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fore. With every step forward we have come nearer the limits, thus leaving less room for future advance. There is a certain amount of energy stored up in fuel which may possibly be utilized in the application of power. The engineer of to-day who reads Dickens's graphic description of the steamship in which he first crossed the Atlantic, with flame issuing from the top of her funnel, will appreciate the enormous waste of power that must have been incurred. The problem of invention from that time to this has been to save as much as possible of this wasted energy and apply it to the blades of the screw propeller. There is also a limit to the power which can be exerted by an engine of given weight. Inventions of lighter and lighter motors have been steps toward this limit, which is probably not yet reached. Yet we are so much nearer to it in the engines which to-day run Count Zeppelin's airship, and the flyers of Farman and Wright, that we may safely say that it is at least being approached.

The resistance and supporting power of the air are yet more determinate. No progress in invention will increase the weight which a given volume or surface of air will support at a given speed, nor can the resistance experienced by a surface in moving through the air ever be reduced below the point set by physical theory. With these conditions in mind we are prepared to inquire what form an aerial vehicle may take, and what results may be expected from it.

II.

Two systems of navigating the air are now being developed, which are radically different-we might almost say opposite-in their fundamental principles. One is that of the flying machine, which is supported by motion through the air as a bird by its wings. The only form of flyer yet found feasi

ble is the aeroplane, which is supported by a rapid movement of translation, and of which all flying machines now being tried are samples. Of another form, a flyer carried by revolving wings, I need not speak in detail, because success in this form has not yet been reached. Whether it does or does not hereafter supersede the aeroplane, the principle of support through motion alone is common to both.

The other form is the airship proper, floating in the air by its own bouyancy, and not held up by propulsion. It is, in fact, the dirigible balloon, so enlarged and perfected that the term airship may well take the place of balloon in discussing it. For conciseness I shall use the terms "flyer" and "airship" in comparing these two forms of aerial vehicle.

It is much easier to point out the limits to the development of the flyer than to that of the airship. There are several drawbacks to every form of flyer, either of which seems fatal to its extensive use, and which taken together throw it out of the field of competition. One of these is inherent in the theory of its support by the air; the others are purely practical.

Being, as it were, supported upon the air, it must present to the latter a horizontal surface proportional to the entire weight to be carried, including motor, machine, and cargo. If one square yard of surface can be made to carry a certain weight at a certain speed, one thousand square yards will be required to carry one thousand times that weight. Any enlargement of the machine must therefore be in a horizontal direction. The estimate of weight must be so much per square yard of horizontal surface; an addition of weight in the vertical direction can never be possible. Hence, if any enlargement of the flyers is ever madefor example, if they are to carry two men instead of one, as at present-it

must be through enlarging their superficial extent in the same proportion. Reflecting on the present extent of the successful flyers, it will readily be seen that a practically unmanageable area of supporting surface and a consequent weakening of the machine will be required for any important enlargement. Whether the limit be one, two, or three men, every extension of it must, to secure the necessary strength, involve increased weight per square yard, which will be less and less compatible with its performance.

The

A practical difficulty which seems insuperable is that the flyer, supported only by its motion through the air, can never stop in flight to have its machinery repaired or adjusted. It makes toward the ground like a wounded bird the moment any stoppage occurs. navigator may be able to guide its fall, but not to prevent it. He can only choose the point of dropping among trees, houses, rivers, or fields which, within a limited area, will be productive of least damage. No engine yet built by human skill, much less the delicate motors necessary in the flyer, can be guaranteed against accident. The limitations upon a vehicle of transportation, the slightest accident to whose propelling machinery involves in all probability the destruction of the vehicle, as well as danger to the lives and limbs of the passengers, need not be dwelt upon. If a steamship were liable to go to the bottom the moment any accident occurred to her machinery, the twentieth century would have come upon us without steam navigation on the ocean.

Another serious limitation upon the flyer is that it cannot be navigated out of sight of the ground, and must descend at once if enveloped in fog. This necessity arises from the deviation in the apparent direction of gravity which must be produced by any change in the inclination of the supporting surface,

through the consequent acceleration or retardation of the speed. The principle at play is shown in an observation which may be made whenever a railway carriage at high speed is brought rapidly to a stop. A passenger standing well balanced on his feet during the period of retardation will find himself suddenly falling backward at the moment of the complete stop. He has been leaning backward while fancying himself erect.

Neither of the two drawbacks first mentioned is incident to the airship. Her buoyant power is proportional to her cubical contents, and not merely to the surface she presents to the air. She can therefore be enlarged in length, breadth, and thickness, instead of being confined to length and breadth, like the aeroplane. Floating in the air, she may possibly stop for repairs, which the flyer never can. This faculty carries with it a wide range of possibilities, how little soever may be the probabilities of their realization. A comparison with the steamship will show them in the clearest light.

As the ocean steamship has increased in size, she has also increased in speed. At the present moment the two largest ships afloat are also those of highest speed. It may have seemed to many, as it long did to the writer, that in this there was a constantly increasing sacrifice of power. The larger the ship the greater the power, and therefore the greater the consumption of coal, required to drive her at any given speed. It might, therefore, be felt that considerations of economy would suggest that the smaller ships should be built for high speed rather than the larger ones. But the advance is in reality upon correct lines. Leaving out the practical limits set by such conditions as the depth of harbors and the time required to load and unload, the larger the ship the more economically the application of power in driving her

at any given speed. The principle involved is simple. The model remaining the same, the carrying capacity increases as the cube of the length. But the resistance of the water, and therefore the power of the engine and the consumption of coal, increases only as the square of the length. Hence the larger the ship the more economically can a ton of cargo be carried at a given speed.

The same principle applies to the airship. The larger she can be built, the more economically she can be driven when we measure economy by the ratio of carrying power to cost of running. The limits to her possible size cannot be set by any principles of physical science. The question is simply one of constructive engineering— How large can we build her and still keep her manageable?

This view is not presented as opening out a vista of unlimited progress, but rather to avoid ignoring any possible line of progress. An airship of a size not yet dreamed of will require new devices for the application of power which may be utilized in our present system of land and ocean transport. We can never do away with the difference between the ground, the ocean, and the air as supporting agencies, and the solution of the problem must, in the long run, turn upon their respective advantages and drawbacks.

III.

Among the ideas which, inherited from our ancestors or formed in childhood, remain part of our nature through life may be placed the notion we so universally entertain that, if we succeed in navigating the air with a fair approach to safety, an important end will be reached. This notion must have been as deeply felt as one so purely speculative can be from the time that men reflected on the flight of birds. If any child to-day grows up without

many a time longing for the power to fly, and reflecting how much easier its possession would make it to pass from country to country, it must have been from some unusual power of refraining from useless speculation. The notion, justified perhaps in our ancestors, that flight through the air has some inherent element of superiority to locomotion on the surface of the earth or ocean is still a feature of our common nature.

Let us lay aside this notion long enough to inquire whether the cheapening of transportation by steam power during the last century has not practically done away with all the supposed advantages of flight through the air, which appeared in so strong a light to former generations. Probably few of us realize in our daily thought that it now costs less to transport any small light article-a pair of shoes for example-across the Atlantic than to deliver them from a shop to the house of a customer in New York or London. Careful thought may show us that, leaving aside exceptional cases, like that of striving to reach the Pole, the substitution of aerial for land and water transportation is at bottom the substitution for the solid ground of so imperfect a support for moving bodies as the thin air.

We can best judge this view by coming down to concrete facts. Let us take the case of an express train running from London to Edinburgh. When going at high speed the main resistance it has to encounter is that of the air. It is in overcoming this resistance that the greater part of its propulsive power is expended. Now, imagine the highest possible perfection in an aerial vehicle which shall carry passengers and mails from London to Edinburgh in competition with the railway. If the surface presented to the air by the vehicle were no greater than that presented by the train, it would still encounter a large fraction of the same

resistance when going at the same speed. But, as a matter of fact, owing to the necessary size of the flyer, the resisting surface would be vastly greater than in the case of the train, and the means of overcoming this resistance by adequate propulsive power would be more imperfect and expensive. In the case of the train the wheels of the engine are made effective by the reaction of the solid ground. In the airship the reaction is only that of the air, a condition which necessitates propelling surfaces of a superficial extent greater in proportion.

Needless to say, the consumption of fuel must be increased in proportion to the power to be expended. The Royal Mail airship will therefore have to consume several times as much coal as the engine of the Flying Scotchman if she is to carry the same burden. What the multiplier may be admits of at least an approximate estimate, but it may be feared that the most careful mathematical computation would show a disparity so extravagant as to deaden interest in the subject.

This view may appear in conflict with the principle already mentioned, that increased economy will be gained by increasing the size of the airship. But we must remember that the economy is measured by the ratio of cargo or other weight carried to fuel consumed. It must always cost more to run a large ship than to run a small one. Economy is gained only when we increase the dimensions of the airship so that she will carry more cargo than the ocean steamer or the railway train. The projector of an airship who would successfully compete with the steamship in ocean traffic must not permit his modesty to suggest beginning with dimensions less than a length of half a mile and a diameter of 600 feet. His ship might then be able to carry some 10,000 tons of cargo or 15,000 passengers, and it would be only through

these great possibilities that economic success would be reached. If this requirement seems extravagant or impracticable, the fault lies in the problem itself, and not in our treatment of it.

In order to present the case in another wholly practical aspect, it may be remarked that, no matter how high the speed of the airship, the wind would affect it by its entire velocity. A normal speed of 100 miles an hour would be reduced to one-half by meeting a wind blowing in the opposite direction at a rate of fifty miles an hour. It is true that a favoring wind at the same speed would accelerate its motion, and enable it to reach its destination more quickly. But it is needless to describe the practical drawbacks of so uncertain a system of transportation.

When we look carefully into the matter, we see that these are by no means the only drawbacks inherent to the general use of the airship. In addition to her being carried out of her course at the rate of twenty or thirty miles an hour by a wind blowing across her line of motion at this not unusual speed, comes the difficulty, we might say the impossibility, of finding her destination or effecting a landing in foggy weather. To appreciate these drawbacks it must be remembered that they do not arise merely from imperfections in the present development of the airship, but are inherent in any form of aerial vehicle, no matter to what degree it may be perfected. Unless the science of the future discovers some form of action between material masses, of the practical attainment of which the science of to-day gives not even a hint, any method of aerial transportation must be subjected not only to the drawbacks we have mentioned, but to a number of others which we refrain from setting forth merely because the items are all on the debit side.

But let us also in fairness see what

is to be placed on the credit side. First and almost alone among these must be in the reader's mind the fact that steam transportation on land requires the building of railways, which are so expensive that the capital invested in them probably exceeds that invested in all other forms of transportation. Moreover, there are large areas of the earth's surface not yet accessible by rail, among which are the two Poles and the higher mountains. All such regions, the mountains excepted, we may suppose to be attainable by the perfected airship of the future.

The more carefully we analyze these possible advantages, the more we shall find them to diminish in importance. Every part of the earth's surface on which men now live in large numbers, and in which important industries are prosecuted, can be now reached by railways, or will be so reached in time. True, this will involve a constantly increasing investment of capital. But the interest on this investment will be a trifle in comparison with the cost and drawbacks incident to the general introduction of the best system of aerial transportation that is even ideally possible in the present state of our knowl edge.

Let us stop a moment to see the framework of the reasoning on which our conclusions are based. We have not taken either the airship or the flyer of to-day as the measure of what is possible in the future. We have not dwelt upon the great ratio of failure to success or of labor cost to results in the trials hitherto made. The vehicle we have had in mind, and of which we have shown the shortcomings, is an ideal one to be realized, if possible, in the future-a vehicle in which every part shall be so nicely adjusted that the maximum of efficiency shall be reached with the least possible weight,

and the best devices used to diminish friction and insure the application of all the power available in the fuel to the purpose of driving. We have allowed no practical questions of construction to interfere with success. We have shown what would be the more than colossal dimensions of an airship that could successfully compete with the ocean steamship of to-day, without inquiring into the practicability of building her or the problem of managing her in an ocean storm. May we not say, as the outcome of these reflections, that the efforts at aerial navigation now being made are simply most ingenious attempts to substitute, as a support of moving bodies, the thin air for the solid ground? And is it not evident, on careful consideration, that the ground affords a much better base than air ever can? Resting upon it we feel safe and know where we are. In the air we are carried about by every wind that blows. Any use that we can make of the air for the purpose of transportation, even when our machinery attains ideal perfection, will be uncertain, dangerous, expensive, and inefficient, as compared with transportation on the earth and ocean. The glamor which surrounds the idea of flying through the air is the result of ancestral notions, implanted in the minds of our race before steam transportation had attained its present development. Exceptional cases there may be in which the airship will serve a purpose, but they are few and unimportant.

The attitude of the writer is not that of an advocate conducting a case against aerial navigation and leaving it to the other side to present its own views. He cheerfully admits the possibility of exceptional cases in which the airship may be a more effective means of attaining an end than any other yet at our command. The most promising result now in sight is the reaching of the Poles. It may be

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