300 miles an hour, though at such great speeds the factor of air resistance becomes extremely important.

Very high rates of travel by air will scarcely be possible unless a machine can be given a wider variation in speed than is feasible to-day; and, to obtain such a variation, two methods at present suggest themselves. One, which has been experimented with already, is to mount the sustaining surfaces in such a way that a pilot, by a movement of a wheel or lever, can rock the planes to a certain limited extent, and present them to the air at either a steep or a fine angle. When an aviator has climbed to a sufficient altitude, and is flying fast, he adjusts his planes so that they are at a fine angle to the air-exercising, indeed, only just sufficient lift to preserve horizontal flight, and offering at the same time a minimum of drift, or head resistance. Then, when the time comes to descend, the pilot operates the rocking mechanism again, and inclines his planes at a steep angle to the air. The effect of this is twofold; in the first place the planes, being at a steep angle, exercise a greater lift, and support the machine in consequence at the slower speed necessary for alighting; and, in the second place, they act as an air brake, and check forward motion in the same way that a bird, when it wishes to alight on some spot it sees below, swings its wings at an angle in order to reduce speed.

The advantage of this system is that it can be adapted to existing methods of construction, and that planes which are built of nothing more substantial than wood, covered with fabric, can be so connected to the hull of a machine that they will rock in the way required, without imperilling their factor of safety.

But when speeds are sought, as they must be, which will render air-borne transit very considerably faster, even under adverse weather conditions, than that of land or sea, it is scarcely likely that the method described, effective though it may prove to a limited extent, will provide variations which are sufficiently wide. Machines which bear heavy loads, as they will when mails and passengers are carried by air, and are expected at the same time to fly extremely fast, will need a considerable wing-area if, while moving across the ground at a reasonable speed, they are to gain a sufficient lift' to

take them into the air; and in alighting, when the safety and comfort of passengers have to be considered, the speed must be sufficiently low-necessitating again large wing-areas-to ensure a smooth and easy contact with the ground.

Ships reef their sails in a rising wind, and an aeroplane, as it drives ahead with gathering speed, increasing thereby the air pressure on its surfaces, will require to be able to curtail or reef them while actually in flight; and it must also be able, as is the ship, to shake out these reefs again when occasion demands, and resume its full surface. Here, as a matter of fact, if it can be realised, is the ideal-that, at whatever speed an aeroplane may be travelling, it shall expose only just enough surface to maintain it in horizontal flight. One needs only to watch a bird, as it contracts its wings when gliding fast, to realise what this power would give an aeroplane. It would mean more, indeed, to the aeroplane than does the gear-box to a motor-car. Slow alighting speeds could be combined with very high flying speeds; and it might even be possible to satisfy the wish of the aviator who said he desired to ascend from his own garden, instead of from an aerodrome, and to descend again safely on an ordinary lawn.

To build an aeroplane with wing-surfaces which can be contracted or expanded while the machine is actually in flight is, as may be imagined, easier said than done. It appears, indeed, impossible of achievement, so long as wood is the chief material in construction, to evolve any system which, while operating successfully, will provide at the same time an adequate strength. But as soon as aeroplanes cease to become obsolete so rapidly, with types changing almost from day to day, it should be possible on a commercial scale to replace wood by metal; and then, with a high-grade steel, produced in the form of hollow tubes, one may hope by experiment to evolve a wing which will telescope inwards on itself, and provide as required a surface either large or small.

Technical problems arise, naturally. In the outer sections of a telescopic wing it would not be practicable, for instance, in view of this telescopic action, to use either interplane struts or bracing wires. Therefore the strength of these outer wing-sections would have to

be sufficient-and there is little doubt it could be made so-to withstand unsupported the strains imposed by a rapid movement through the air. With a high-grade steel, such as would be used in the construction of the hollow spars, very great strength should be obtained with a comparatively light weight. Already in wood construction it is possible, thanks mainly to experience, to give planes such an inherent strength that the use of struts and wires is very considerably reduced, which means, of course, a lessening in head resistance; and with variable-surface wings of the future the fact must be borne in mind that, when they are moving through the air at very high speeds, and are receiving their heaviest pressure, they will have been reefed to such an extent that they are exposing only a small amount of surface. This will of course lessen the strains to which they are subjected, and also the resistance which they offer to their own movement through the air, while the steel main-spars, as they telescope one within another, will gain automatically a greater strength.

It is possible, already, to form a mental picture of the passenger aircraft of the future-a machine which is amphibious, alighting either on land or water; navigating, indeed, three elements-earth, sea, and air. It will be driven, perhaps, by petrol turbines, delivering many thousands of horse-power; and its air-screws will have blades which can be varied in the pitch or angle which they present to the air, so that they may adapt themselves automatically while in motion to any speed, high or low.

The air-screw, it may be explained, has blades so designed, and placed at such an angle, that they will deal most efficiently with the air when moving forward through it at some specific speed-this speed being the maximum at which the craft to which the propeller is fitted has been designed to fly. When an aircraft is moving at a speed considerably lower than this maximum, its air-screw or screws tend to churn up the air and slip, instead of dealing with the air-stream smoothly. Apart, however, from the fact that the weight of a variable-pitch mechanism has to be reckoned with seriously, the difference between the high speed and the

low speed of an aeroplane is not yet sufficiently marked to render really imperative any variable-pitch system. But in the future, when machines will need to ascend rapidly and alight slowly, and to fly also at extremely high maximum speeds, the variable-pitch air-screw will, in the absence of some better device, become practically indispensable.

The hull of the large passenger aircraft, resembling in some respects that of a ship, will represent in its shape the latest that science can teach as to lessening the air resistance which is encountered when at very high speeds. It will be smooth-surfaced and tapering, with the dome of its conning-tower projecting only a short distance above the level of the hull. When on the water the machine will ride like a ship, and will be sufficiently strong constructionally to withstand heavy seas. It will be fitted also with an alighting mechanism, which will draw up and disappear within the hull when the machine is in flight or resting on the surface of the water, but will be lowered into position when it is necessary to descend on land. The ability of such a craft to alight either on land or water will on occasion give it a considerable advantage, apart from any question of speed, in the transport of passengers and mails. A trans-Atlantic machine, for instance, carrying passengers from New York to London, instead of having to put in at Liverpool or Southampton (as would a ship) and transfer its passengers and luggage to a train-causing thereby delay as well as inconvenience-will fly straight on overland to an air-station just on the outskirts of London, from which the passengers will be carried into the city in a few minutes by means of high-speed tubes.

The machines of the future will not be biplanes or triplanes, but craft employing a large number of sustaining surfaces, superposed in such a way that they interfere with each other as little as possible. The ideal is for a plane to work in a perfectly undisturbed air stream. If, therefore, surfaces are fitted directly one above another, and close together, as was the case with early experimental triplanes, the air-flow is disturbed as it passes between them, and a loss in lift and efficiency is the result. But any such loss can be minimised by setting the planes some distance apart, and also by what

is known as 'staggering-that is to say, by placing one plane out a little way in front of, or behind, another.

The advantage of using a number of fairly small surfaces, instead, say, of one or two large ones, is that the span of the planes, or their width from one wing-tip to the other, is kept within a reasonable limit. With machines having an extremely wide span, such as would be necessary, say, with biplanes carrying a very heavy load, the difficulty is to give these wide-spread wings sufficient strength, and to prevent them at the same time from becoming too heavy.

Another advantage to be obtained by using a number of planes, each narrow in chord, is that, when a considerable amount of sustaining area has to be used, as in weight-carrying machines, its lifting power is likely to be greater, when disposed in this way, than would be the case were only one or two large surfaces employed. The reason for this is that most of the sustaining power of a plane is obtained near its entering or forward edge; therefore, if an attempt should be made, in endeavouring to avoid a wide span, to use planes which have a deep chord, it may be found that their rear sections are inefficient.

With large machines, employing a number of surfaces which are telescopic, and also of a moderate span, it should be possible, when on the water, to draw these surfaces inwards till they are almost hidden within the hull; and this would permit a machine, whenever necessary, to ride out a rough sea without any risk of damage to the planes through the rolling of the machine bringing them in contact with the tops of the waves.

In ascending, either from land or sea, the machine of the future will speed up her engines, and at the same time extend her wings till the moment comes when she has enough surface exposed, for the pace at which she is moving, to lift her into the air. After this she will climb, using a large amount of surface to ensure a rapid ascent; then, having gained altitude, she will cease to ascend and will begin to move horizontally, using the full power of her motors, and gradually reducing her sustaining surface till just enough is left to maintain her at any chosen height. The faster she travels, under the thrust of her turbines and air-screws, the less sustaining area