SOARING FLIGHT BY MAN

In a Meteoric Shower

A third and rare type of bird flight has been calledsailing. The bird faces the wind, and with wings outspread and their forward edge elevated rises while being forced backward under the action of the breeze. As soon as the windsomewhat subsides, the bird turns andsoarsin the desired direction. Flight is thus accomplished without muscular effort other than that necessary to properly incline the wings and to make the turns. It is practicable only in squally winds, and the birds which practice “sailing”—the albatross and frigate bird—are those which live in the lower and more disturbed regions of the atmosphere. This form of flight has been approximately imitated in the manœuvering of aeroplanes.

Comparison of flying machines and ships suggests many points of difference. Water is a fluid of great density, with a definite upper surface, on which marine structures naturally rest. A vessel in the air may be at any elevation in the surrounding rarefied fluid, and great attention is necessary to keep it at the elevation desired. The air has no surface. The air ship is like a submarine—the dirigible balloon of the sea—and perhaps rather more safe. An ordinary ship is only partially immersed; the resistance of the fluid medium is exerted over a portion only of its head end: but the submarine or the flying machine is wholly exposed to this resistance. The submarine is subjected to ocean currents of a very few miles per hour, at most; the currents to which the flying machine may be exposed exceed a mile a minute. Put a submarine in the Whirlpool Rapids at Niagara and you will have possible air ship conditions.

A marine vessel maytack,i.e., may sail partially against the wind that propels it, by skillful utilization of the resistanceto sidewise movement of the ship through the water: but the flying machine is wholly immersed in a single fluid, and a head wind is nothing else than a head wind, producing an absolute subtraction from the proper speed of the vessel.

How a Boat Tacks

The wind always exerts a pressure, perpendicular to the sail, which tends to drift the boat sidewise (R) and also to propel it forward (L). Sidewise movement is resisted by the hull. An air ship cannot tack because there is no such resistance to drift.

Aerial navigation is thus a new art, particularly when heavier-than-air machines are used. We have no heavier-than-waterships. The flying machine must work out its own salvation.

SOARING FLIGHT BY MAN

Flying machines have been classified as follows:—

Lighter than Air

Heavier than Air

We will fall in with the present current of popular interest and consider the aeroplane—that mechanical grasshopper—first.

Octave Chanute (died 1910)

Octave Chanute(died 1910)To the researches of Chanute and Langley must be ascribed much of American progress in aviation.

When a flat surface like the side of a house is exposed to the breeze, the velocity of the wind exerts a force or pressure directly against the surface. This principle is taken into account in the design of buildings, bridges, and otherstructures. The pressure exerted per square foot of surface is equal (approximately) to the square of the wind velocity in miles per hour, divided by 300. Thus, if the wind velocity is thirty miles, the pressure against a house wall on which it acts directly is 30 × 30 ÷ 300 = 3 pounds per square foot: if the wind velocity is sixty miles, the pressure is 60 × 60 ÷ 300 = 12 pounds: if the velocity is ninety miles, the pressure is 90 × 90 ÷ 300 = 27 pounds, and so on.

Pressure of the Wind

If the wind blows obliquely toward the surface, instead of directly, the pressure at any given velocity is reduced, but may still be considerable. Thus, in the sketch, letabrepresent a wall, toward which we are looking downward, and let the arrowVrepresent the direction of the wind. The air particles will follow some such paths as those indicated, being deflected so as to finally escape around the ends of the wall. The result is that a pressure is produced which may be considered to act along the dottedlineP, perpendicular to the wall. This is the invariable law: that no matter how oblique the surface may be, with reference to the direction of the wind, there is always a pressure produced against the surface by the wind, and this pressure always actsin a direction perpendicular to the surface. The amount of pressure will depend upon the wind velocity and the obliquity or inclination of the surface (ab) with the wind (V).

Now let us consider a kite—the “immediate ancestor” of the aeroplane. The surfaceabis that of the kite itself, held by its stringcd. We are standing at one side and looking at theedgeof the kite. The wind is moving horizontally against the face of the kite, and produces a pressurePdirectly against the latter. The pressure tends both to move it toward the left and to lift it. If the tendency to move toward the left be overcome by the string, then the tendency toward lifting may be offset—and in practiceisoffset—by the weight of the kite and tail.

Forces Acting on a Kite

We may represent the two tendencies to movement produced by the forceP, by drawing additional dotted lines, one horizontally to the left (R) and the other vertically (L);and it is known that if we let the length of the linePrepresent to some convenient scale the amount of direct pressure, then the lengths ofRandLwill also represent to the same scale the amounts of horizontal and vertical force due to the pressure. If the weight of kite and tail exceeds the vertical forceL, the kite will descend: if these weights are less than that force, the kite will ascend. If they are precisely equal to it, the kite will neither ascend nor descend. The ratio ofLtoRis determined by the slope ofP; and this is fixed by the slope ofab; so that we have the most important conclusion:not only does the amount of direct pressure (P) depend upon the obliquity of the surface with the breeze (as has already been shown), but the relation of vertical force (which sustains the kite) to horizontal force also depends on the same obliquity. For example, if the kite were flying almost directly above the boy who held the string, so thatabbecame almost horizontal,Pwould be nearly vertical andLwould be much greater thanR. On the other hand, ifabwere nearly vertical, the kite flying at low elevation, the string and the direct pressure would be nearly horizontal andLwould be much less thanR. The forceLwhich lifts the kite seems to increase whileRdecreases, as the kite ascends: butLmay not actually increase, because it depends upon the amount of direct pressure,P, as well as upon the direction of this pressure; and the amount of direct pressure steadily decreases during ascent, on account of the increasing obliquity ofabwithV. All of this is of course dependenton the assumption that the kite always has the same inclination to the string, and the described resolution of the forces, although answering for illustrative purposes, is technically incorrect.

It seems to be the wind velocity, then, which holds up the kite: but in reality the string is just as necessary as the wind. If there is no string, and the wind blows the kite with it, the kite comes down, because the pressure is wholly due to a relative velocity as between kite and wind. The wind exerts a pressure against the rear of a railway train, if it happens to be blowing in that direction, and if we stood on the rear platform of a stationary train we should feel that pressure: but if the train is started up and caused to move at the same speed as the wind there would be no pressure whatever.

One of the very first heavier-than-air flights ever recorded is said to have been made by a Japanese who dropped bombs from an immense man-carrying kite during the Satsuma rebellion of 1869. The kite as a flying machine has, however, two drawbacks: it needs the wind—it cannot fly in a calm—and it stands still. One early effort to improve on this situation was made in 1856, when a man was towed in a sort of kite which was hauled by a vehicle moving on the ground. In February of the present year, Lieut. John Rodgers, U.S.N., was lifted 400 feet from the deck of the cruiserPennsylvaniaby a train of eleven large kites, the vessel steaming at twelve knots against an eight-knot breeze. The aviator made observationsand took photographs for about fifteen minutes, while suspended from a tail cable about 100 feet astern. In the absence of a sufficient natural breeze, an artificial wind was thus produced by the motion imparted to the kite; and the device permitted of reaching some destination. The next step was obviously to get rid of the tractive vehicle and tow rope by carrying propelling machinery on the kite. This had been accomplished by Langley in 1896, who flew a thirty-pound model nearly a mile, using a steam engine for power. The gasoline engine, first employed by Santos-Dumont (in a dirigible balloon) in 1901, has made possible the present dayaeroplane.

Sustaining Force in the Aeroplane

What “keeps it up”, in the case of this device, is likewise its velocity. Looking from the side,abis the sail of the aeroplane, which is moving toward the right at such speed as to produce the equivalent of an air velocityVto the left. This velocity causes the direct pressureP, equivalent to a lifting forceLand a retarding forceR. The latter is the force which must be overcome by the motor: theformer must suffice to overcome the whole weight of the apparatus. Travel in an aeroplane is like skating rapidly over very thin ice: the air literally “doesn’t have time to get away from underneath.”

Direct, Lifting, and Resisting Forces

If the pressure is 10 lbs. when the wind blows directly toward the surface (at an angle of 90 degrees), then the forces for other angles of direction are as shown on the diagram. Theamountsof all forces depend upon the wind velocity: that assumed in drawing the diagram was about 55 miles per hour. But therelationsof the forces are the same for the various angles, no matter what the velocity.

If we designate the angle made by the wings (ab) with the horizontal (V) asB, thenPincreases asBincreases, while (as has been stated) the ratio ofLtoRdecreases. When the angleBis a right angle, the wings being in the positiona´b´,Phas its maximum value for direct wind—1/300 of the square of the velocity, in pounds per square foot; butLis zero andRis equal toP. The plane would have nolifting power. When the angleBbecomes zero, positiona´´b´´, wings being horizontal,Pbecomes zero and (so far as we can now judge) the plane has neither lifting power nor retarding force. At some intermediate position, likeab, there will be appreciable lifting and retarding forces. The chart shows the approximate lifting force, in pounds per square foot, for various angles. This force becomes a maximum at an angle of 45° (half a right angle). We are not yet prepared to consider why in all actual aeroplanes the angle of inclination is much less than this. The reason will be shown presently. At this stage of the discussion we may note that the lifting power per square foot of sail area varies with

the square of the velocity,andthe angle of inclination.

The total lifting power of the whole plane will also vary with its area. As we do not wish this whole lifting power to be consumed in overcoming the dead weight of the machine itself, we must keep the parts light, and in particular must use for the wings a fabric of light weight per unit of surface. These fabrics are frequently the same as those used for the envelopes of balloons.

Since the total supporting power varies both with the sail area and with the velocity, we may attain a given capacity either by employing large sails or by using high speed. The size of sails for a given machine varies inversely as the square of the speed. The original Wright machine had 500 square feet of wings and a speed of fortymiles per hour. At eighty miles per hour the necessary sail area for this machine would be only 125 square feet; and at 160 miles per hour it would be only 31-1/4 square feet: while if we attempted to run the machine at ten miles per hour we should need a sail area of 8000 square feet. This explains why the aeroplane cannot go slowly.

It would seem as if when two or more superposed sails were used, as in biplanes, the full effect of the air would not be realized, one sail becalming the other. Experiments have shown this to be the case; but there is no great reduction in lifting power unless the distance apart is considerably less than the width of the planes.

In all present aeroplanes the sails are concaved on the under side. This serves to keep the air from escaping from underneath as rapidly as it otherwise would, and increases the lifting power from one-fourth to one-half over that given by our 1/300 rule: the divisor becoming roughly about 230 instead of 300.

Shapes of Planes

Why are the wings placed crosswise of the machine, when the other arrangement—the greatest dimension in the line of flight—would seem to be stronger? Thisis also done in order to “keep the air from escaping from underneath.” The sketch shows how much less easily the air will get away from below a wing of the bird-like spread-out form than from one relatively long and narrow but of the same area.

A sustaining force of two pounds per square foot of area has been common in ordinary aeroplanes and is perhaps comparable with the results of bird studies: but this figure is steadily increasing as velocities increase.

Thus far a single wing or pair of wings would seem to fully answer for practicable flight: yet every actual aeroplane has several small wings at various points. The necessity for one of these had already been discovered in the kite, which is built with a balancing tail. In the sketch on page 18 it appears that the particles of air which are near the upper edge of the surface are more obstructed in their effort to get around and past than those near the lower edge. They have to turn almost completely about, while the others are merely deflected. This means that on the whole the upper air particles will exert more pressure than the lower particles and that the “center of pressure” (the point where the entire force of the wind may be assumed to act) will be, not at the center of the surface, but at a point some distanceabovethis center. This action is described as the “displacement of the center of pressure.” It is known that the displacement is greatest for leastinclinations of surface (as might be surmised from the sketch already referred to), and that it is always proportional to the dimension of the surface in the direction of movement;i.e., to the length of the lineab.

Balancing Sail

If the weightWof the aeroplane acts downward at the center of the wing (atoin the accompanying sketch), while the direct pressurePacts at some pointcfarther along toward the upper edge of the wing, the two forcesWandPtend to revolve the whole wing in the direction indicated by the curved arrow. This rotation, in an aeroplane, is resisted by the use of a tail plane or planes, such asmn. The velocity produces a direct pressureP´on the tail plane, which opposes, like a lever, any rotation due to the action ofP. It may be considered a matter of rather nice calculation to get the area and location of the tail plane just right: but we must remember that the amount of pressureP´can be greatly varied by changing the inclination of the surfacemn. This change of inclination is effected by the operator, who has access to wires which are attached to the pivoted tail plane. It is of course permissible to place the tail planein frontof the main planes—asin the original Wright machine illustrated: but in this case, with the relative positions ofWandPalready shown, the forward edge of the tail plane would have to be depressed instead of elevated. The illustration shows the tail built as a biplane, just as are the principal wings (page141).

Suppose the machine to be started with the tail plane in a horizontal position. As its speed increases, it rises and at the same time (if the weight is suspended from the center of the main planes) tilts backward. The tilting can be stopped by swinging the tail plane on its pivot so as to oppose the rotative tendency. If this control is not carried too far, the main planes will be allowed to maintain some of their excessive inclination and ascent will continue. When the desired altitude has been attained, the inclination of the main planes will, by further swinging of the tail plane, be reduced to the normal amount, at which the supporting power is precisely equal to the load; and the machine will be in vertical equilibrium: an equilibrium which demands at every moment, however, the attention of the operator.

In many machines, ascent and tilting are separately controlled by using two sets of transverse planes, one set placed forward, and the other set aft, of the main planes. In any case, quick ascent can be produced only by an increase in the lifting forceL(see sketch, page24) of the main planes: and this force is increased by enlarging the angle of inclination of the main planes, that is, by a controlledand partial tilting. The forward transverse wing which produces this tilting is therefore called theelevating rudderor elevating plane. The rear transverse plane which checks the tilting and steadies the machine is often described as thestabilizing plane.Descentis of course produced bydecreasingthe angle of inclination of the main planes.

Roe’s Triplane at Wembley

Roe’s Triplane at Wembley(From Brewer’sArt of Aviation)

If we need extra sails for stability and ascent or descent, we need them also for changes of horizontal direction. Letabbe the top view of the main plane of a machine, following the coursexy. Atrsis a vertical plane called thesteering rudder. This is pivoted, and controlled by theoperator by means of the wirest,u. Let the rudder be suddenly shifted to the positionr´s´. It will then be subjected to a pressureP´which will swing the whole machine into the new position shown by the dotted lines, its course becomingx´y´. The steering rudder may of course be double, forming a vertical biplane, as in the Wright machine shown below.

Action of the Steering Rudder

Successful steering necessitates lateral resistance to drift,i.e., a fulcrum. This is provided, to some extent, by the stays and frame of the machine; and in a much more ample way by the vertical planes of the original Voisin cellular biplane. A recent Wright machine had vertical planes forward probably intended for this purpose.

Recent Type of Wright Biplane

It now begins to appear that the aviator has a great many things to look after. There are many more things requiring his attention than have yet been suggested. No one has any business to attempt flying unless he is superlatively cool-headed and has the happy faculty of instinctively doing the right thing in an emergency. Give a chauffeur a high power automobile running at maximum speed on a rough and unfamiliar road, and you have some conception of the position of the operator of an aeroplane. It is perhaps not too much to say that to make the two positions fairly comparable we shouldblindfoldthe chauffeur.

Broadly speaking, designers may be classed in one of two groups—those who, like the Wrights, believe in training the aviator so as to qualify him to properly handle his complicated machine; and those who aim to simplify the whole question of control so that to acquire the necessary ability will not be impossible for the average man. If aviation is to become a popular sport, the latter ideal must prevail. The machines must be more automatic and the aviator must have time to enjoy the scenery. In France, where amateur aviation is of some importance, progress has already been made in this direction. The universal steering head, for example, which not only revolves like that of an automobile, but is hinged to permit of additional movements, provides for simultaneous control of the steering rudder and the main plane warping, while scarcely demanding the conscious thought of the operator.

TURNING CORNERS

A year elapsed after the first successful flight at Kitty Hawk before the aviator became able to describe a circle in the air. A later date, 1907, is recorded for the first European half-circular flight: and the first complete circuit, on the other side of the water, was made a year after that; by both biplane and monoplane. It was in the same year that Louis Blériot made the pioneer cross-country trip of twenty-one miles, stopping at willen routeand returning to his starting point.

Circular Flight

We are looking downward on an aeroplaneabwhich has been moving along the straight pathcd. Atdit begins to describe the circlede, the radius of which isod, aroundthe centero. The outer portion of the plane, at the edgeb, must then move faster than the inner edgea. We have seen that the direct air pressure on the plane is proportional to the square of the velocity. The direct pressureP(see sketch on page22) will then be greater at the outer than at the inner limb; the lifting forceLwill also be greater and the outer limb will tend to rise, so that the plane (viewed from the rear) will take the inclined position shown in the lower view: and this inclination will increase as long as the outer limb travels faster than the inner limb; that is, as long as the orbit continues to be curved. Very soon, then, the plane will be completely tipped over.

Necessarily, the two velocities have the ratioom:om´; the respective lifting forces must then be proportional to the squares of these distances. The difference of lifting forces, and the tendency to overturn, will be more important as the distances most greatly differ: which is the case when the distanceomis small as compared withmm´. The shorter the radius of curvature, the more dangerous, for a given machine, is a circling flight: and in rounding a curve of given radius the most danger is attached to the machine of greatest spread of wing.

This particular difficulty has considerably delayed the development of the aeroplane. It may, however, be overcome by very simple methods—simple, at least as far as their mechanical features are concerned. If the outerlimb of the plane is tilted upward, it is because the wind pressure is greater there. The wind pressure is greater because the velocity is greater. We have only to increase the wind pressure at the inner limb, in order to restore equilibrium. This cannot be done by adjusting the velocity, because the velocity is fixed by the curvature of path required: but the total wind pressure depends upon thesail areaas well as the velocity; so that by increasing the surface at the inner limb we may equalize the value ofL, the lifting force, at the two ends of the plane. This increase of surface must be a temporary affair, to be discontinued when moving along a straight course.

The Aileron

Let us stand in the rear of an aeroplane, the main wing of which is represented byab. Let the small fan-shaped wingscanddbe attached near the ends, and let the control wires,e,f, passing to the operator atg, be employed to close and unclasp the fans. If these fans are given a forward inclination at the top, as indicated in the end view, they will when spread out exert an extra lifting force. A fanwill be placed at each end. They will be ordinarily folded up: but when rounding a curve the aviator will open the fan on the inner or more slowly moving limb of the main plane. This represents one of the first forms of theaileronor wing-tip for lateral control.

The more common present form of aileron is that shown in the lower sketch, atsandt. The method of control is the same.

Wing Tipping

The cellular Voisin biplanes illustrate an attempt at self-sufficing control, without the interposition of the aviator. Between the upper and lower sails of the machine there were fore and aft vertical partitions. The idea was that when the machine started to revolve, the velocity of rotation would produce a pressure against these partitions which would obstruct the tipping. But rotation may take place slowly, so as to produce an insufficient pressure for control, and yet be amply sufficient to wreck the apparatus. The use of extra vertical rudder planes, hinged on a horizontal longitudinal axis, is open to the same objection.

In some monoplanes with the invertedVwing arrangement, a dipping of one wing answers, so to speak, to increaseits concavity and thus to augment the lifting force on that side. The sketch shows the normal and distorted arrangement of wings: the inner limb being the one bent down in rounding a curve. An equivalent plan was to change the angle of inclination of one-half the sail by swinging it about a horizontal pivot at the center or at the rear edge: some machines have been built with sails divided in the center. The obvious objection to both of these plans is that too much mechanism is necessary in order to distort what amounts to nearly half the whole machine. They remind one of Charles Lamb’s story of the discovery of roast pig.

Wing Warping

The distinctive feature of the Wright machines lies in the warping or distorting of theends onlyof the main planes. This is made possible, not by hinging the wings in halves, but by the flexibility of the framework, which is sufficiently pliable to permit of a considerable bending without danger. The operator, by pulling on a stout wire linkage, may tip up (or down) the cornerscc´of the sails at one limb, thus decreasing or increasing the effective surface acted on by the wind, as the case may require.The only objection is that the scheme provides one more thing for the aviator to think about and manipulate.

Let us consider again the condition of things when rounding a curve, as in the sketch on page32. As long as the machine is moving forward in a straight line, the operator sits upright. When it begins to tip, he will unconsciously tip himself the other way, as represented by the linexyin the rear view. Any bicyclist will recognize this as plausible. Why not take advantage of this involuntary movement to provide a stabilizing force? If operating wires are attached to the aviator’s belt and from thence connected with ailerons or wing-warping devices, then by a proper proportioning of levers and surfaces to the probable swaying of the man, the control may become automatic. The idea is not new; it has even been made the subject of a patent.

The Gyroscope

This device for automatic control is being steadily developed and may ultimately supersede all others. It uses the inertia of a fast-moving fly wheel for control, in a manner not unlike that contemplated in proposed methods of automatic balancing by the action of a suspended pendulum. Every one has seen the toy gyroscope and perhaps has wondered at its mysterious ways. The mathematical analysis of its action fills volumes: but some idea of what it does, and why, may perhaps be gathered at the expenseof a very small amount of careful attention. The wheelacbd, a thin disc, is spinning rapidly about the axleo. In the side view,abshows the edge of the wheel, andoo´the axle. This axle is not fixed, but may be conceived as held in some one’s fingers. Now suppose the right-hand end of the axle (o´) to be suddenly moved toward us (away from the paper) and the left-hand (o) to be moved away.The wheel will now appear in both views as an ellipse, and it has been so represented, asafbe. Now, any particle, likex, on the rim of the wheel, will have been regularly moving in the circular orbitcb. The tendency of any body in motion is to move indefinitely in a straight line. The cohesion of the metal of the disc prevents the particlexfrom flying off at a straight line tangent,xy, and it is constrained, therefore, to move in a circular orbit. Unless some additional constraint is imposed, it will at least remain in this orbit and will try to remain in its plane of rotation. When the disc is tipped, the plane of rotation is changed, and the particle is required, instead of (so to speak) remaining in the plane of the paper—in the side view—to approach and pass through that plane atband afterward to continue receding from us. Under ordinary circumstances, this is just what it would do: but if, as in the gyroscope, the axleoo´is perfectly free to move in any direction, the particlexwill refuse to change its direction of rotation. Its position has been shifted: it no longer lies in the plane of the paper: but it will at least persist in rotating in a parallel plane: and this persistence forces the revolving disc to swing into the new position indicated by the curvehg, the axis being tipped into the positionpq. The whole effect of all particles likexin the entire wheel will be found to produce precisely this condition of things: if we undertake to change the plane of rotation by shifting the axle in a horizontal plane, the device itself will (if not prevented) make afurther change in the plane of rotation by shifting the axle in a vertical plane.

A revolving disc mounted on the gyroscopic framework therefore resists influences tending to change its plane of rotation. If the device is placed on a steamship, so that when the vessel rolls a change of rotative plane is produced, the action of the gyroscope will resist the rolling tendency of the vessel. All that is necessary is to have the wheel revolving in a fore and aft plane on the center line of the vessel, the axle being transverse and firmly attached to the vessel itself. A small amount of power (consumed in revolving the wheel) gives a marked steadying effect. The same location and arrangement on an aeroplane will suffice to overcome tendencies to transverse rotation when rounding curves. The device itself is automatic, and requires no attention, but it does unfortunately require power to drive it and it adds some weight.

The gyroscope is being tested at the present time on some of the aeroplanes at the temporary army camps near San Antonio, Texas.

This feature of aeronautics is particularly important, because any device which will give automatic stability when turning corners will go far toward making aviation a safe amusement. Inequalities of velocity exist not only on curves, but also when the wind is blowing at anything but uniform velocity across the whole front of the machine.The slightest “flaw” in the wind means an at least temporary variation in lifting force of the two arms. Here is a pregnant source of danger, and one which cannot be left for the aviator to meet by conscious thought and action. It is this, then, that blindfolds him: he cannot see the wind conditions in advance. The conditions are upon him, and may have done their destructive work, before he can prepare to control them. We must now study what these conditions are and what their influence may be on various forms of aerial navigation: after which, a return to our present subject will be possible.

AIR AND THE WIND

The air that surrounds us weighs about one-thirteenth of a pound per cubic foot and exerts a pressure, at sea level, of nearly fifteen pounds per square inch. Its temperature varies from 30° below to 100° above the Fahrenheit zero. The pressure of the air decreases about one-half pound for each thousand feet of altitude; at the top of Mt. Blanc it would be, therefore, only about six pounds per square inch. The temperature also decreases with the altitude. The weight of a cubic foot, ordensity, which, as has been stated, is one-thirteenth of a pound ordinarily, varies with the pressure and with the temperature. The variation with pressure may be described by saying that thequotientof the pressure by the density is constant: one varies in the same ratio as the other. Thus, at the top of Mt. Blanc (if the temperature were the same as at sea level), the density of air would be about 6/15 × 1/13 = 2/65: less than half what it is at sea level. As to temperature, if we call our Fahrenheit zero 460°, and correspondingly describe other temperatures—for instance, say that water boils at 672°—then (pressure being unchanged) theproductof the density and the temperature is constant. If the density at sea level and zero temperature is one-thirteenth pound, then that at sea level and 460° Fahrenheit would be

(0 + 460) / (460 + 460) × 1/13 = 1/26.

These relations are particularly important in the design of all balloons, and in computations relating to aeroplane flight at high altitudes. We shall be prepared to appreciate some of their applications presently.

Generally speaking, the atmosphere is always in motion, and moving air is called wind. Our meteorologists first studied winds near the surface of the ground: it is only of late years that high altitude measurements have been considered practically desirable. Now, records are obtained by the aid of kites up to a height of nearly four miles: estimates of cloud movements have given data on wind velocities at heights above six miles: and much greater heights have been obtained by free balloons equipped with instruments for recording temperatures, pressures, altitude, time, and other data.

When the Eiffel Tower was completed, it was found that the average wind velocity at its summit was about four times that at the base. Since that time, much attention has been given to the contrasting conditions of surface and upper breezes as to direction and velocity.

Air is easily impeded in its movement, and the well-known uncertainties of the weather are closely related to local variations in atmospheric pressure and temperature. When near the surface of the ground, impingement against irregularities therein—hills, cliffs, and buildings—makes the atmospheric currents turbulent and irregular. Wherethere are no surface irregularities, as on a smooth plain or over water, the friction of the air particles passing over the surface still results in a stratification of velocities. Even on a mountain top, the direction and speed of the wind are less steady than in the open where measured by a captive balloon. The stronger the wind, the greater, relatively, is the irregularity produced by surface conditions. Further, the earth’s surface and its features form a vast sponge for sun heat, which they transfer in turn to the air in an irregular way, producing those convectional currents peculiar to low altitudes, the upper limit of which is marked by the elevation of the cumulus clouds. Near the surface, therefore, wind velocities are lowest in the early morning, rising to a maximum in the afternoon.

Diurnal Temperatures at Different Heights

Diurnal Temperatures at Different Heights(From Rotch’sThe Conquest of the Air)

Every locality has its so-called “prevailing winds.” Considering the compass as having eight points, one ofthose points may describe as many as 40% of all the winds at a given place. The direction of prevalence varies with the season. The range of wind velocities is also a matter of local peculiarity. In Paris, the wind speed exceeds thirty-four miles per hour on only sixty-eight days in the average year, and exceeds fifty-four miles on only fifteen days. Observations at Boston show that the velocity of the wind exceeds twenty miles per hour on half the days in winter and on only one-sixth the days in summer. Our largest present dirigible balloons have independent speeds of about thirty-four miles per hour and are therefore available (at some degree of effectiveness) for nearly ten months of the year, in the vicinity of Paris. In a region of low wind velocities—like western Washington, in this country—they would be available a much greater proportion of the time. To make the dirigible able to at least move nearly every day in the average year—in Paris—it must be given a speed of about fifty-five miles per hour.

Figures as to wind velocity mean little to one unaccustomed to using them. A five-mile breeze is just “pleasant.” Twelve miles means a brisk gale. Thirty miles is a high wind: fifty miles a serious storm (these are the winds the aviator constantly meets): one hundred miles is perhaps about the maximum hurricane velocity.

As we ascend from the surface of the earth, the wind velocity steadily increases; and the excess velocity of winter winds over summer winds is as steadily augmented. Thus, Professor Rotch found the following variations:

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