portion of the earth’s surface is periodically more heated above its
normal temperature, or average for the year, than the neighboring
region, the resulting abnormal temperature gradient causes a periodic
surface wind tending toward the excessively heated place, and a counter
wind above. That is, the cooler and heavier column of air sinking and
uplifting the lighter, results in a lowering of the common center of
gravity of the two columns of air, and thus furnishes the driving
power of the wind. For example, an island or a peninsula may be
considerably hotter by day and cooler by night than the surrounding
water; a continent may be much hotter in summer and much colder in
winter than the bordering ocean. Thus during the hot period a moist
wind blows landward; during the cold period a dry wind blows seaward.
If the land has vast and lofty slopes the uprush of air during the hot
period and the downrush during the cool period may be very powerful.
The currents so produced by the aggregate of local agencies, including
the deviation caused by the earth’s rotation, combine with the general
circulation of the atmosphere to form the actual wind of the place.
Thus the periodic current may conspire with the general circulation,
or oppose it; may intensify, weaken or obliterate it; may overmaster,
reverse or mask it completely.
Of the various continental monsoons of the globe the most powerful
spring from the annual flux and reflux of the atmosphere over the vast
declivities and table-lands of Asia. Here the conditions are especially
favorable. As the sun approaches Cancer, the burning deserts and high
plateaus, combining their force with the draft on the mountain sides,
generate a continental uprush that sucks in all the aërial currents of
the surrounding seas, hurling them aloft to the isothermal layer whence
they radiate as the four winds of heaven; for here at this season the
planetary circulation is disrupted, obliterated or reversed, appearing
merely as a perturbation of the monsoon at its height. In India the
force is particularly effective. Along the north the Himalayas stretch
1,300 miles in latitude, with an average height of 18,000 feet and
with sunburned areas on either side. North of this range are the lofty
plateaus of Thibet and Cashmere, south of it the desert of Gobi and
the borders of the Indian Ocean. Over this watery tract from beyond the
equatorial line, from the isles of Oceanica and from the wintry plains
of Australia, the air flows in with accumulated strength, sweeping the
Bay of Bengal and the Arabian Sea in a continuous gale bearing up the
mountain slopes incredible floods of water. Over the Arabian Sea in
summer the gale is so steady and swift that no ordinary ship can force
a passage from Bombay to the Gulf of Aden. Above the Bay of Bengal the
moist south winds, converging between the coast and headlands, pour
cloud laden up the Himalayan slopes, precipitating their whole vapor
in prodigious torrents seldom seen elsewhere. Khasia at this season
sustains a Noachian deluge, the rain at times falling nearly a yard
deep in one day and night.[65] Quite appropriately, therefore, the
summer monsoon over India, especially its component southwest wind from
the Arabian Sea, and southerly wind from the Bengal Bay and farther
east, is called the wet monsoon.
The winter monsoon of Asia, is the reverse of the summer one, both
in direction of gradient and in physical character. It is a cold
flood of air pouring from the frigid table-lands and wintry depths
of the desert, down the mountains and valleys in continual overflow
on all sides of the continent, and then far out over the sea, where
it reascends to complete its long cycle. In its descent all moisture
vanishes by heating, and no intensive temperature gradient occurs, as
in summer, to accelerate its gently modulated tide. In India the winds
from Cashmere and Thibet pour down the Himalayas toward the Arabian Sea
a clear current of air which unites with the trade-wind, increasing
its force, and forming the moderate winter monsoon of that region, or
as it is commonly called, from its lack of moisture, the dry monsoon.
The kinematic character, and the extent of both summer and winter
currents, are well portrayed in charts 47 and 48 for all the south and
southeast of Asia. Across the islands of Japan, it will be observed,
the winds blow in opposite directions summer and winter. In Siberia
the monsoon winds trend along her great rivers and valleys, generally
northward in the winter and the reverse in the summer, combining in
both seasons with the prevailing westerlies, due to the rotation of the
earth.
All the other continents have their monsoons, though less powerful
than those of Asia. In the great desert of Sahara, for example, there
is an ascending hot current in the summer, causing a strong indraught
from the Atlantic and the Mediterranean; but this is far less intense
than if its action were fortified by lofty slopes and table-lands. In
winter when the Sahara cools to nearly the oceanic temperature, little
monsoon effect is perceptible, and the general circulation continues
unperturbed. In Australia the monsoon influence is still feebler,
owing to the limited extent of the country and to the general lowness
and flatness of the land. Over parts of South America, the annual
ebb and flow of the atmosphere is considerable, particularly along
the northeastern coast, and in the whole Amazon Valley, whose aërial
currents in general conspire with the trade-winds, strengthening them
materially in the southern summer, though it is less in winter when the
continental temperature more nearly approximates that of the ocean. The
monsoons of North America have been described in some detail by Ferrel
as follows:
“On the continent of North America we have monsoon influences
similar to those of Asia, but not nearly so strong, because
the extent of the continent, and consequently the annual range
of temperature, are not so great. They are, for the most part,
not sufficiently strong to completely overcome and reverse the
current of the general circulation of the atmosphere, and so
to produce a real monsoon, but they cause great differences
between the prevailing directions of the winter and summer
winds.
“In the summer the whole interior of the continent becomes
heated up to a temperature much above that of the oceans on the
same latitudes on each side—indeed, above that of the Gulf of
Mexico and the Pacific Ocean on its southern and southwestern
borders. The consequence is that the air over the interior of
the continent becomes more rare than over the oceans, rises
up and flows out in all directions above while the barometric
pressure is diminished, and the air from all sides, from the
Atlantic on the east to Pacific Ocean on the west, the Gulf of
Mexico on the south, and the polar sea on the north, flows in
below to supply its place. On the east the tendency to flow in
is not strong enough to counteract the general easterly motion
of the air at the earth’s surface in the middle latitudes, and
to cause a westerly current, but it simply retards the general
easterly current and gives rise to a greater prevalence of
easterly winds along the Atlantic sea-coast during the summer
season....
“In winter the thermal conditions over the continent are
reversed. The interior of the continent is now the coldest
part, and it is especially colder than the surrounding oceans
at that season. It has also very high plateaus and mountain
ranges. The air, therefore, of the lower strata, and especially
those next the earth’s surface, now tends to flow out in all
directions to the warmer oceans and the Gulf of Mexico, and
especially to run down the long slope of plateau from the
Rocky Mountains into the Mississippi Valley. The effect over
the whole of the United States east of the Rocky Mountains
is to cause the winds, which otherwise would be westerly and
southwesterly, to become generally northwesterly winds, instead
of southerly and southwesterly ones, as in summer. There is not
a complete monsoon effect, but simply a great change between
summer and winter in the prevailing directions of the winds. In
Texas, however, and farther east along the northern border of
the Gulf, the effect is somewhat that of a complete monsoon. In
New England and farther south in the Eastern States the monsoon
effect is to cause the prevailing winds to be from some point
north of west, instead of south of west as in summer.
“In summer, Central America and Mexico have a much higher
temperature than that of the adjacent tropical sea on the
southwest, and having high mountain ranges and elevated
plateaus, there is consequently a strong tendency to draw in
air from the southwest at this season, which not only entirely
counteracts the regular trade-winds of these latitudes, but
even reverses them and causes southwest winds. The effect is to
cause in midsummer a large area here, extending far westward,
of calms and irregular and light winds, mostly southwesterly
ones, and an apparent widening of the equatorial calm-belt at
this season so as to make its northern limit reach up, along
the coast, nearly to the parallel of 20°. The effect is similar
to that in the Atlantic west of the Gulf of Guinea and Liberia,
except that it here appears to be some greater, and causes
a true monsoon effect, since during the winter the regular
northeasterly trade-winds prevail, but strengthened by the
reverse thermal conditions of the winter season. On the eastern
side, and over the western end of the Gulf of Mexico, there is
a somewhat regular monsoon effect, the prevailing winds being
easterly, or blowing toward the land, during the summer, and
the reverse in winter.
“Along the west coast of North America in the middle latitudes
there is a strong monsoon influence; for the interior of the
continent becomes heated in summer to a much higher temperature
than that of the southwesterly ocean, and hence a strong
current is drawn in from this direction, at right angles to
the general trend of the coast which, combining with the
general southwesterly winds of these latitudes in the general
circulation of the atmosphere, causes the strong and steady
westerly and southwesterly winds of this region during the
summer. Farther north, up toward Alaska, the summer monsoon
effect is combined with the current caused by the deflection
of the continent as well as the general easterly current of
high latitudes, so that the winds here are generally southerly,
but still have somewhat of a monsoon character, being southerly
and southwesterly in summer and easterly and southeasterly
during the winter.
“Along the northern coast of America, as along that of Siberia,
the monsoon tendency is to draw the air from the colder land
to the warmer ocean in winter, and the reverse in summer; and
these effects, combined with the general easterly motion of
the atmosphere in these latitudes, gives rise to prevailing
southwesterly winds in winter and northwesterly ones in summer.
The winter monsoon influence, however, is small here—much
more so than in Siberia, for the ocean contains so many large
islands that it has rather a continental than an oceanic winter
temperature; and besides, it has not the influence of a warm
current—such as the continuation of a part of the Gulf Stream
along the northern coast of Europe and Asia.”
Similar to the monsoons in essential nature are the diurnal winds of
seacoast and mountain side. They begin with the heating of the land
in the morning, attain their maximum intensity about mid afternoon,
or during the hottest of the day, and finally are reversed at night.
Besides being so much briefer than monsoons, they are also in general
feebler and less extensive. They may be quite noticeable on calm
days, especially in clear weather and in hot climates; but usually
they are masked or entirely overwhelmed where other marked currents
occur—currents due either to the general circulation or monsoons, or
other powerful disturbing agencies.
In land-and-sea breezes, which usually extend not far inland, there is
a surface inflow of sea air during the forenoon and early afternoon,
balanced by an outflow of warm air above, rising from the heated soil.
After sundown this is reversed, the chilled air from inland pouring
out to sea, while overhead the warmer sea air is forced landward at a
higher level. These currents are strongest where the diurnal range of
temperature is greatest and where the local topography is of suitable
configuration. Particularly favorable are steeply declining shores,
narrow bays and inlets, girded by mountains or lofty hills. During
the day heated air ascends such declivities with alacrity, like smoke
through an inclined flue, while at night, when cooled by radiation
and contact with the soil, it rushes torrentlike down the valleys and
hillsides, passing out to sea, often in sudden squalls that embarrass,
or endanger, small sailing craft. Circulatory currents like the above
have sometimes been used by aëronauts to carry them out to sea and back
again to land at a different level.
In like manner the mountain-and-valley winds may be used by the
skillful aëronaut. It is well known that these flow up the courses of
rivers, cañons and land slopes generally by day, but at night reverse
their course and pour down again with considerable force. For this
reason experienced hunters place their camp fires below tent in a
sloping valley. The strength of the breeze depends, of course, upon
the daily range of temperature, and the steepness and expanse of the
slope. Such winds are deftly used by the masters of soaring flight, the
great robber and scavenger birds, and no doubt may be used by men in
motorless aëroplanes, to gain elevation, and journey great distances
without expenditure of energy.
CHAPTER XVI
CYCLONES, TORNADOES, WATERSPOUTS
Besides the periodic winds so far treated, there are prominent aërial
movements having no regular course or season. These are the nonperiodic
winds which so exercise or perplex the weather forecaster and those who
confide in him. In general such winds are of a temporary character,
arising from an unstable condition of the air in some locality, or
from unequal heating, either of which causes may generate, or briefly
sustain, an updraught, with its attendant gyration. Owing to the
whirling character of such ascending currents, they have received
various significant names, such as cyclone, tornado, whirlwind; the
three terms applying to vortices in decreasing order of magnitude. Each
in turn may be treated briefly.
The cyclone is a temporary large gyratory wind. It may last a few hours
or a few days. It may measure fifty to a hundred miles across, or it
may measure more than a thousand miles. On the weather map it is in
general marked by a group of closed isobars, showing a considerable
pressure gradient toward a small internal area where the pressure is
a minimum. To an observer looking about the earth’s surface and lower
levels of the atmosphere, the cyclone appears merely as an ordinary
wind, accompanied perhaps by rain or snow. It is not a swiftly rotating
narrow column, or cone of air, like a tornado or whirlwind, full of
gyrating dust and débris.
The motive power of a cyclone, though in general due to the buoyancy
of heated air, may spring from more than one set of conditions. Notice
has already been taken of vortices due to a hot column of air at lower
barometric pressure than its lateral environment. Take another case.
If a dry atmosphere is of uniform temperature and pressure at various
levels, but has a vertical temperature gradient a little greater than
the normal cooling of an ascending gas, a portion of air started upward
in any casual way becomes warmer than its lateral environment, and
hence continues to rise until the unstable condition due to abnormal
temperature gradient ceases. Again, while the surface stratum is in
stable equilibrium, it may happen that the second mile of air is
abnormally hot, and the third mile abnormally cold, and thus a vortex
may occur in mid air, without disturbing the face of the earth.
Whatever be the initial atmospheric condition causing the vertical
uprush, the nature of the resulting circulation is in general that of
the cyclone, illustrated, in part, by the whirling vortex of water in
a basin. As the current ascends, an indraught occurs in all the lower
regions of air, and an outflow in all directions above, sometimes
at the height of a mile or two, again in all the region next to the
isothermal layer. As the earth has at all places above the equator
a component of rotation about the vertical line, it follows that in
northern latitudes all the air flowing toward the vortex is in a whirl
opposite in motion to the hands of a watch lying face upward, and all
the outflowing air above has a like angular motion, but gradually
diminishing until it is reversed. At the lower portion of the vortex
the air whirls inward and upward with increasing velocity, while
above, it whirls outward and upward, with waning velocity, thus moving
in a double-spiral path shaped like a cord wound on an hourglass. In
the constricted part, or neutral plane of the vortex, the air moves
neither outward nor inward, but spirals straight upward. To match the
upflow, and complete the closed circulation, there must be a downflow
on the exterior of the cyclone, and since the whirl is reversed
in direction, this outer mass of downflowing reverse-whirling air
embracing the cyclone is called the anticyclone.
Between the inner and outer vortex the air is comparatively calm and
the pressure is a maximum, with steepest gradient toward the center
of the cyclone. Also the air is calm just at the axis of the vortex,
while for some distance away its speed increases as the radius of its
whirl, so that the central mass rotates practically as a solid column,
thus still further lowering the pressure near the axis. This solidly
rotating central column of air is sometimes called the core of the
vortex.
High above the center of the cyclone, where perhaps the air is sucked
downward, clarified by compression, then whirled outward, the sky is
usually clear, or thinly fogged, while without this central patch are
heavy clouds. The obscure or clear central part is called the “eye[66]
of the storm.” Through this the cirrus clouds may sometimes be seen
high above, either stationary or radiating away, if the vortex extends
so high. Sailors on the deck of a vessel passing through a cyclone have
often noticed the eye of the storm overhead, perhaps ten or twelve
degrees in diameter, and with special clearness in the tropics. To the
white, feathery cirrus clouds, scurrying away radially from the top of
the vortex, they have given the name “plumes of the storm,” or “mares’
tails.” In sailing their vessel through the center of a cyclone, they
have observed the circulatory motion of the winds and clouds, and
frequently have found the deck covered or surrounded with cyclone
sweepings, such as land and water birds, insects, butterflies, etc.,
brought into the quiet core of the vortex from the incurving winds
beyond. Further details of the motion in a cyclone vortex are given as
follows by Ferrel, §178:
“In Fig. 49 is given a graphic representation of the resultant
motions and of the barometric pressures for both the surface
of the earth and for some level high up in the atmosphere and
above the neutral plane, where the motions in the vertical
circulation are outward from the center. The solid circles
represent isobars at the earth’s surface and the solid arrows
the directions, and in some measure, by their different
lengths, the relative velocities of the wind. The heavy circle
represents the circle of greatest barometric pressure at the
earth’s surface, say 765 mm., while the pressure of the outer
border is 760 mm., and the dividing line between the cyclone
and the anticyclonic gyrations. Within this limit the pressure
diminishes to the center, and the gyrations are cyclonic, and
the direction of the resultant of motion inclines in toward the
center, but beyond that limit the gyrations are anticyclonic,
and the direction of resultant motion inclines toward the outer
border of these gyrations. The heavy dotted circle represents
the circle of maximum pressure at some high level, and is much
nearer the center than that at the earth’s surface. It is
also the dividing line between the cyclonic and anticyclonic
gyrations at that level. The dotted arrows indicate the
directions and in some measure the relative velocities, of the
wind at this level. The arrows in the cyclonic part represent
the direction of the wind as declining outward, because the
plane here considered is supposed to be above the neutral
plane, where the radial component of motion is outward, but
for any level below the neutral plane the inclination is still
inward. The arrows are shorter above in the cyclonic part and
longer in the anticyclonic part than they are at the earth’s
surface, since the cyclonic gyratory velocities decrease and
the anticyclonic increase with increase of altitude.
[Illustration: FIG. 49.—VELOCITY DIAGRAM IN HORIZONTAL SECTION OF A
CYCLONE.]
“The upper part of the figure is a representation of a vertical
section of the air, very much exaggerated in altitude, in
which the solid curved line represents a section of an isobaric
surface near the earth’s surface, say of 740 mm. barometric
pressure. The lowest part corresponds with the center of the
cyclone and the highest part with the heavy circle in the
lower part of the figure, and the steepest gradients with the
longest solid arrows, since the greater the gyratory velocities
at the earth’s surface the greater the gradients, though they
are not strictly proportional. The second dotted curved line
from the top represents a section of the isobaric surface of
high altitudes, in which the highest parts correspond with
the heavy dotted circle below, since the highest pressure at
all altitudes is very nearly where the cyclonic gyrations
vanish and change to the anti-cyclonic. The depression here
is smaller because the cyclonic area is smaller, and the
gyratory velocities less, than at the earth’s surface. The
upper dotted line belongs to an isobaric surface still higher,
where the gyrations are supposed to be all anti-cyclonic, and
here, consequently, the greatest pressure is in the center, as
indicated by the curved line.
“As the interior of the whole cyclonic system is warmer
than the exterior, and consequently the air less dense, the
distances between the isobaric surfaces are necessarily greater
in the interior than the exterior part, and so, however much
the isobaric surface at or near the earth’s surface may be
depressed by the cyclone gyration there, at a considerable
altitude, if the temperature difference is great enough, it
must become convex instead of concave.
“The track of any given particle of air in a cyclone, resulting
from the vertical and gyratory circulation, is that of a
large converging and ascending spiral in the lower part, but
of a diverging and ascending spiral in the upper strata of
the atmosphere, and the nearer the earth’s surface the more
nearly horizontal is the motion, since the vertical component
gradually decreases and vanishes at the surface.
“The whole energy of the system by which the inertia of the
air and the frictional resistance are overcome and the motions
maintained, is in the greater interior temperature and the
temperature gradients, by which the circulation is maintained.
This being kept up, the deflections and gyrations are merely
the result of the modifying influence of the earth’s rotation,
which is not a real force, since it does not give rise to
kinetic energy, but merely to changes of direction.
“It must be borne in mind that the preceding is a
representation of the motions and pressures of a cyclone
resulting from perfectly regular conditions, in an atmosphere
otherwise undisturbed, and having a uniform temperature,
except so far as it is affected by the temperature disturbance
arising from the cyclonic conditions. Accordingly results so
regular are not to be found in Nature, but generally only rough
approximations to them.
“Since the wind inclines less and less toward the center of the
cyclone below the neutral plane and declines from the center
above it, the upper currents above this plane in a cyclone are
always from a direction, in the northern hemisphere, a little
to the right of that of the lower currents, when not affected
by abnormal circumstances.”
Observation of cyclones in Nature very well confirms the leading
features set forth on theoretical grounds. If the vortex pass centrally
over an observatory there is noted first a high barometer and calm air,
attended perhaps by scurrying cirrus clouds; next a rapidly falling
pressure and increasing wind, with dark clouds and precipitation,
commonly accompanied by thunder and lightning; then the hushing of
the storm to a dead calm, and low barometer and thinning or clearing
of the clouds overhead; then a rising barometer with renewed winds in
the reverse direction, and finally subsiding winds, rising barometer
and clearing weather. These phenomena are the more definitely presented
if the whirl is strong while its travel along the earth is slow. But
owing to their progressive easterly motions, cyclones in the north have
their moist hot southern masses elevated, chilled and precipitated
on their eastern fronts and beyond, while their rear experiences the
opposite action and is called the clearing side. Conversely in the
tropics the westerly moving cyclones have cloudy and wet rears, because
the easterly drift on high carries the precipitating masses toward the
rear. The general hygrometric appearance of a centrally passing cyclone
in middle latitude is thus described by Ferrel, §207:
“In the regular progression of a cyclone in the middle
latitudes somewhat centrally over a place, the cloud and rain
area of the front part, extending far toward the east, first
passes over, occupying a half-day, or a day and more, and
then the front part of the ring of dense cloud with a heavy
shower of rainfall. After this there are indications of a
clearing up, and even the sun may break through the cloud for
an hour or two; but presently there is an apparent gathering
and thickening of the cloud and a second shower. This is at
the time of the passage of the rear side of the ring of denser
cloud. After this there is the final clearing up.”
Except for special conditions, cyclones are never stationary, but drift
along with the general march of the atmosphere, like dimpling eddies in
a stately flowing river. In general, therefore, their trend is westward
in lower latitudes, eastward in middle and higher latitudes, with a
pace slow or swift according to the prevailing current. Notably also
they have a poleward trend. Thus, if the path extends from tropic to
temperate clime, it is frequently concave toward the east and sensibly
parabolic in form. This is markedly true of those swift-whirling, small
cyclones called hurricanes,[67] and particularly those vigorous ones
blowing past the West Indies and the Philippines, and those that vex
the Indian Ocean.
As to the speed of travel of cyclones, that may be judged, at least for
northern latitudes, from the accompanying table, taken from Loomis,[68]
and showing the average monthly rate of progression in miles per hour,
of cyclone centers over the United States, the Atlantic Ocean and
Europe. In general, beyond the tropics tall cyclones travel faster than
short ones, owing to the faster drift of the higher strata.
----------+----------+--------------+---------
| | Atlantic |
| United | Ocean Middle | Europe.
Month. | States. | Latitudes. |
----------+----------+--------------+---------
January | 33.8 | 17.4 | 17.4
February | 34.2 | 19.5 | 18.0
March | 31.5 | 19.7 | 17.5
April | 27.5 | 19.4 | 16.2
May | 25.5 | 16.6 | 14.7
June | 24.4 | 17.5 | 15.8
July | 24.6 | 15.8 | 14.2
August | 22.6 | 16.3 | 14.0
September | 24.7 | 17.2 | 17.3
October | 27.6 | 18.7 | 19.0
November | 29.9 | 20.0 | 18.6
December | 33.4 | 18.3 | 17.9
----------+----------+--------------+---------
Year | 28.4 | 18.0 | 16.7
----------+----------+--------------+---------
To find the actual speed of the wind at a place, of course, the linear
velocities of whirl and of translation must be combined; or, vice
versa, if one of these be known it can be graphically subtracted from
the observed wind velocity to find the other. This combination of two
wind components to find their resultant, or, vice versa, can easily be
done by laying off on paper, arrows of suitable length and direction
to represent the two known velocities, placing the head of one arrow
to the tail of the other, then completing the triangle, and taking its
third side to represent the required wind velocity, in magnitude and
direction. Obviously if the cyclone moves eastward, whirling oppositely
to the hands of a watch, the swiftest wind is on its right side,
which consequently is known as the dangerous side. In the northern
hemisphere, therefore, the rule for dodging a great whirlwind is to run
north, if that be practicable.
Stationary cyclones occur under favorable conditions. At least that
name has been applied to columns of hot air streaming up from a fixed
base, more or less circular. Every island in the ocean generates such
a vortex on a clear, hot summer day, since its temperature far exceeds
that of the surrounding water. All day long this uprush continues
whatever be the humidity. And if the soil slopes upward steeply, the
vortex is so much the stronger, particularly if the island be in a calm
region. Above such a tract the gulls and vultures, and possibly even
man, might soar all day without motive power. This condition and its
interesting possibility deserve investigation.
Cyclones may occur at any season, but in general they are most abundant
when the greatest temperature disturbances occur. The relative
frequency of tropical cyclones for various localities and for the
twelve months of the year is seen in the following table[69]:
_The Yearly Periods of Cyclone Frequency in Several Seas_
------------+---------+---------+----------+----------+-------+-------
| Arabian | Bay of | S. Indian| Java | China |
| Sea. | Bengal. | Ocean. | Sea. | Sea. |Havana.
------------+---------+---------+----------+----------+-------+-------
No. of years| 234 | 139 | 40 | -- | 85 | 363
No. of | | | | | |
cyclones | 70 | 115 | 53 | 12 | 214 | 355
| | |Piddington|Piddington|
Authority. |Chambers.|Blanford.|Thom and |and |Schuck.| Poey.
| | |Reid. |Thom. | |
------------+---------+---------+----------+----------+-------+-------
Jan | 6 | 2 | 17 | 25 | 2 | 1
Feb | 4 | 0 | 25 | 42 | 0 | 2
Mar | 3 | 2 | 19 | 8 | 2 | 3
April | 13 | 8 | 15 | 8 | 2 | 3
May | 18 | 16 | 7 | 0 | 5 | 1
June | 29 | 9 | 0 | 0 | 5 | 3
July | 3 | 3 | 0 | 0 | 10 | 12
Aug | 3 | 4 | 0 | 0 | 19 | 27
Sept | 4 | 5 | 2 | 0 | 27 | 23
Oct | 6 | 27 | 2 | 0 | 16 | 17
Nov | 14 | 16 | 7 | 0 | 8 | 5
Dec | 3 | 8 | 6 | 17 | 3 | 2
------------+---------+---------+----------+----------+-------+-------
The tornado is a slender cyclone or hurricane. It is usually but a
few yards or rods in diameter, and seldom exceeds one mile across its
active column, whereas a cyclone may cover an area of any size from
fifty to one or two thousand miles in diameter. Moreover, the cyclone
requires for its inception an extensive pressure gradient marked by
closed isobars, and once generated may last several days. A tornado per
contra may spring into action where the lateral pressure is uniform,
spend its force in a few moments, and leave a uniform barometric field
in its wake. In shape the tornado is usually of greater height than
width. The cyclone is far-flung laterally, but in height may not exceed
the narrow tornado, since both must terminate beneath the isothermal
layer, and commonly do not extend so high. Both vortices are caused
by the ascensional force of hot air. In both the air spirals in and
upward at the bottom, out and upward at the top, constantly cooling
by expansion, and finally descends on the outside to complete the
closed circulation. In general the tornado is the more violent and
destructive, though limited to a brief and narrow path. More aptly,
perhaps, the tornado may be called a slender hurricane of brief
duration; both of them being small cyclones, or aërial vortices, of
minor size and concentrated intensity. The relation of the tornado and
cyclone has been defined as follows, by Professor Moore:
“The cyclone is a horizontally revolving disk of air of
probably 1,000 miles in diameter, while the tornado is a
revolving mass of air of only about 1,000 yards in diameter,
and is simply an incident of the cyclone, nearly always
occurring in its southeast quadrant. The cyclone may cause
moderate or high winds through a vast expanse of territory,
while the tornado, with a vortical motion almost unmeasurable,
always leaves a trail of destruction in an area infinitesimal
in comparison with the area covered by the cyclone.”
Two initial conditions seem essential to the genesis of a substantial
tornado. In the first place, the atmosphere of its immediate locality
must have appreciable gyration. Of course, in all extra equatorial
regions the air has some incipient whirl due to the earth’s rotation,
and this whirl is magnified as the fluid is sucked into the vortex. But
the magnification may be slight owing to the brief lateral displacement
of the air feeding the tornado. If, however, the fluid be drawn from a
considerable distance, and have from local conditions some additional
whirl superadded to that due to the earth’s rotation, the gyratory
flow in the medium near the vortical axis may be very swift. On the
other hand, the additional whirl, due to local conditions, may tend
to neutralize that due to the earth’s component, thereby leaving a
very feeble gyration, if any. But in general the rotation of tornadoes
is observed to be in the direction of the earth’s component; to the
left north of the equator, to the right south of it. This observation
is doubtless the more striking because when the accidental local spin
conspires with the permanent terrestrial one, the resultant whirl
is intensified, while in the opposite case it is so enfeebled as to
attract scant, if any attention.
In the second place, the genesis of a tornado requires unstable
equilibrium in the local atmosphere. This instability, as in cyclones,
may arise from abnormal temperature gradation. Thus, if along any
vertical the temperature falls more than six degrees Centigrade for one
thousand meters ascent, a mass of air started upward will continue to
rise, since it cools less rapidly than the environing medium. In this
way there will ensue a continuous uprush of air so long as the unstable
state endures; and the action may be very vigorous if a large stratum
of air is greatly heated before it disrupts into the cold upper layers.
In general, the loftier the tornado the more violent it is, just as
the taller flue generates the stronger draft with the same temperature
gradient.
Dynamically, the tornado may be treated as a rotating pillar of air
in which each mass of fluid fairly retains its angular momentum. This
means that for any mass of the whirling air the radius of its path,
multiplied by its circular speed, remains a constant product; in
other words, the velocity of whirl varies inversely as the radius.
Accordingly, the circular velocity is exceedingly rapid where the
radius is very small. Now, when any mass runs round a circle its
centrifugal force is known to be directly as the square of the speed of
its centroid and inversely as the radius. But by the above assumption
the speed itself is inversely as the radius. Hence, the centrifugal
force varies inversely as the cube of the radius of the inflowing
mass of air. This centrifugal force, acting on the inner layers of
air of the rotating column, must be supported by the pressure against
them exerted by the outer layers as they pass inward. Thus there is a
strong barometric gradient from the remote still air toward the swiftly
whirling parts of the vortex.
It follows from the above argument that inside a tornado the barometric
pressure may be much below the normal; and it is easy to see that if
a barometer, starting from some point on the tornado base, be moved
vertically upward it must show a declining pressure, but if moved
upward and outward it may be made to show a constant pressure all the
way to the upper portion of the vortex. The instrument would thus
travel along an isobaric, bell-shaped surface opening upward. On a
series, therefore, of concentric circles on the base of a tornado,
we may erect a family of coaxial bell-shaped surfaces to mark the
points of equal pressure, and thus map out the isobars of the vortex.
Inside these coaxial surfaces reaching to earth, others of still
lower pressure may be drawn tapering downward to a rounded point and
terminating at various places on the axis. In an actual tornado one of
these infinitely numerous funnel-shaped isobaric surfaces may become
distinctly outlined and visible, if the air has sufficient moisture to
start precipitation when it reaches a surface of suitably low pressure.
This quite usually occurs in Nature, the funnel sometimes reaching to
earth, sometimes only part way, according to the pressure at which
precipitation begins, this pressure depending, of course, on the
percentage of humidity of the uprushing air.
The form of the funnel-like cloud ere it reaches the earth is
interesting. Being an isobaric surface, it would support in static
equilibrium a free particle resting on it and sharing its rotatory
motion. The lower rounded part of the funnel is parabolic, the upper
outer part hyperbolic; the two together delineating the well-known
Rankine double vortex of hydrodynamics. Students of hydrostatics know
that when a glass of water is spun round its axis at a fixed velocity,
the dimple observed is of parabolic form, and if frozen will sustain
in repose a small shot resting on its surface and whirling with it.
Similarly the lower part of the funnel is parabolic because in it the
air rotates, as one solid body, while the broader part of the funnel is
hyperbolic because in it the air has a speed inversely proportional to
its radius of motion.
If everywhere in a tornado the circular velocity of the inflowing air
were inversely proportional to the radius, as above assumed, the speed
near the axis would be indefinitely great. This cannot be admitted.
Practically, the inflow ceases when the centrifugal force of the
gyrating stratum equals the pressure urging it toward the axis. Within
this stratum is a column of air rotating everywhere with constant
angular velocity about the vortical axis, and thus having quite calm
air at its center. Outside this solidly rotating core the air spirals
radially inward and upward. Some idea of the stream lines in such
spiral flow may be obtained from Fig. 50 if a rapid circular motion be
added to the inward and upward velocity represented by the arrows.
In the foregoing discussion no account of friction was taken. Near the
earth’s surface this dampens the whirl and centrifugal force, so that
the air flows more directly into the vortex, while farther aloft the
centrifugal force near the axis so effectually checks the inflow as
to allow the central core of air to rush up nearly unimpeded, as in
a walled flue, taking its draught mostly from the lower part. As a
consequence, the upward speed of the heated air in the tornado tube may
be enormous, supporting in its stream objects of considerable mass.
[Illustration: FIG. 50.—FUNNEL-LIKE CLOUD SOMETIMES OBSERVED IN A
TORNADO.]
The true horizontal speed anywhere in a tornado is compounded of the
velocities of gyration and of translation, as in the cyclone. Hence the
advancing side may be considerably the swifter and more destructive,
particularly more destructive since the impact of air increases as
the square of the velocity. If the vortex were stationary it would
be equally dangerous on all sides, standing erect and symmetrical;
but it drifts with the whole mass of air, sometimes quite swiftly and
often with varying speed of travel at different levels; thus, in its
slenderest forms, appearing bent and not infrequently twisted, as
it advances writhing serpentlike through the sky. Furthermore, the
intensity of whirl may fluctuate momentarily, with consequent shifting
of the isobaric surface, including that one whose form is visible
by reason of incipient condensation; and thus the funnel-like misty
tongue appears to dart earthward as a foggy downshoot from the cloud
above, whereas its parts are really rushing upward at all times very
swiftly, whether visible or not. This agile protrusion of the nimbus,
now a tongue, now a dark and mighty tower, is the strenuous part of
the storm, the abominated “twister” which the Kansan farmer sedulously
shuns, or peeps at from a hole in the ground. Unwelcome, indeed, are
its visitations, when, with mickle and multitudinous roar, it claps
his house in sudden darkness, hurls it aloft and sows its sacred relics
over all the adjoining township, “that with the hurly-burly hell itself
awakes.”
Theory, as well as experience, accredits the tornado with vast energy
and power. For, suppose a surface stratum of air one mile in area
and one thousand feet thick to increase in absolute temperature one
per cent, thus uplifting the superincumbent atmosphere ten feet. The
total energy stored in this way equals the weight lifted multiplied by
its upward displacement. The weight is a ton per square foot and the
displacement is ten feet; hence the stored energy is ten-foot tons per
square foot of the heated tract, or about 280,000,000 foot tons for
the square mile of heated air. This is equivalent to the work of one
million horses for over a quarter of an hour. A goodly percentage of
this stored work may be converted into kinetic energy in the active
part of the dry tornado. It is the energy of a vast reservoir suddenly
gushing through a tall penstock. It is a colossal upward cataract, an
aërial Niagara, a Johnstown flood suddenly liberated and quickly spent.
A vortex of that description possesses enormous devastating power, for
it is endowed with four destructive elements: rapid onset for razing,
violent spin for distorting, swift uprush for lifting, low pressure for
disrupting. These four grim powers may operate at once and in accord.
When, for example, they assault a house, the horizontal blasts push and
wrench it on the foundation, the cellar air suddenly expanding puffs it
aloft, the internal air bursts its walls or windows, the uprush carries
its members on high and scatters them wantonly to the four winds.
These powers are abundantly attested by authentic reports from many
localities.
When the tornado appears as a misty column it is familiarly called
a “waterspout,” particularly if it appears over a sea or lake. As
already explained, the visible and cloudy portion of the column is
due to condensation of the aqueous vapor in the air, as it rushes
expanding and cooling into the low pressure part of the vortex. From
the lashed and rippling sea surface, where it upcones into the base of
the spout, some water is carried aloft as spray mingling with the mist
of the chilled vapor, but not necessarily in very large proportion,
and never rising in solid body to the cloud, as popularly supposed. On
the contrary, waterspouts, however massive and formidable looking, are
very tenuous, and may occur on land or water indifferently. Doubtless
they are better defined, more regular and more familiar over water,
and hence their name; but essentially they are vapor spouts, though
mingled at times with dust or spray. Owing to rapid precipitation of
the uprushing aqueous vapor, there may be heavy rainfall on all sides
of the waterspout, so that at sea it may be difficult for the observer
to ascertain how much of the downpour is salt water and how much is
fresh. On land the downpour is sometimes mingled with débris, and even
with live fish and frogs caught up from neighboring bodies Of water.
Copious hail also may fall with the rain, if the vortex be a lofty one.
[Illustration: FIG. 51.—VERTICAL SECTION OF THE ST. LOUIS, MO., TORNADO
OF MAY 27, 1896, SHOWING THE VORTEX TUBES IN A THEORETICAL, TRUNCATED,
DUMBBELL-SHAPED VORTEX.]
[Illustration: FIG. 52.—HORIZONTAL SECTION OF ST. LOUIS TORNADO OF MAY
27, 1896.]
The following description and analysis of a representative spout is due
to Professor Bigelow of the U. S. Weather Bureau:[70]
“The tornado may be illustrated by the St. Louis storm of May
27, 1896. It is a truncated dumbbell vortex out off at the
ground on the plane where the inflowing angle is about 30°.
This vortex is much smaller than the hurricane, although of
the same type. It is about 1,200 meters high and about 2,000
meters in diameter on the surface. The vortex tubes are shown
in Figs. 51 and 52. In these figures can be seen the vortex
tubes, geometrically spaced, through each of which the same
amount of air rises. The rotating velocity is greatest about
300 meters above the ground, but the dimensions are such as to
produce enormous velocities in the lower levels. The radius in
the outer tube is taken to be 960 meters, and the inner tube
55 meters. The radial inward velocity on the outer tube is—8
meters per second; on the outer tube the tangential velocity
is 13 meters per second, and on the inner 224 meters per
second; on the outer tube the vertical velocity is 0.27, and
on the inner tube it is 80 meters per second. On the outer
tube the total velocity is 15 meters per second, and on the
inner tube 270 meters per second. The volume of air ascending
in each tube is 774,500 cubic meters per second. On account
of the distortion of the theoretical vortex, due to the
cutting of the lower portion by the truncated plane, and to the
progressive motion of the whole system that constitutes the
tornado, there is difficulty in computing the pressure to fit
these observed velocities and radii.
“Tornadoes occur in the southern and southeastern quadrants of
areas of low pressure, along the borders of the cold and the
warm masses which entered into the structure of the cyclone.
When a cold mass is superposed upon a warm mass, as was the
case at St. Louis, a tornado will occur if the difference in
specific gravity be sufficient to inaugurate a violent mixing,
and the rotation be about a vertical axis, instead of about a
horizontal axis, as in the case of thunderstorms.”
[Illustration: FIG. 53.—VERTICAL SECTION OF SHORT TORNADO.]
The size and form of waterspouts alter greatly with the state of the
atmosphere. As Ferrel observes, they may vary “from that of a cloud
brought down over a large area of the earth’s surface in a tornado
where the air is nearly saturated with vapor and the general base of
the clouds very low, somewhat as represented in Fig. 53, to that which
occurs when the air is very dry, and when the tornadic action is barely
able to bring the cloud down from a great height into a slender spout
of small diameter, somewhat as represented in Fig. 54. Horner says that
their diameters range from 2 to 200 feet, and their heights from 30
to 1,500 feet. Dr. Reye states that their diameters on land, at base,
are sometimes more than 1,000 feet. Oersted puts the usual height of
waterspouts from 1,500 feet to 2,000 feet, but states that in some rare
cases they cannot be much less than 5,000 or 6,000 feet. On the 14th of
August, 1847, Professor Loomis observed a waterspout on Lake Erie, the
height of which, by a rough estimate, was a half mile, and the diameter
about 10 rods at the base and 20 rods above.
“Judge Williams, in speaking of the tornado of Lee’s Summit, where he
saw it, says: ‘It seemed to be about the size of a man’s body where it
touched the clouds above, and then tapered down to the size of a mere
rod.’”
[Illustration: FIG. 54.—VERTICAL SECTION OF A TALL TORNADO.]
When the tornado vortex is so tall and strong as to carry raindrops up
to freezing strata it is commonly known as a hailstorm. The congealing
occurs usually in those isobaric surfaces which dip down in the center
of the vortex, but reach only part way to the earth. As indicated
in Fig. 55, the clear aqueous vapor near the earth is condensed to
cloud on crossing an isobaric surface of sufficiently low pressure
and temperature; then it proceeds as mingled cloud and rain till it
crosses the freezing isobar into the region of snow and hail formation;
thence finally curves outwardly to stiller air and descends as a cloud
of mingled vapor, rain and frozen parts. Of this frozen shower one
part may come to earth as hail or rain, the snow and sleet melting on
the way; while another part may be redrawn into the swift uprush, and
carried aloft till its frozen drops, or pellets, have grown so large by
accretion as to plunge to earth by sheer bulk, even though they must
traverse a furious ascending wind. A good illustration from Nature of
this cycle in the center of a hailstorm is presented in the following
by Mr. John Wise, America’s adventurous pioneer balloonist:
“This storm originated over the town of Carlisle, Pa., on
the 17th of June, 1843. I entered it just as it was forming.
The nucleus cloud was just spreading out as I entered the
vortex unsuspectingly. I was hurled into it so quickly that I
had no opportunity of viewing the surroundings outside, and
must therefore confine this relation to its internal action.
On entering it the motions of the air swung the balloon to
and fro and around in a circle, and a dismal, howling noise
accompanied the unpleasant and sickening motion, and in a
few minutes thereafter was heard the falling of heavy rain
below, resembling in sound a cataract. The color of the cloud
internally was of a milky hue, somewhat like a dense body
of steam in the open air, and the cold was so sharp that my
beard became bushy with hoar frost. As there were no electric
explosions in this storm during my incarceration, it might
have been borne comfortably enough but for the seasickness
occasioned by the agitated air-storm. Still, I could hear
and see, and even smell, everything close by and around.
Little pellets of snow (with an icy nucleus when broken) were
pattering profusely around me in promiscuous and confused
disorder, and slight blasts of wind seemed occasionally to
penetrate this cloud laterally, notwithstanding there was an
upmoving column of wind all the while. This upmoving stream
would carry the balloon up to a point in the upper clouds,
where its force was expended by the outspreading of its
vapor, whence the balloon would be thrown outward, fall down
some distance, then be drawn into the vortex, again be carried
upward to perform the same revolution, until I had gone through
the cold furnace seven or eight times; and all this time the
smell of sulphur, or what is now termed ozone, was perceptible,
and I was sweating profusely from some cause unknown to me,
unless it was from undue excitement. The last time of descent
in this cloud brought the balloon through its base, where,
instead of pellets of snow, there was encountered a drenching
rain, with which I came into a clear field, and the storm
passed on.”
As might be expected the hailstones vary much in form, size and
quantity. If by chance any stones become slightly flattened they ride
level in the ascending current, and hence by aggelation grow most
rapidly on the periphery which is a line of diminished pressure. At
times they are more or less oval, and again they appear as fragments of
considerable masses of ice, broken perhaps by collision in the violent
parts of the tornado tube. Their great variety in shape and bulk may be
appreciated from the following extracts taken from the records of the
Signal Service:
[Illustration: FIG. 55.—VERTICAL SECTION OF A HAIL TORNADO.]
In _Professional Paper of the Signal Service No. 4_, describing the
tornadoes of May 29th and 30th, 1879, in Kansas, Nebraska, Missouri,
and Iowa, this passage occurs relative to a tornado at Delphos, Mo.:
“On the farm of Mr. Peter Bock, in the adjoining township of
Fountain, about 4 miles W. of the storm’s centre, and during
the hailstorm that preceded the tornado, masses of ice fell as
large as a man’s head, breaking in pieces as they struck the
earth. One measured 13 inches in circumference, another 15, and
a hole made by one that fell near the place of Mr. J. H. Kams
measured 7 inches across one way and 8 the other. This immense
fragment of aërial ice broke into small pieces, so that its
exact size could not be determined.”
The following description is given of the tornado that visited Lincoln
County, Neb., at that time:
“At first the hailstones were about the size of marbles, but they
rapidly increased in diameter until they were as large as hens’ eggs
and very uniform in shape. After the precipitation had continued about
fifteen minutes, the wind ceased and the small hail nearly stopped,
when there commenced to fall perpendicularly large bodies of frozen
snow and ice, some round and smooth and as large as a pint bowl, others
inclined to be flat, with scalloped edges, and others resembled rough
sea-shells. One of the latter, after being exposed an hour to the sun,
measured fourteen inches in circumference.”
The following was reported by the Signal Service observer at Fort
Elliott, Tex., 1888:
“A thunder-storm began at 4.10 P.M. and ended at 7.40 P.M.,
moving from southwest to northwest. Hail began at 5.18 P.M.
and ended at 5.26 P.M., the hailstones being spheroidal in
shape and about two inches in diameter; formation, solid snow.
The ‘break’ (hills) at the foot of the plains several miles
northwest of station were absolutely white with hailstones for
three hours after the storm. This was observed by everybody
at the station; on the morning of the 26th I walked down to
the Sweetwater Creek, three fourths of a mile distant, and saw
great banks of hailstones which had been washed down during the
night. The bottoms along the Sweetwater were literally covered
with banks of hailstones from six to eight feet in depth. It
was estimated that there was enough hail to cover ten acres to
a depth of six feet. The hailstones killed five horses which
were out on the prairie on a ranch six miles north of station.
The Sweetwater Creek was higher than ever known before, the
freshet destroying nearly the entire post garden. The high
water is supposed to have been caused by a ‘cloud-burst’ at
or near the foot of the plains, where the Sweetwater has its
source; there was only 0.36 inch of rainfall at the station. On
Sunday, May 27th, hailstones were collected on the banks of the
Sweetwater, which had been washed down and lay in drifts 6 feet
deep, actual measurement by the observer.”
When, after imprisonment and long sustention in a powerful tornadic
vortex, the accumulated rain or hail finally breaks through and pours
down to earth, in solid cataract, the phenomenon is commonly called
a cloud-burst. The foregoing example is a partial illustration.
The following is quoted from Espy, describing a cloud-burst near
Hollidaysburg, Penn., in which the water seems to have poured down
nearly in a solid stream:
“On examining the northern side of this ridge, large masses
of gravel and rocks and trees and earth, to the number of 22,
were found lying at the base on the plain below, having been
washed down from the side of the ridge by running water. The
places from which these masses started could easily be seen
from the base, being only about 30 yards up the side. On going
to the head of these washes they were found to be nearly round
basins from 1 to 6 feet deep, without any drains leading into
them from above. The old leaves of last year’s growth, and
other light materials, were lying undisturbed above, within an
inch of the rim of these basins, which were generally cut down
nearly perpendicularly on the upper side, and washed out clean
on the lower. The greater part of these basins were nearly of
the same diameter, about 20 feet, and the trees that stood in
their places were all washed out. Those below the basin were
generally standing, and showed by the leaves and grass drifted
on their upper side how high the water was in running down the
side of the ridge; on some it was as high as three feet. It
probably, however, dashed up on the trees above its general
level.”
Dry whirlwinds of moderate size, but sometimes of considerable
violence, frequently occur in clear weather when the percentage of
humidity is small and when the vertical temperature gradient is
unusually pronounced. In this case there may be strong agitation of
the air, rendered visible at the earth’s surface by light débris on
land, or boiling of the water at sea; but the main body of the tube is
invisible and free from mist except high up where precipitation begins,
capped by a growing patch of white cloud in a clear sky, and which
may gradually broaden and condense sufficiently to cause a shower of
rain. On land the dry whirlwind may be delineated as a tall column, by
whirling dust or sand. In this case, if the gyration is violent, the
central core may appear clean and clear owing to the centrifugal force
which keeps the grains out where they are balanced by the pressure of
the inrushing air. In such vortices the sand spout may appear to be
hollow as in the case of waterspouts whose interior cores are free
from cloud or condensed vapor. On the other hand, myriads of mild
transparent whirlwinds unmarked, except by down or humanly invisible
dust, or dim aërial refractions, may frisk and play in the boundless
sky unnoticed by the blunt eyes of men, yet constantly engaged in
generating or marshaling the clouds and in buoying upward the ponderous
eagles, the vultures and the whole brood of passive flyers whom we
have not yet learned to emulate. Thus when we remember that an upward
trend of air of scarcely one yard per second, and too feeble to support
a falling hair, is yet sufficient to carry the condor and albatross
without wing beat, it seems important to explore these minor vortices
and to ascertain their availability and practical usefulness for human
soaring.
CHAPTER XVII
THUNDERSTORMS, WIND GUSTS
Still another interesting kind of aërial disturbance is the familiar
heat thunderstorm. This is not synonymous with those electrified
tornadoes and cyclones which are accompanied by thunder and lightning,
sometimes of great violence. Most tornadoes are thunderstorms, but
not vice versa. The thunderstorm is not essentially a vortex, but
rather a wind squall marked by sudden changes of temperature and
pressure, bearing with it massive clouds fraught with rain, or hail,
and disruptive electric charges flashing frequently to earth, or
from point to point in the sky. Its approach is usually announced by
rumbling thunder and heavy black clouds along the horizon. Its duration
is brief, varying from a few minutes to an hour or two. Further
characteristics are thus expressed by Moore:
“On land, thunderstorms occur most frequently at specific hours
of the day or night, such as 3 to 5 in the afternoon or 9 to 10
in the evening and sometimes even at 2 or 3 A.M., but no such
diurnal period is observed in midocean. The phenomena usually
occur in a pretty regular order of succession. After several
hours of fair weather, with gentle winds, there comes a calm;
the cumulus clouds grow larger, the lower stratum of clouds is
seen to be moving rapidly; gusts of wind start up with clouds
of dust, rain is seen to be falling at a distance; the movement
of rain and dust shows that the wind is blowing out from this
rain cloud near the ground no matter which way the rainy region
is advancing; a few large drops fall from slight clouds and
then suddenly the heavy rain begins. Lightning that may have
occurred during the preceding few minutes becomes more frequent
and more severe as the rain increases. After the maximum
severity of rain and wind, the lightning also diminishes or
entirely ceases, and we are soon able to say that the storm has
passed by. If we watch its retreat from us in the afternoon
we shall see the rear of a great cumulus on which the sun is
shining, but through whose dark-blue curtain of cloud and rain
nothing save occasional lightning is visible. After the storm
has passed, the lower atmosphere soon becomes appreciably
cooler and drier, the sky is nearly clear of clouds, and the
wind has shifted to some other point of the compass than that
which prevailed before the storm.”
The genesis of thunderstorms is varied and manifold. In one simple
type, a large tract of heated air in the unstable state and with a high
percentage of humidity swells upward at the center, the ascending moist
air forming, at the precipitation altitude, a growing cloud which may
become very broad, dark and bulky, drifting along over the earth with
the prevailing current. Eventually rain begins to form, or may be hail
or snow, if the heated column reaches to a great height. The falling
shower cools the air from the cloud down to the earth, increasing its
density and materially weighting it with the descending liquid or
solid particles. The showery column then sinks, especially along its
inner part where it is maturest, thus causing an outrush of cool air
along the earth, the immediate forerunner and herald of the rain. This
outrushing current pushes upward the environing clear moist air, thus
forming new margins of massive cumuli around the older nimbus widening
within, showering, cooling and sinking. Thus the rain area is broadened
and propagated, sometimes with nearly equal speed in all directions,
but generally fastest in the direction of the most unstable condition,
or of the then prevailing drift of the atmosphere. Indeed, the forward
cloud ranks may far outspeed the wind, seeming by their imperious
bluster and gigantic gloom to commandeer new recruits, as if by magic,
out of the clear sky. Before this solemn mustering and turbulent front
of the storm the black vapors suddenly startled into visible shape,
rush buoyantly upward in ragged shreds, like smoke from unseen fires,
and quickly blend with the general array of compact cloud expanding
across the sky. Again, several thunderstorms, merged like a mountain
range in solid phalanx, may sweep abreast over a continent, with long
horizontal[71] roll, ever rising in front and upheaving the sultry
air, thus replenishing perpetually the ponderous cumuli which form
the vanguard of this far-flung and titanic march of the clouds. Such
a storm is usually powerful and persistent, commonly enduring until
the sun’s decline and the shades of night have cooled the lower air,
and thus allayed the commotion by enfeebling the forces that favor its
progress.
The speed of rise of the air beneath the base of the thunderhead is
a question of some interest in aëronautics. If the ascent be so much
as a foot or two per second, one may expect the vultures to prefer
soaring beneath the thundercloud during its formative period. Here
also the aëroplanist might attempt a record flight, if the cloud were
high enough to be out of his way. But if he ventured to penetrate the
base of the thunderhead, he might find the turmoil too irregular and
strenuous for his comfort.
Of like interest is the long aërial swell that leads the advancing
storm. When will aviators make this the theater of their adventurous
frolic, careering playfully before the brow of the tempest and the
harmless rage of the lightning, gay-winged heralds of the coming
tumult, sailing perhaps with slackened motive power, yet swift and
secure as the storm-riding petrels at sea?
Besides the winds and aërial currents commonly studied by
meteorologists, are the minor disturbances which affect more
particularly the wayfarers of the sky, whether birds or men. The
atmosphere quite usually is vexed with invisible turmoils; most
sensible, indeed, over rough territory, but conspicuous also above
the smooth terrene, and at all elevations from earth to the highest
cloudland. Before sunrise, and generally in weather uniformly overcast,
these miscellaneous and nondescript movements of the air are least
active, for any given speed of the general drift of the atmosphere;
but when the sun shines and the soil is nonuniformly heated, the
disturbances become most pronounced. A whole troop of playful zephyrs
rise and set with the sun, in addition to the diurnal winds already
studied. Over the dusty plain they reveal their presence and shape
in those coiling columns that constitute the safety vents of the
atmosphere, and obviate the disruptive violence of the uprush that
would occur should a considerable region of surface air become
excessively heated. Over the city, particularly in winter, the local
turmoils of the atmospheric surf are revealed in the play of a thousand
smoky columns, and better still, when it snows, by the incessant
swell and veering of the flaky flood whose surges and eddies bewilder
the vision by their complexity. Over the water the clouds of fog
and steaming vapor are the best index of the local zephyrs, where,
it must be remembered, the rising and veering of the vapor wreaths
accompany like motions in the atmosphere. Over the forest, field and
meadow the interminable wandering of thistle down and gauzy shreds of
vegetation, now fast, now slow, now high aloft, then sheer earthward,
indicate what erratic and perpetual motions prevail throughout the open
country even on the stillest days. In the deep bosom of the atmosphere,
the parallel ranks of the cirri all across the sky mark the crests
of undulations quite as regular and tumultuous as the billows of a
wind-swept sea; while the fierce seething and upsurging of the separate
cumuli manifest the operation of vortices of prodigious energy. These
visible billows and whirlwinds suggest an infinitude of transparent
ones hardly less powerful, at the various levels unmarked by clouds.
For wherever two streams of abnormally graded densities neighbor each
other, a readjustment may occur agitating the entire region with a host
of pulsations, squalls, cataracts and fountains which the bird and
navigator must parry with proportionate care and skill.
And it is because of the amazing resistance of these wandering zephyrs,
waves and eddies that they demand the attention of aëronauts; nay,
more, it is because of the substantial labor they can perform when
adroitly encountered and duly employed. For the simplest elements of
aërodynamic science make clear that a rising zephyr hardly strong
enough to support a falling leaf is adequate to sustain the heaviest
soaring birds and aëroplanes gliding swiftly through it. In fact, the
sailors of fast air ships feel a heavy impulse and distinct shock in
plowing those mild cross winds which, to the fixed observer, seem not
like blasts, but rather as gentle swells or harmless currents. These,
therefore, have been made the subject of investigation by various
students of aëronautics.
The first incentive to the instrumental study of the fluctuations
of the wind in speed and direction seems to have been the hope to
furnish a quantitative basis for various theories of soaring flight.
Pénaud,[72] in 1875, had explained this phenomenon by postulating
an upward current. Lord Rayleigh,[73] in 1883, had made the more
general assumption of a wind having either a variable speed or a
variable direction as a necessary and sufficient condition for such
flight. Marey,[74] in 1889, and Langley,[75] in 1893, gave elementary
qualitative explanations of soaring in a horizontal wind of variable
velocity, though neither adduced concrete data to prove that the feat
could be performed in an actual wind. Each and all of those theories
may be sound enough in the abstract, but to show that they represent
realities of art or Nature they should be applied to a concrete
instance of soaring of a machine or a bird of known resistance, in a
wind of known variability.
To such end the writer in 1892 devised an anemograph for recording
simultaneously the speed of the wind and its horizontal and vertical
components of direction, while Dr. Langley devised a very light and
delicate cup anemometer for recording the variations of wind speed in
a horizontal plane, but not the changes of direction. Both instruments
were set up in January, 1893, and both investigations were published
with the _Proceedings of the International Conference on Aërial
Navigation_ of that year; but neither investigation was pushed far
enough to prove conclusively the possibility of a particular bird
or model soaring in the particular wind recorded. The two together
did, however, reveal quite astonishing fluctuations of the wind in
both speed and direction, results that have since received ample
exemplification in the more extended records of other observers.
[Illustration: FIG. 56.—UNIVERSAL ANEMOGRAPH. (The vanes are high above
the point indicated by the break in the vertical pipe.)]
Fig. 56 shows the recording anemometer for speed and double direction
constructed by the writer in 1892. A large weather vane was firmly
strapped to a vertical pipe which turned freely on ball bearings and,
by means of a small crank actuating a chronograph pencil, recorded its
fluctuations on a long sheet of paper winding on the drum from a roll
behind. On top of the pipe and about fifteen feet from the ground,
was mounted a carefully balanced horizontal vane, from which a fine
steel wire ran down the axis of the pipe to a fixed pulley, thence
to a second recording pencil. A third pencil recorded the beats of a
pendulum, thus standardizing the speed of the paper. A fourth pencil,
not shown, was designed to record the turns of an anemometer mounted
near the top of the pipe. The records of the wind speed thus secured
are omitted for lack of standardization, as the experiments were
prematurely terminated.
[Illustration: FIG. 57.—RECORDS OF WIND VARIATION IN HORIZONTAL AND
VERTICAL DIRECTION.]
Typical records of the wind direction are shown in Fig. 57 in which
the circles represent the paths swept by the wind-vane cranks that
operated the corresponding pencils. Both vanes, as shown by their
diagrams, veered quite frequently ten degrees in a short interval of
time, and not seldom twenty to thirty degrees. Frequently, also, it was
observed, in scanning the various records, that a rise or lull in the
wind speed was accompanied by a corresponding variation in direction;
but the observations were not sufficiently numerous and extended to
establish this phenomenon as a general occurrence. But as it can be
shown theoretically that a horizontal stream of air of constant cross
section and uniform velocity at each section, can not greatly fluctuate
in velocity from point to point, without more pronounced changes of
density than the barometer records, it naturally follows that the
stream must broaden where the air speed lags, and narrow where it
accelerates; in other words, it follows that there must be some change
in direction. The records were taken in the middle of a clear open
space of two hundred acres at Notre Dame University on a sunless day in
January, 1893, when the temperature was 24° F., and the wind eight to
twelve miles per hour. Their application to the theory of soaring need
not be considered here.
Further studies of the wind pulsations were made by use of a toy
balloon attached to a long thread. The first trials are thus recounted
in the paper above cited:
“After some preliminary tests from the top of the Physical
Laboratory of the Johns Hopkins University, during the Easter
vacation of 1893, I ascended the Washington Monument at
Baltimore, where I paid out the exploring line at a height
of 200 feet. The wind was blowing toward the southeast at
the speed of 25 to 35 miles per hour, and the sky, which had
remained clear till 3 o’clock, was rapidly darkening, with
indications of approaching rain. The balloon, when let forth,
immediately fell to a depth of 30 or 40 feet, being caught
in the eddy of the monument, then presently encountering
the unbiased current, sailed in it toward the southeast,
approximately level with the spool end of the thread. After
the balloon had drawn out 100 feet of thread I checked it to
observe the behavior of this much of the exploring line. The
balloon rose and fell with the tossing of the wind, but did
not flutter like a flag, as it would do if formed of irregular
outline. Neither did the thread flutter, nor do I believe there
is ever a tendency in a line greatly to flutter in a current
as does a flag or sail. Presently I paid out 300 feet of the
exploring line, whereupon the waves in the thread became quite
remarkable. The thread then, as a rule, was never approximately
straight. Sometimes it was blown into the form of a helix of
enormous pitch; at other times into the form of a wavy figure
lying nearly in a single vertical plane; and again, the entire
exploring line should veer through an angle of 40° to 60°,
either vertically or horizontally. The balloon, of course,
seldom remained quiet for more than a few seconds at a time,
but tossed about on the great billows like a ship in a storm.
Quite usually the billows could be seen running along the
line from the spool to the balloon, and, as a rule, several
different billows occupied the string at one time.
“The observations just delineated, however curious they may
be, afford no adequate conception of the behavior of the air
currents over an open plane, nor at a great height above the
earth, because the Washington Monument at Baltimore stands but
100 feet above the surrounding buildings, which undoubtedly
send disturbances to a greater height than 200 feet. To
supplement these explorations, therefore, I determined to
have them repeated from the top of the Washington Monument at
Washington and the Eiffel Tower at Paris.”
Some months later in the year, the experiment was repeated at the top
of the Washington Monument in Washington, at a height of five hundred
feet. The balloon, with a stone attached, was paid out from the north
window of the monument till it reached the ground. Then the stone was
removed by an assistant who drew the balloon well away from the huge
eddy of the great shaft, and let it fly toward the east, drawing the
thread after it like a mariner’s log in the wake of a ship. When six
hundred feet of the thread had been let out, it was observed to veer in
all directions under the varying surges of the wind. These variations
seemed larger than could be expected from the wake of the shaft alone
near its summit, where it measures about thirty feet in thickness.
Such qualitative observations, though interesting and suggestive,
are not wholly satisfactory. The same may be said of the study of
air currents by aid of smoke from tall chimneys. The eddy about such
columns may extend to a considerable height above them, and the wake
is farreaching. The experiments would therefore best be made from high
open-work towers above plane country or a broad sheet of water.
A better method perhaps would be to liberate a pilot balloon, or
discharge a bomb giving a bright compact cloud, and to trace its path
by means of two cameras, as it floats from point to point in the
aërial current. The instruments, if suitably stationed, would give the
continuous space history of the floating object; that is, its actual
path and the speed at each part thereof, or, in other words, the
magnitude and direction of the velocity at each point. But, of course,
this method would not reveal the wind’s history at any given fixed
point, as recorded by the anemograph above described.
[Illustration: FIG. 58.—RECORDS OF WIND SPEED OBTAINED BY LANGLEY.]
Fig. 58 is a typical wind-speed record obtained by Langley in January,
1893, by means of a very light cup anemometer mounted eleven feet
above the north tower of the Smithsonian Institution, and 153 feet
from the ground. The abscissæ represent time in minutes, the ordinates
wind speed in miles per hour. The records were taken in cloudy weather
and in a south-southeast wind. Other records were taken during the
month of February, showing like deviations from the mean, though at
times more pronounced; for Dr. Langley noted that “the higher the
absolute velocity of the wind, the greater the relative fluctuations
which occur in it.”
It will be observed from this record that, when the average speed was
about twelve miles an hour, the extreme fluctuation was rarely one
third greater or less than that, and on the average varied hardly
one sixth. It must be further added that the air on approaching the
anemometer had traversed a mile of the lower residential section of
the city, then crossed the body of the Smithsonian building, which
itself is half as high as the tower. It should be expected, therefore,
that this wind was, other things equal, naturally more turbulent than
if flowing in from a level plain. This surmise is justified by the
more extensive records of wind speeds shown in meteorological records
taken respectively in clear and in obstructed places. On the other
hand, even in level places where no obstruction is visible for several
miles, the wind, though it may be steady at one time, can at another
time be gustier than that shown in Langley’s record, according to the
state of the weather; for the gusts are not all due to neighboring
obstacles, but may be transmitted from afar, even from the depths of
the atmosphere.
Assuming the wind speed at any instant to vary by one sixth of the
mean, its impactual pressure will then vary by thirty-six per cent of
the pressure of the mean wind, remembering that the pressure varies as
the square of the speed. This fluctuation of the impactual pressure
tallies fairly well with that found by Professor Marvin at the top of
Mount Washington, in 1890, by means of a pressure plate.[76] He found
the variation to be approximately thirty-five per cent of the mean
pressure. Professor Hazen, however, reports but little variation in
the wind speed in the free atmosphere well above the earth. In several
balloon ascensions he suspended from the basket a lead weight by means
of a cord to which was looped the thread of a toy balloon. He found
that the little balloon sometimes moved ahead if the weight sometimes
followed it, but that in general the relative motion was very feeble,
thus indicating that the fluctuations of the velocity in the depth of
the atmosphere at those times were very slight.[77] However this be for
such distances from the earth and its protuberances, the fluctuations
of wind speed found at meteorological stations sufficiently resemble
those reported by Dr. Langley. As corroborative evidence, the reader
may be referred to the wind records published in the _Interim Report_
for 1909, of the British Advisory Committee for Aëronautics.
Without the material evidence of commotion in the atmosphere, a
moment’s reflection will make clear that such turmoil must exist, even
over a vast, smooth plain, especially in bright weather, and more
particularly over bare ground in dry weather. For it is well known
that clear, dry air transmits radiation with very slight absorption,
when the sun is well toward the zenith, and hence that the temperature
in the depth of the atmosphere is but little changed from moment
to moment, due to the passage of sunlight. At the earth’s surface,
however, the air by contact with heating or cooling soil may change
temperature rapidly. The direct sunlight falling perpendicularly upon
a perfectly absorbent material transmits nearly two calories of heat
per minute to each square centimeter of the receiving surface. It
would, therefore, under favorable circumstances, elevate by nearly
two degrees C. per minute a layer of water one centimeter deep, or
a layer of air something over a hundred feet thick, if all the heat
falling on the assumed surface were communicated to the neighboring air
stratum. In practice, a large percentage of the incident sunlight is
reflected and radiated by the soil, into sidereal space without heating
the air. But every one per cent of it caught up by the air in contact
with the earth is sufficient to heat a layer roughly one foot thick
one degree per minute. Hence, unless the heated air streamed upward
continually, the layer next the earth would quickly be raised to a
very abnormal temperature, which would result in a violent uprush. The
gradual ascension of the surface air may take place in large or small
columns, or in both kinds at once. In either case, the composition of
the ascensional motion with the general movement of the wind due to
barometric gradient must cause gustiness and marked irregularity of
speed and direction.
Various causes have been assigned for the gustiness of the winds.
Ferrel and many other writers assume that the air, especially near
the earth, is full of small vortices rotating about axes of various
inclination. These whirls, on passing squarely across a weather vane,
cause it to point one way for a moment, then presently the opposite
way, while if they cross obliquely they cause a like sudden veering of
the vane, but less extensive.
Helmholtz has proved that in the atmosphere strata of different
densities come at regular intervals to be contiguous one above
the other, and thus to beget conditions favorable to the formation
of aërial waves, sometimes so large as to set the lower regions of
air into violent commotion and thereby generate the so-called gusty
weather. He has summarized as follows some of the important conclusions
of his dynamic analysis.[78]
“As soon as a lighter fluid lies above a denser one with well-defined
boundary, then evidently the conditions exist at this boundary for
the origin and regular propagation of waves, such as we are familiar
with on the surface of water. This case of waves, as ordinarily
observed on the boundary surfaces between water and air, is only to be
distinguished from the system of waves that may exist between different
strata of air, in that in the former the difference of density of the
two fluids is much greater than in the latter case. It appeared to me
of interest to investigate what other differences result from this in
the phenomena of air waves and water waves.
“It appears to me not doubtful that such systems of waves occur with
remarkable frequency at the bounding surfaces of strata of air of
different densities, even although in most cases they remain invisible
to us. Evidently we see them only when the lower stratum is so nearly
saturated with aqueous vapor that the summit of the wave, within which
the pressure is less, begins to form a haze. Then there appear streaky,
parallel trains of clouds of very different breadths, occasionally
stretching over the broad surface of the sky in regular patterns.
Moreover, it seems to me probable that this, which we thus observe
under special conditions that have rather the character of exceptional
cases, is present in innumerable other cases when we do not see it.
“The calculations performed by me show, further, that for the observed
velocities of the wind there may be formed in the atmosphere not only
small waves, but also those whose wave lengths are many kilometers
which, when they approach the earth’s surface to within an altitude
of one or several kilometers, set the lower strata of air into
violent motion and must bring about the so-called gusty weather. The
peculiarity of such weather (as I look at it) consists in this, that
gusts of wind often accompanied by rain are repeated at the same place,
many times a day, at nearly equal intervals and nearly uniform order of
succession.”
Commandant Le Clement de Saint-Marcq has drawn some interesting
conclusions from the hypothesis that an ordinary wind consists of a
uniform current on which is superposed periodic motions in the wind’s
main direction and also at right angles thereto. But he has not
established his hypothesis by adequate observations. He assumes the
pulsations to be simple harmonic motions, which of course they would be
if they were plane compressional waves; but at the same time he shows
that the fluctuations are too large to be compressional waves, with the
concurrent slight variations of the barometric pressure.
It is still a question whether the pulsations of the natural wind be
harmonic. If so, the speed records should be sine curves, and the to
and fro acceleration of any mass of moving air should be variable for
any given pulsation. But the few records available show in many parts a
constant acceleration of the wind speed throughout a particular swell
or lull of velocity, indicating that the pulsations are not generally
simple harmonic ones.
In scanning the wind-speed records published by Langley, so many
instances of uniform wind acceleration are noticed that one naturally
inquires whether the rate of gain of velocity be sufficient to sustain
in soaring flight an aëroplane or bird held to the wind solely by
its inertia, as Langley believed to be possible. The total forward
resistance of a well-formed aërial glider, or bird, may be taken as
one eighth of its weight; hence, if poised stationary in its normal
attitude of flight, it will just be sustained by a direct head wind
having a horizontal acceleration of one eighth that of gravity, or
four feet per second. Now, the most favorable parts of the record here
shown (Fig. 58) exhibit nowhere an acceleration so great as four feet
per second, and on the average far less than that, as may be proved
by sealing the diagram. Hence, the wind here recorded was wholly
inadequate to support by its pulsative force either bird or man. But
as this record is a fair representative of all those published by Dr.
Langley, it follows that such pulsations can at best merely aid in
soaring when happily and adroitly encountered; but that they cannot
fully sustain soaring at any level, much less during ascensional flight
to great altitudes, or migrational flight to vast distances. It still
remains, therefore, to ascertain what kind of aërial currents are
adequate to sustain those marvelous feats of soaring on passive pinions
which for ages have been the delight and wonder of all keen observers,
and which are of such enduring interest to mankind. This investigation,
however, appertains more particularly to the science of applied
aërodynamics.
APPENDICES
APPENDIX I
STRESS IN A VACUUM BALLOON[79]
_By A. F. Zahm_
As inventors frequently propose the construction of a vacuum balloon,
to secure buoyancy without the use of gas, it may be desirable to
estimate the strength of material required to resist crushing, say in a
spherical balloon.
The unit stress in the wall of a thin, hollow, spherical balloon
subject to uniform hydrostatic pressure, which is prevented from
buckling, is given by equating the total stress on a diametral section
of the shell to the total hydrostatic pressure across a diametral
section of the sphere, thus:
2π_rtS_ = π_pr^2_
in which _S_ may be the stress in pounds per square inch, _p_ the
resultant hydrostatic pressure in pounds per square inch, _r_ the
radius of the sphere, _t_ the wall thickness.
The greatest allowable mass of the shell is found by equating it to the
mass of the displaced air, thus:
4π_r_^2_t_ς_{1} = 4π_r_^3ς_{2}/3
in which ς_{1} is the density of the wall material, ς_{2} the density
of the atmosphere outside.
Now, assuming _p_ = 15, ς_{1}/ς_{2} = 6,000, for steel and air, the
equations give:
_S_ = 3_p_ς_{1}/2ς_{2} = 45 × 6,000/2 = 135,000 pounds
per square inch as the stress in a steel vacuum balloon.
For aluminum ς_{1} is less, but the permissible value of _S_ is also
less in about the same proportion.
The last equation shows that for a given material and atmospheric
environment, the stress in the shell or wall of the spherical balloon
is independent of the radius of the surface. It is also well known
that the stress is less for the sphere than for any other surface.
Hence, no surface can be constructed in which _S_ will be less than
3_p_ς_{1}/2ς_{2}. The argument is easily seen to apply to a partial
vacuum balloon, since a balloon of one nth vacuum will float a cover of
but one nth the mass and strength.
The above result was obtained on the assumption that the shell was
prevented from buckling. As a matter of fact, it would buckle long
before the crushing stress could be attained. We must conclude,
therefore, that while a vacuum balloon has alluring features, the
materials of engineering are not strong enough to favor such a
structure. Perhaps it is nearer the truth to say that such a project is
visionary, with the materials now available.
• • • • •
A like argument applies to the balloon reservoir in which it has
been proposed to compress the surplus gas taken from a balloon hull
on expansion of its contents by change of level or temperature. If a
given mass of gas obeying Boyle’s law be pumped into a receiver of
given shape and mass, the resultant stress in the receiver wall will be
independent of the size. Hence the material of the proposed reservoir,
if expanded to the size of the hull itself, will weigh the same, and
suffer the same increment of unit stress, for a given mass increment of
gas. Hence, instead of pumping the above-mentioned gas surplus from the
hull into the reservoir, this latter may be discarded and its mass of
material spread over the hull itself. This argument applies only if the
shapes of hull and reservoir be equally effective, as, for example, if
both be cylindrical.
APPENDIX II
AËRONAUTIC LETTERS OF BENJAMIN FRANKLIN
PASSY, Aug. 30, 1783.
On Wednesday, the 27th instant, the new aërostatic Experiment, invented
by Messrs. Montgolfier of Annonay, was repeated by M. Charles,
Professor of experimental Philosophy at Paris.
A hollow Globe 12 feet Diameter was formed of what is called in England
Oiled Silk, here Taffetas gommé, the Silk being impregnated with a
Solution of Gum elastic in Linseed Oil, as he said. The Parts were
sewed together while wet with the Gum, and some of it was afterwards
passed over the Seam, to render it as tight as possible.
It was afterwards filled with inflammable Air that is produced by
pouring Oil of Vitriol upon Filings of Iron, when it was found to have
a tendency upwards so strong as to be capable of lifting a Weight of 39
Pounds, exclusive of its own Weight which was 25 lbs and the Weight of
the Air contain’d.
It was brought early in the morning to the Champ de Mars, a Field in
which Reviews are sometimes made, lying between the military School and
the River. There it was held down by a Cord till 5 in the afternoon,
when it was to let loose. Care was taken before the Hour to replace
what Portion had been lost, of the inflammable Air, or of its Force, by
injecting more.
It is supposed that not less than 50,000 People were assembled to see
the Experiment, The Champ de Mars being surrounded by multitudes, and
vast Numbers on the opposite Side of the River.
At 5 O’clock Notice was given to the Spectators by the Firing of two
Cannon, that the Cord was about to be cut. And presently the Globe was
seen to rise, and that as fast as a Body of 12 feet Diameter, with a
force only of 39 Pounds, could be suppos’d to remove the resisting Air
out of its Way. There was some Wind, but not very strong. A little
Rain had wet it, so that it shone, and made an agreeable appearance.
It diminished in Apparent Magnitude as it rose, till it enter’d the
Clouds, when it seem’d to me scarce bigger than an Orange, and soon
after became invisible, the Clouds concealing it.
The multitude separated, all well satisfied and delighted with the
Success of the Experiment, and amusing one another with discourses of
the various uses it may possibly be apply’d to, among which many were
very extravagant. But possibly it may pave the Way to some Discoveries
in Natural Philosophy of which at present we have no conception.
A Note secur’d from the Weather had been affix’d to the Globe,
signifying the Time & Place of its Departure, and praying those who
might happen to find it, to send an account of its state to certain
Persons at Paris. No News was learned of it till the next Day, when
information was received that it fell a little after 6 o’clock, at
Gonesse, a Place about four Leagues Distance, and that it was rent
open, and some say had ice in it. It is suppos’d to have burst by the
Elasticity of the contain’d Air when no longer compress’d by so heavy
an Atmosphere.
One of 38 feet Diameter is preparing by Mr. Montgolfier himself, at the
Expence of the Academy, which is to go up in a few days. I am told it
is constructed of Linen & Paper, and is to be filled with different
Air, not yet made public, but cheaper than that produc’d by the Oil of
Vitriol, of which 200 Paris Pints were consum’d in filling the other.
It is said that for some Days after its being fill’d the Ball was found
to lose an eighth Part of its Force of Levity in 24 Hours; Whether this
was from Imperfection in the Tightness of the Ball, or a Change in the
Nature of the Air, Experiments may easily discover....
M. Montgolfier’s Air to fill the Globe has hitherto been kept secret;
some suppose it to be only common Air heated by passing thro’ the Flame
of burning Straw, and thereby extreamly rarefied. If so, its Levity
will soon be deminish’d by Condensation, when it comes into the cooler
Region above....
P. S. I just now learned that some observers say, the Ball was 150
Seconds in rising, from the cutting of the Cord till hid in the Clouds;
that its height was then about 500 Toises, but, being moved out of
the Perpendicular by the Wind, it had made a Slant so as to form a
Triangle, whose base on the Earth was about 200 Toises. It is said the
Country People who saw it fall were frightened, conceiv’d from its
bounding a little, when it touched the Ground, that there was some
living Animal in it, and attack’d with Stones and Knives, so that it
was much mangled; but it is now brought to Town and will be repair’d.
The great one of M. Montgolfier is to go up, as is said, from
Versailles, in about 8 or 10 days. It is not a Globe but of a different
Form, more convenient for penetrating the Air.
It contains 50,000 cubic Feet, and is supposed to have Force of Levity
equal to 1,500 pounds weight. A Philosopher here, M. Pilâtre du Rozier,
has seriously apply’d to the Academy for leave to go up with it, in
order to make some experiments. He was complimented on his Zeal and
Courage for the Promotion of Science, but advis’d to wait till the
management of these Balls was made by Experience more certain & safe.
They say the filling of it in Montgolfier’s Way will not cost more
than half a Crown. One is talk’d of to be 110 feet Diameter. Several
gentlemen have ordered small ones to be made for their Amusement.
One has ordered four of 15 feet Diameter each; I know not with what
Purpose; but such is the present Enthusiasm for promoting and improving
this Discovery, that probably we shall soon make considerable Progress
in the art of constructing and using the Machines.
Among the Pleasanteries Conversation produces on this subject, some
suppose Flying to be now invented, and that since Men may be supported
in the Air, nothing is wanted but some light handy instrument to give
and direct Motion. Some think Progressive Motion on the Earth may
be advanc’d by it, and that a Running Footman or a Horse slung and
suspended under such a Globe so as to have no more of Weight pressing
the Earth with their Feet, then Perhaps 8 or 10 pounds, might with a
fair Wind run in a straight Line across Countries as fast as that Wind,
and over Hedges, Ditches & even Waters. It has been even fancied that
in time People will keep such Globes anchored in the Air, to which by
Pullies they may draw up Game to be preserved in the Cool & Water to be
frozen when Ice is wanted. And that to get Money, it will be contriv’d
to give People an extensive View of the Country, by running them up in
an Elbow Chair a Mile high for a Guinea, &c., &c.
B. FRANKLIN.
PASSY, Nov. 22d, 1783.
... Enclosed is a copy of the Proces verbal taken of the Experiment
yesterday in the Garden of the Queen’s Palace la Muette, where the
Dauphin now resides, which being near my House I was present. This
Paper was drawn up hastily, and may in some Places appear to you
obscure; therefore I shall add a few explanatory Observations.
This Balloon was larger than that which went up from Versailles and
carried the Sheep, &c. Its bottom was open, and in the middle of the
Opening was fixed a kind of Basket Grate, in which Faggots and Sheaves
of Straw were burnt. The Air rarefied in passing thro’ this Flame rose
in the Balloon, swell’d out its sides, and Fill’d it.
The Persons who were plac’d in the Gallery made of Wicker, and attached
to the Outside near the Bottom, had each of them a Port thro’ which
they could pass Sheaves of Straw into the Grate to keep up the Flame,
& thereby keep the Balloon full. When it went over our Heads, we could
see the Fire which was very considerable. As the Flame slackens, the
rarefied Air cools and condenses, the Bulk of the Balloon diminishes
and it begins to descend. If those in the Gallery see it likely to
descend in an improper Place, they can by throwing on more Straw, &
renewing the Flame, make it rise again, and the Wind carries it farther.
One of these courageous Philosophers, the Marquis d’Arlandes, did me
the honour to call upon me in the Evening after the Experiment, with
Mr. Montgolfier, the very ingenious Inventor. I was happy to see him
safe. He informed me that they lit gently, without the least Shock, and
the Balloon was very little damaged.
This method of filling the Balloon with hot Air is cheap and
expeditious, and it is supposed may be sufficient for certain
purposes, such as elevating an Engineer to take a view of an Enemy’s
Army, Works, &c., conveying Intelligence into, or out of a besieged
Town, giving Signals to distant places, or the like.
The other method of filling a Balloon with permanently elastic
inflammable Air, and then closing it is a tedious Operation, and
very expensive; Yet we are to have one of that kind sent up in a few
days. It is a Globe of 26 feet diameter. The Gores that compose it
are red and white Silk, so that it makes a beautiful appearance. A
very handsome triumphal Car will be suspended to it, in which Messrs.
Roberts, two Brothers, very ingenious Men, who have made it in concert
with Mr. Charles, propose to go up. There is room in this Car for a
little Table to be placed between them, on which they can write and
keep their journal, that is, take Notes of everything they observe, the
State of their Thermometer, Barometer, Hygrometer, &c., which they will
have more leisure to do than the others, having no fire to take care
of. They say they have a contrivance which will enable them to descend
at Pleasure. I know not what it is. But the Expence of this machine,
Filling included, will exceed, it is said, 10,000 Livres.
This Balloon of only 26 feet diameter, being filled with Air ten times
lighter than common Air, will carry up a greater Weight than the other,
which tho’ vastly bigger, was filled with an Air that could scarcely be
more than twice as light. Thus the great Bulk of one of these Machines,
with the short duration of its Power, & the great Expence of filling
the other will prevent the Inventions being of so much Use as some may
expect, till Chemistry can invent a cheaper light Air producible with
more Expedition.
But the Emulation between the two Parties running high, the Improvement
in the Construction and Management of the Balloons had already made
a rapid Progress; and one cannot say how far it may go. A few Months
since the idea of Witches riding thro’ the Air upon a Broomstick, and
that of Philosophers upon a Bag of Smoke, would have appeared equally
impossible and ridiculous.
These Machines must always be subject to be driven by the Winds.
Perhaps Mechanic Art may find easy means to give them progressive
Motion in a Calm, and to slant them a little in the Wind.
I am sorry this Experiment is totally neglected in England, where
mechanic Genius is so strong. I wish I could see the same Emulation
between the two Nations as I see between the two Parties here. Your
Philosophy seems to be too bashful. In this Country we are not so
much afraid of being laught at. If we do a foolish thing, we are the
first to laugh at it ourselves, and are almost as much pleased with
a Bon Mot or a Chanson, that ridicules well the Disappointment of a
Project, as we might have been with its Success. It does not seem to me
a good reason to decline prosecuting a new Experiment which apparently
increases the power of a Man over Matter, till we can see to what use
that power can be applied. When we have learnt to manage it, we may
hope some time or other to find Uses for it, as men have done for
Magnetism and Electricity, of which the first Experiments were mere
Matters of Amusement.
This Experience is by no means a trifling one. It may be attended with
important Consequences that no one can foresee. We should not suffer
Pride to prevent our progress in Science.
Beings of a Rank and Nature far superior to ours have not disdained
to amuse themselves with making and launching Balloons, otherwise we
should never have enjoyed the Light of those glorious objects that rule
our Day & Night, nor have had the Pleasure of riding round the Sun
ourselves upon the Balloon we now inhabit.
B. FRANKLIN.
PASSY, Dec. 1, 1783.
In mine of yesterday I promised to give you an account of Messrs.
Charles & Roberts’ Experiment, which was to have been made this Day,
and at which I intended to be present. Being a little indispos’d, &
the Air cool, and the Ground damp, I declin’d going into the Garden
of the Tuilleries where the Balloon was plac’d, not knowing how long
I might be oblig’d to wait there before it was ready to depart; and
chose to stay in my Carriage near the Statue of Louis XV, from whence I
could well see it rise, & have an extensive View of the Region of Air
thro’ which, as the Wind sat, it was likely to pass. The Morning was
foggy, but about one o’clock the Air became tolerably clear; to the
great satisfaction of spectators, who were infinite. Notice having been
given of the intended Experiment several days before in the Papers,
so that all Paris was out, either about the Tuilleries, on the Quays
& Bridges, in the Fields, the Streets, at the Windows, or on the Tops
of Houses, besides the inhabitants of all the Towns & Villages of the
Environs. Never before was a philosophical Experiment so magnificently
attended. Some Guns were fired to give Notice that the departure of
the great Balloon was near, and a small one was discharg’d which went
to an amazing height, there being but little Wind to make it deviate
from its perpendicular Course, and at length the Sight of it was lost.
Means were used, I am told, to prevent the great Balloon’s rising so
high as might endanger its Bursting. Several Bags of Sand were taken
on board before the Cord that held it down was cut, and the whole
Weight being then too much to be lifted, such a Quantity was discharg’d
as to permit its Rising slowly. Thus it would sooner arrive at that
Region where it would be in equilibrio with the surrounding Air, and
by discharging more Sand afterwards, it might go higher if desired.
Between One & Two o’Clock, all Eyes were gratified with seeing it
rise majestically from among the Trees and ascend gradually above the
Buildings, a most beautiful Spectacle! When it was about 200 feet high,
the brave Adventurers held out and wav’d a little white Pennant, on
both sides their Car, to salute the Spectators, who return’d loud Claps
of Applause. The Wind was very little, so that the Object, tho’ moving
to the Northward, continued long in View; and it was a great while
before the admiring People began to disperse. The persons embark’d
were Mr. Charles, Professor of Experimental Philosophy, & zealous
Promotor of that Science; and one of the Messieurs Robert, the very
ingenious Constructors of the Machine. When it arrived at its height,
which I suppose might be 3 or 400 Toises, it appeared to have only
horizontal Motion. I had a Pocket Glass, with which I follow’d it,
till I lost Sight first of the Men, then of the Car, and when I last
saw the Balloon, it appear’d no bigger than a Walnut. I write this at
7 in the evening. What became of them is not yet known here. I hope
they descended by Day-light, so as to see and avoid falling among
Trees or on Houses, and that the Experience was completed without any
mischievous Accident, which the Novelty of it & the want of Experience
might well occasion. I am the more anxious for the Event, because I am
not well informed of the Means provided for letting themselves gently
down, and the Loss of these very ingenious Men would not only be a
Discouragement to the Progress of the Art, but be a sensible Loss to
Science and Society.
Tuesday Morning, December 2,—I am reliev’d from my Anxiety by hearing
that the Adventurers descended well near l’Isle Adam, before Sunset.
This Place is near 7 Leagues from Paris. Had the Wind blown fresh, they
might have gone much farther.
P.S. Tuesday Evening ... I hear farther that the Travellers had perfect
Command of the Carriage, descending as they pleas’d by letting some of
the inflammable Air escape, and rising again by discharging some Sand;
that they descended over a Field so low as to talk with Labourers in
passing and mounted again to pass a Hill. The little Balloon falling at
Vincennes shows that mounting higher it met with a Current of Air in a
contrary Direction; an Observation that may be of use to future aërial
Voyagers.
B. FRANKLIN.
APPENDIX III
SUCCESSFUL MILITARY DIRIGIBLE BALLOONS
FRANCE
_The Clément-Bayard II_[80]
The _Clément-Bayard II_ may be classed among the airships usually
called “flexible.” The shape of its hull is preserved not by any rigid
framing, but by internal gas pressure maintained by ballonets fed by
ventilating fans. Moreover, the suspension which binds envelope and car
together as one solid is composed wholly of flexible elements, without
any rigid intermediary structure.
The general plan, then, of the craft comprises three prominent
features, well marked and distinct in character:
(_a_) The fish-shaped envelope with major section well forward, a form
favorable to both speed and stability.
(_b_) The trussed girderlike car whose length allows the load to be
distributed over the hull, thus preserving its nicety of outline.
The most minute and technical and mechanical details were studied
for eighteen months by M. Clément and his devoted collaborator, the
engineer Sabathier. The girder car, as will be seen presently, is
particularly well designed to serve as car, sustainer and stiffener. No
stabilizing device is attached to the envelope; all are fixed to the
car, on which is mounted also the complete propulsion plant.
(_c_) The suspension which binds the buoyant envelope to the car serves
no other purpose. Note also the ingenious arrangement of two motors
and two propellers, forming two independent systems, yet unitable
under certain conditions. The placement of the propellers, rudders and
stabilizing surfaces well above the bottom of the car, insures them
against dangerous contact on landing, or while maneuvering near the
ground.
The envelope is of rubberized Continental cloth. Its volume is 7,000
cubic meters, length 76.5 meters and major diameter 13.22 meters, or
an elongation of 5.76 diameters. Inside the gas envelope is an air bag
of 2,200 cubic meters. It is divided into two compartments, _Q_ and
_Q´_, which can be filled with air together or separately through the
air duct, _Q_, joined to a blower, _P_, run by the two motors, or by
hand when so desired. The balloon proper comprises two gas valves, _R_.
Each compartment of the ballonet has one air valve, _S_. The valves of
the type Clément-Bayard-Chauvière are automatic. Their construction is
so perfect that for the first time in France, at least on a balloon of
so large bulk, the blower runs continuously in constant communication
with the ballonet, the pressure in the envelope remaining invariable,
due to the regular play of the valves, which yield at the pressure for
which they are set. They may also be Worked by hand from the pilot’s
bridge in case of emergency. The envelope has on its upper side three
ripping seams, one in the middle, the others toward either end. These
rip panels can be worked together or separately, and permit the rapid
deflation of the balloon.
The long car is attached to the hull by hempen duck feet fastened to
a bolt rope running along the envelope below the equator; these duck
feet terminate below in steel suspension cables fixed to the car. Below
the principal bolt rope are others to which are fastened the duck
feet of the oblique cords, which assure the perfect solidarity of the
envelope and car. The steel cable sustainers have an ingenious patented
regulating windlass. The girder car consists of a latticed girder,
built of steel tubes united with cast-iron joints and steel-tie wires.
Its whole length is 45 meters, of which 14.5 meters constitute the car
proper. It is divided into segments which are easily demountable, thus
rendering it easily transportable by truck or railway. The forward
segment, _A_, tapers toward the front to a sharp point and is of
triangular cross-section. The mid segment, _B_, constituting the car,
has a quadrangular section of variable size. The rear segment, _D_, is
of triangular section, diminishing progressively toward the rear, which
rises to a sort of tail supporting the empennage and the direction
rudders. The entire girder car when resting on the ground is supported
by two pneumatic shock absorbers, _U_, _U_, projecting from its floor.
The car proper comprises three parts: in front, the motor and machine
room, 2.5 meters wide; in the middle, the elevated bridge, _N_, for
the pilot and his aide; in the rear, the passenger cabin, 8 meters
long, 1.3 meters wide and 2 meters high for the observers and wireless
telegraphy plant. The two reservoirs of essence, _M_, _m_, are placed
above the passenger about the center of pressure. The blower _P_, for
the ballonets, and the guide ropes _T_, are placed above the pilot’s
bridge.
In the motor room are symmetrically arranged two Bayard-Clément
engines, _G G_, separated enough to allow free passage between them.
Each motor is elastically supported to obviate vibrations, and connects
with the transmission shaft by a variable speed gear. The engines
can be run separately or together by a connecting sprocket chain, and
develop 100 to 130 horse-power each. The cooling of each motor is
effected by an aluminum radiator, _L L_, of large surface.
The Chauvière propellers, _K K_, six feet in diameter, are driven by
shafting and gear wheels at a normal speed of 250 rotations per minute.
A special recording device serves to show their thrust at each instant,
as also the torque of the motors.
The pilot, standing on the bridge where he enjoys a clear view, has
immediate charge of the vessel’s movements. Before him are the various
controls which he must operate, and the divers indicators which he must
consult. These are the direction wheel, the manometers, the aneroid
and registering barometers, the clinometer, the blower control to
regulate the amount and distribution of pressure, the elevating-rudder
wheel, the spark control, the ripping cord, the release string of the
guide-rope, and the system of transmitting orders to the mechanicians
whereby he can control the engines and the blowers which furnish air to
the radiator and ballonet.
The direction and poise of the vessel in flight are controlled by the
rudders and empennage at the rear, and its altitude from minute to
minute is governed by the elevating biplane _E´_, of 30 square meters
above the car in the mid region of the vessel.
The _Patrie_[81]
The _Patrie_, the third of its type, was first operated in 1906. The
gas bag of the first balloon was built by Surcouf at Billancourt,
Paris. The mechanical part was built at the Lebaudy Sugar Refinery.
Since then the gas bags have been built at the Lebaudy balloon shed at
Moisson, near Paris, under the direction of their aëronaut, Juchmes.
The gas bag of the _Patrie_ was 197 feet long with a maximum diameter
of 33 feet, 9 inches, situated about 2/5 of the length from the front;
volume 111,250 cubic feet; length approximately six diameters. This
relation, together with the cigar shape, is in accordance with the
plans of Colonel Renard’s dirigible, built and operated in France in
1884; the same general shape and proportions being found in the Ville
de Paris.
The first Lebaudy was pointed at the rear, which is generally admitted
to be the proper shape for the least resistance, but to maintain
stability it was found necessary to put a horizontal and vertical plane
there, so that it had to be made an ellipsoid of revolution to give
attachment for these planes.
The ballonet for air had a capacity of 22,958 cubic feet or about 1/5
of the total volume. This is calculated to permit reaching a height of
about one mile and to be able to return to the earth, keeping the gas
bag always rigid. To descend from a height of one mile, gas would be
released by the valve, then air pumped into the ballonet to keep the
gas bag rigid, these two operations being carried on alternately. On
reaching the ground from the height of one mile, the air would be at
the middle of the lower part of the gas bag and would not entirely fill
the ballonet. To prevent the air from rolling from one end to the other
when the air ship pitches, thus producing instability, the ballonet
was divided into three compartments by impermeable cloth partitions.
Numerous small holes were pierced in these partitions, through which
the air finally reached the two end compartments.
In September, 1907, the _Patrie_ was enlarged by 17,660 cubic feet
by the addition of a cylindrical section at the maximum diameter,
increasing the length but not the maximum diameter.
_The Gas Bag._—The gas bag is cut in panels; the material is a rubber
cloth made by the Continental Tire Company at Hanover, Germany. It
consists of four layers arranged as follows:
Weight oz. per
square yard.
_a._ Outer layer of cotton cloth covered with lead chromate 2.5
_b._ Layer of vulcanized rubber 2.5
_c._ Layer of cotton cloth 2.5
_d._ Inner layer of vulcanized rubber 2.21
------
Total weight 9.71
A strip of this cloth one foot wide tears at a tension of about 934
pounds. A pressure of about one inch of water can be maintained in the
gas bag without danger. The lead chromate on the outside is to prevent
the entrance of the actinic rays of the sun, which would cause the
rubber to deteriorate. The heavy layer of rubber is to prevent the
leaking of the gas. The inner layer of rubber is merely to prevent
deterioration of the cloth by impurities in the gas. This material
has the warp of the two layers of cotton cloth running in the same
direction and is called straight thread. The material in the ballonet
weighs only about 7¾ ounces per square yard, and has a strength of
about 336 pounds per running foot. When the _Patrie_ was enlarged in
September, 1907, the specifications of the material allowed a maximum
weight of 10 ounces per square yard, a minimum strength of 907 pounds
per running foot, and a loss of 5.1 cubic inches of hydrogen per square
yard in twenty-four hours at a pressure of 1.18 inches of water. Bands
of cloth are pasted over the seams inside and out with a solution of
rubber to prevent leaking through the stitches.
_Suspension._—One of the characteristics of the _Patrie_ is the “short”
suspension. The weight of the car is distributed over only about 70
feet of the length of the gas bag. To do this, an elliptical-shaped
frame of nickel-steel tubes is attached to the bottom of the gas
bag; steel cables run from this down to the car. A small hemp net
is attached to the gas bag by means of short wooden cross-pieces,
or toggles, which are let into holes in a strong canvas band which
is sewed directly on the gas bag. The metal frame, or platform, is
attached to this net by means of toggles, so that it can be quickly
removed in dismounting the air ship for transportation. The frame can
also be taken apart, 28 steel cables about 0.2 inches in diameter run
from the frame down to the car, and are arranged in triangles. Due
to the impossibility of deforming a triangle, rigidity is maintained
between the car and gas bag.
The objection to the “short” suspension of the _Patrie_ is the
deformation of the gas bag. A distinct curve can be seen in the middle.
_The Car._—The car is made of nickel-steel tubes (12 per cent nickel).
This metal gives the greatest strength for minimum weight. The car is
boat-shaped, about 16 feet long, about 5 feet wide and 2½ feet high.
About 11 feet separate the car from the gas bag. To prevent any chance
of the fire from the engine communicating with the hydrogen, the steel
framework under the gas bag is covered with a noncombustible material.
The pilot stands at the front of the car, the engine is in the
middle, the engineer at the rear. Provision is made for mounting a
telephotographic apparatus, and for a 100-candle-power acetylene
searchlight. A strong pyramidal structure of steel is built under the
car, pointing downward. In landing the point comes to the ground first
and this protects the car, and especially the propellers, from being
damaged. The car is covered to reduce air resistance. It is so low,
however, that part of the equipment and most of the bodies of those
inside are exposed, so that the total resistance of the car is large.
_The Motor._—The first Lebaudy had a 40-horse-power Daimler-Mercedes
benzine motor. The _Patrie_ was driven by a 60 to 70-horse-power
4-cylinder Panhard and Levassor benzine motor, making 1,000 r. p. m.
_The Propellers._—There are two steel propellers 8½ feet in diameter
(two blades each) placed at each side of the engine, this giving the
shortest and most economical transmission. To avoid any tendency to
twist the car, the propellers turn in opposite directions. They are
“high speed,” making 1,000 to 1,200 r. p. m.