.” When estimated straight across from
surface to surface, that is, in the direction of the liveliest change
of pressure, it is the maximum pressure gradient. Along this normal
direction the air tends to flow with an acceleration proportional to
the gradient. The velocity thus acquired by any portion of air in
being pushed along the line of falling pressure, combined with its
velocity due to other causes, gives its true velocity. A most important
consideration, therefore, in a scientific study of the wind is the
pressure distribution.
In practical meteorology, observations of the barometric pressure are
made simultaneously at many points on the earth’s surface, and the
readings then plotted on a map, after “reduction to sea level.” This
reduction is made by adding to each barometric reading the weight of a
column of air between the barometer level and the sea level, according
to tables prepared for this purpose. Lines called “isobars”[61] are
then drawn, at regular intervals, through all points of like sea-level
pressure, the indicated change of pressure between consecutive isobars
on the U. S. weather map being usually one-tenth of an inch of mercury.
These exhibit at once, over the entire field of observation, the
horizontal pressure gradient reduced to sea level, and commonly called
the “barometric gradient.” In meteorology, the pressure normal to the
isobar is called the gradient, and is expressed in millimeters of
mercury per degree of a great circle. On the same weather chart are
mapped the isothermal lines and wind directions for all the stations
of the weather service. From these data and the reported moisture
conditions, the meteorologist forecasts the probable weather some hours
or days in advance.
No perfectly comprehensive formula can be given for the barometric
pressure at any place and altitude, but certain general laws may be
observed. Where, for example, the speed of the air is increased along
any level of an air stream, the pressure is lessened, and conversely.
Thus, if the wind blows squarely against the front of an isolated
house, the speed will be greatly checked at the center front, and
accelerated at both sides and over the roof, thereby increasing the
apparent barometric pressure on the front, and lessening it on the
sides and over the top. A similar effect may be observed when the air
flows round the hull and framing of air craft.
Again, if the atmosphere over any locality is heated appreciably more
than its environment, the heated column tends to expand upward and
overflow aloft in all directions toward the cooler neighborhood, thus
lessening the pressure throughout the heated column, and increasing
the pressure throughout the environing atmosphere laterally. When this
effect is marked the plotted isobars often form a series of closed
curves about the heated region, manifesting a pressure gradient at the
lower levels in all directions toward the heated area. This grouping of
the isobars exhibits the familiar low pressure area of the weather map.
On the other hand, if any locality be cooled appreciably more than its
environment, the cooled column sinks, so that the surrounding warmer
air aloft flows in over it, thereby increasing the pressure over the
cooled area, and diminishing it throughout the environment. The isobars
may then form a series of closed curves about the cooled region, with a
pressure gradient along the higher levels in all directions away from
the cooled area. Of course, if heat were the only agency disturbing
the earth’s barometric pressure, there should be a parallelism between
the heat and pressure gradients; but, as already noted, the speed
or momentum of the aërial currents is also a substantial agency in
modifying the pressure lines.
It is well to remember that, while the base of a warm column of air
may, due to the overflow aloft, have less pressure than the base of
the cool environing column which receives the overflow, the high part
of the column may have greater pressure than the equally high part of
the cool. For if the columns be initially of the same temperature and
pressure, heating one of them uplifts its levels of given pressure
above those of its neighbor. When the overflow begins, a partial
equalization of pressure levels occurs, but not a complete one so long
as the flow has any head.
An interesting hygrometric feature of these highs and lows may here
be observed in passing. As already explained, when a column of air
ascends it cools by expansion, and tends to precipitate its water
content as cloud or rain; and conversely, when the air sinks it heats
by compression, thus acquiring greater moisture capacity and tending
to clarify. As a consequence, the areas of low pressure and a rising
atmosphere are usually marked by clouds and rainfall, while the areas
of high pressure and falling atmosphere are marked by clear, or
clearing weather. In the low, damp areas, then, the air feels heavy
while it is really light; in the high and dry area the air feels light,
while it is really dense, and most favorable to air men for carrying
heavy loads in their balloons or flyers. Similarly when air flows over
a mountain range the ascending stream precipitates moisture, due to
cooling by expansion, while the descending stream, on the other side,
comes down hot and dry, due to compression.
A characteristic mechanical feature of the high and low pressure areas
is the closed circulation between them, involving practically the whole
atmosphere below the isothermal layer. If we conceive the entire globe
spotted with high and low areas, we may picture the air surging upward
in the lows, flowing outward under the isothermal layer, descending in
the highs, then flowing outward along the earth’s surface toward the
lows in a continuous cycle. Thus, chiefly is maintained the vast and
multifold circulation of the atmosphere over the entire world.
In general the motion is of a vortical nature, by which is meant that
the masses of air as they flow along stream suffer more or less change
of orientation in space, the rotation at times being so slight as to
be undetectable, and again so marked as to excite wonder, as in the
whirlwind. Many of these atmospheric vortices, even though varying
in diameter from a few yards to hundreds of miles, resemble in their
behavior the gyrating column of water in a common circular basin
emptying through an orifice at its bottom. If the water is very still
when the drain opens, the column descends with imperceptible, if any,
rotation; but if the column has an initial whirl, or angular velocity,
this is magnified as the water approaches the axis of the vortex,
the tendency of the mass being to preserve its angular momentum, or
fly wheel property. A like action obtains in the great atmospheric
vortices, though here the motion far from the axis may seem like a
straight-blowing wind, rather than part of a vast whirl covering
thousands of square miles.
But even if all the air started directly for the axis of the ascending
column, like still water in a basin, it would promptly acquire vortex
motion, because it flows on the surface of a rotating sphere. The
deflection so produced is evidently greatest at the poles, and for
other places equals the polar value multiplied by the sine of the
latitude. The effect is similar to what occurs when a basin, rotating
about a vertical axis and carrying water with the same angular
velocity, is opened at the bottom. In this case the water at once
begins to gyrate within the basin, as the particles move toward its
axis.
With these preliminary generalities we may proceed to study the more
prominent movements in the atmosphere.
CHAPTER XV
PERMANENT AND PERIODIC WINDS
The winds of the world are commonly classified as the permanent, the
periodic and the nonperiodic, according to their genesis and character.
Their chief features may be briefly outlined.
The most conspicuous and important aërial current on the globe is
the permanent double vortex playing between the equator and the
poles. The heated air of the equatorial belt, uplifted by expansion,
overflows beneath the isothermal layer toward the north and south,
thereby increasing the pressure in the higher latitudes sufficiently
to generate a surface inflow along the earth, and thus maintaining a
perpetual closed circulation which is felt all over the globe. The
main features of this motion have been determined mathematically by
Ferrel,[62] and summarized as follows:
“In the preceding part of this chapter it has been shown that,
if all parts of the atmosphere had the same temperature, there
would be a complete calm over all parts of the earth’s surface.
But that, in consequence of the difference of temperature
between the equatorial and polar regions of the globe, and
the consequent temperature gradient, there arise pressure
gradients and forces which give rise to and maintain a vertical
circulation of the atmosphere, with a motion of the air of
the upper strata of the atmosphere from the equator toward
the poles, and a counter current in the lower part from the
poles toward the equator, as represented by the arrows in the
following figure, and that this of course requires a gradual
settling down of the air from the higher to the lower strata in
the middle and higher latitudes and the reverse in the lower
latitudes. It has also been shown that in case the earth had
no rotation on its axis, this would be exclusively a vertical
circulation in the planes of the meridians without any east or
west components of motion in any part; but that, in consequence
of the deflecting forces arising from the earth’s rotation, the
atmosphere at the earth’s surface has also an east component
of motion in the middle and higher latitudes, and the reverse
in the lower latitudes, and that the velocities of the east
components increase with increase of elevation, so that at
great altitudes they become very much greater than those
at the earth’s surface; while those of the west components
decrease with increase of altitude up to a certain altitude,
where they vanish and change signs and become east velocities,
now increasing with increase of altitude to the top of the
atmosphere.
“It has been further shown that the deflecting forces arising
from the east components of motion of each hemisphere from the
earth’s surface to the top of the atmosphere, in the middle
and higher latitudes and of the upper part of the atmosphere
in the lower latitudes, drives the atmosphere from the polar
regions toward the equator, while those arising from the west
components of motion in the lower part of the atmosphere in
the lower latitudes, having a contrary effect, but small in
comparison with the other on account of the weakness of these
forces near the equator, tend to drive the air a little from
the equator toward the poles. There is, therefore, a depression
of the isobaric surfaces at all altitudes in the polar
regions, especially in the southern hemisphere, a much smaller
depression in the equatorial regions, and a bulging up of the
isobaric surfaces in the vicinity of the parallel of 30° in
the lower part of the atmosphere, the maximum being nearer the
equator as the altitude increases, as represented in Fig. 45,
but at high altitudes there is a minimum of barometric pressure
at the poles and a maximum at the equator.
[Illustration: FIG. 45.—GENERAL CIRCULATION OF THE ATMOSPHERE.]
“In the accompanying figure the solid arrows in the interior
part represent the resultant motions of the winds (longer
arrows indicating greater velocities), in case of an earth
with a homogeneous surface over both hemispheres, in which
the motions would be symmetrical in both and the same at all
longitudes, and the equatorial and tropical calm belts would
be situated at equal distances from each pole. The dotted
arrows indicate the strong, almost eastern motion of the air
at all latitudes at some high altitude, as that of the cirrus
clouds.
“The outline of the outer part of the figure represents an
isobaric surface high up where the bulging up near the parallel
of 30° disappears and the maximum pressure at the same altitude
is transferred to the equator. For lower altitudes the isobaric
surfaces have a bulging up at the parallel of 30°, and a slight
depression at and near the equator. The arrows in this part
represent the polar and equatorial components of motion, the
former above and the latter below, except near the earth’s
surface on the polar sides of the tropical calm-belts, where
there is a polar component of motion arising from the air’s
being pressed out from under the belt of high pressure. This,
perhaps, does not extend beyond the polar circles, beyond
which there can be little motion in any direction, except from
abnormal disturbances.
“For reasons given in §103, the actual mean position of the
equatorial and tropical calm-belts are not precisely as here
represented, but are all a little displaced toward the north
pole, and the polar depression of the isobaric surfaces is
greater in the southern than in the northern hemisphere.”
The conclusions from this approximate analysis are in the main
supported by observation, except as modified by the heterogeneity
of the earth’s surface. The sea-level distribution of barometric
pressure between the equator and poles, as found by Ross’ long series
of measurements, manifests a variation of about one inch of mercury,
with maxima at about 30° of latitude, north and south, as required
by Ferrel’s theory. As a further cause of the depression toward the
poles, may be mentioned the greater speed of the permanent east wind
with the consequent centrifugal lift in the atmosphere.
As to the general easterly direction of the winds at middle and
higher latitudes, that is well known from observation of the motion
of clouds and of the air near the earth. At the cirrus level the
velocity in those latitudes is almost exactly eastward. But the flow
in longitude, illustrated by the outer arrows in Fig. 45, has not
been fully determined by observation. Moreover, as Ferrel himself
showed, the unequal heating of continents and oceans sets up gradients
in longitude, especially in the northern hemisphere, thus adding
considerable disturbance to the general circulation. To this agency
must be added also the latitudinal shifting of insolation, due to the
annual march of the sun across the equator, entailing an oscillatory
seasonal shift of the hot belt, and therefore of the twin-hemispheric
cycle of the atmosphere.
Some currents of the general and permanent circulation are sufficiently
prominent to have special names, such as the _trade-winds_, the
_antitrade-winds_, the _prevailing westerlies_, and, in the lower
latitudes, the _calm belts_, where the flow is exceptionally feeble.
All these currents have been known to sailors since early times, and
have been of considerable importance in marine navigation. Eventually,
perhaps, they may be of like importance in aërial navigation.
The trade-winds are mild tropical surface currents of remarkably steady
speed and direction. Springing from the high-pressure belts in either
hemisphere, at about latitude 30°, they blow toward the equator with
increasing westerly trend. As shown in charts 46 and 47 for midwinter
and midsummer, the trade winds cover a large portion of the tropical
zones in both oceans, and shift slightly in latitude with the sun.
They are separated at the heat equator by the equatorial calm belts, or
doldrums, and are bounded north and south respectively by the calms of
Cancer and of Capricorn. Particularly interesting are the trade-winds
blowing from Spain to the West Indies, which favored Columbus on his
westward voyage, and which certain adventurous Germans have proposed
using to duplicate that memorable voyage, in air ships.
[Illustration: FIG. 46.—NORMAL WIND DIRECTION AND VELOCITY FOR JANUARY
AND FEBRUARY. (KÖPPEN.)]
The antitrade-winds, or counter trades, are lofty winds blowing over
and contrary to the trade winds. As some doubt regarding the direction
of these counter trades had existed, an expedition was sent in 1905, by
two distinguished meteorologists, Teisserenc de Bort of France, and A.
Lawrence Rotch of America, to explore the atmosphere above the tropical
Atlantic. Mr. Rotch has summarized their measurements and conclusions
as follows:[63]
“Pilot balloons, dispatched from the island of Teneriffe and
St. Vincent, were observed with theodolites at the ends of a
base-line, and in this way the heights at which the balloons
changed direction could be ascertained. Later the balloons
were sent up from the yacht itself, which steamed after them,
measurements being made of their angular elevation. The
observations which are plotted in Fig. 46 prove conclusively
the existence of the upper counter-trade. The courses of
the balloons are represented as if projected upon the
surface of the sea and show that the northeast trade-wind
extended only to the height of 3,200 or 4,000 meters, and
then gradually turned into a southerly current which, higher
up, came from the southwest. The width of the dotted band
represents approximately the varying velocity of the trade and
counter-trade. Similar proofs of the northwest trade-wind,
south of the equator were obtained by the same expedition
during the following year, but the above suffices to show that
it would be possible for an aëronaut in the ordinary balloon
to start from the African coast, or from some of the islands
in the trade-wind region, and, after drifting towards the
southwest, to rise a few miles into the current, which would
carry the balloon north and eventually northeast back to land.
Nevertheless, it does happen in certain atmospheric situations
over the tropical north Atlantic that the winds from the
general northwesterly direction prevail up to great heights
without any evidence of the return-trade. Near the equator the
winds are easterly up to the greatest heights which have been
attained.”
[Illustration: FIG. 47.—NORMAL WIND DIRECTION AND VELOCITY FOR JULY AND
AUGUST. (KÖPPEN.)]
[Illustration: FIG. 48.—TRADE AND COUNTER TRADE-WINDS.]
The prevailing westerlies are high-latitude surface winds of the
permanent circulation. In the southern hemisphere they are particularly
strong and steady owing to the comparatively unbroken stretch of
ocean. In the north also they are strong and persistent, but variable
in direction because of disturbances by local winds due to unequal
heating of tracts of land and sea. These features are well illustrated
in charts 47 and 48. Of particular interest in aëronautics is the
prevailing wind blowing from the United States to Europe, which has
been considered a suitable current for transoceanic balloon voyages.[64]
The periodic winds are those whose gradient alternates annually or
daily, due to annual or daily fluctuations of temperature on sloping
or on heterogeneous parts of the globe. The annually fluctuating winds
due to alternate heating and cooling of continents, or large land
areas, bear the general name of monsoon. Among diurnal winds the most
prominent are the land-and-sea breezes, and the mountain-and-valley
breezes. Both kinds are practically available in aëronautics; the
monsoons for long-distance travel, the diurnal winds for local use.