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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 prohably 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.
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 larged and perfected that the term air. ship 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 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; addition of weight in the vertical direction can never be possible. Hence, if any enlargement of the flyers is ever made for example, if they are to carry two men instead of one, as at present-it
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
must be through enlarging their super- through the consequent acceleration or ficial extent in the same proportion. retardation of the speed. The princiReflecting on the present extent of the ple at play is shown in an observation successful ilyers, it will readily be seen which may be made whenever a railthat a practically unmanageable area way carriage at high speed is brought of supporting surface and a consequent rapidly to a stop. A passenger standweakening of the machine will be re- ing well balanced on his feet during quired for any important enlargement. the period of retardation will find himWhether the limit be one, two, or three self suddenly falling backward at the men, every extension of it must, to se- moment of the complete stop. He has cure the necessary strength, involve in- been leaning backward while fancying creased weight per square yard, which himself erect. will be less and less compatible with Neither of the two drawbacks first its performance.
mentioned is incident to the airship. A practical difficulty which seems in- Her buoyant power is proportional to superable is that the flyer, supported her cubical contents, and not merely to only by its motion through the air, can the surface she presents to the air. never stop in flight to have its machin- She can therefore be enlarged in length, ery repaired or adjusted. It makes to- breadth, and thickness, instead of being ward the ground like a wounded bird confined to length and breadth, like the the moment any stoppage occurs. The aeroplane. Floating in the air, she navigator may be able to guide its fall, may possibly stop for repairs, which but not to prevent it. He can only the flyer never can. This faculty carchoose the point of dropping among ries with it a wide range of possibilitrees, houses, rivers, or fields which, ties, how little soever may be the probwithin a limited area, will be produc- abilities of their realization. A comtive of least damage. No engine yet parison with the steamship will show built by human skill, much less the them in the clearest light. delicate motors necessary in the flyer, As the ocean steamship has increased can be guaranteed against accident. in size, she has also increased in speed. The limitations upon a vehicle of trans- At the present moment the two largest portation, the slightest accident to ships afloat are also those of highest whose propelling machinery involves speed. It may have seemed to many, as in all probability the destruction of the it long did to the writer, that in this vehicle, as well as danger to the lives there was a constantly increasing sacand limbs of the passengers, need not rifice of power. The larger the ship be dwelt upon. If a steamship were the greater the power, and therefore liable to go to the bottom the moment the greater the consumption of coal, any accident occurred to her machin- required to drive her at any given ery, the twentieth century would have speed. It might, therefore, be felt that come upon us without steam navigation considerations of economy would sugon the ocean.
gest that the smaller ships should be Another serious limitation upon the built for high speed rather than the flyer is that it cannot be navigated out larger ones. But the advance is in of sight of the ground, and must de reality upon correct lines. Leaving scend at once if enveloped in fog. This out the practical limits set by such connecessity arises from the deviation in ditions as the depth of harbors and the the apparent direction of gravity which time required to load and unload, the must be produced by any change in the larger the ship the more economically inclination of the supporting surface, the application of power in driving her
at any given speed. The principle in- many a time longing for the power to volved is simple. The model remain- fly, and reflecting how much easier its ing the same, the carrying capacity in- possession would make it to pass from creases as the cube of the length. But country to country, it must have been the resistance of the water, and there- from some unusual power of refrainfore the power of the engine and the ing from useless speculation. The noconsumption of coal, increases only as tion, justified perhaps in our ancestors, the square of the length. Hence the that flight through the air has some inlarger the ship the more economically herent element of superiority to locomocan a ton of cargo be carried at a given tion on the surface of the earth or ocean speed.
is still a feature of our common nature. The same principle applies to the Let us lay aside this notion long airship. The larger she can be built, enough to inquire whether the cheapenthe more economically she be ing of transportation by steam power driven when we measure economy by during the last century has not practithe ratio of carrying power to cost cally done away with all the supposed of running The limits to her possible advantages of flight through the air, size cannot be set by any principles of which appeared in so strong a light to physical science. The question is sim- former generations. Probably few of ply one of constructive engineering- us realize in our daily thoughỉ that it How large can we build her and still now costs less to transport any small keep her manageable?
light article-a pair of shoes for exThis view is not presented as open- ample--across the Atlantic than to deing out a vista of unlimited progress, liver them from a shop to the house of but rather to avoid ignoring any pos- a customer in New York or London. sible line of progress. An airship of Careful thought may show us that, a size not yet dreamed of will require leaving aside exceptional cases, like new devices for the application of that of striving to reach the Pole, the power which may be utilized in our substitution of aerial for land and present system of land and ocean trans- water transportation is at bottom the port. We can never do away with substitution for the solid ground of so the difference between the ground, the imperfect a support for moving bodies ocean, and the air as supporting agen- as the thin air. cies, and the solution of the problem We can best judge this view by commust, in the long run, turn upon their ing down to concrete facts. Let us respective advantages and drawbacks. take the case of an express train run
ping from London to Edinburgh. When III.
going at high speed the main resistance Among the ideas which, inherited it mits to encounter is that of the air. from our ancestors or formed in child- It is in overcoming this resistance that hood, remain part of our nature through the greater part of its propulsive power life may be placed the notion we so uni. is expended. Now, imagine the highversally entertain that, if we succeed est possible perfection in an aerial vein navigating the air with a fair ap- hicle which shall carry passengers and proach to safety, an important end will mails from London to Edinburgh in be reached. This notion must have competition with the railway. If the been as deeply felt as one so purely surface presented to the air by the speculative can be from the time that vehicle were no greater than that premen reflected on the flight of birds. If sented by the train, it would still enany child to-day grows up without counter a large fraction of the same resistance when going at the same these great possibilities that economic speed. But, as a matter of fact, owing success would be reached. If this reto the necessary size of the flyer, the quirement seems extravagant or imresisting surface would be vastly practicable, the fault lies in the problem greater than in the case of the train, itself, and not in our treatment of it. and the means of overcoming this re- In order to present the case in ansistance by adequate propulsive power other wholly practical aspect, it may would be more imperfect and expen- be remarked that, no matter how high sive. In the case of the train the the speed of the airship, the wind wheels of the engine are made effective would affect it by its entire velocity. by the reaction of the solid ground. A normal speed of 100 miles an hour In the airship the reaction is only that would be reduced to one-half by meetof the air, a condition which necessi- ing a wind blowing in the opposite ditates propelling surfaces of a superfi- rection at a rate of fifty miles an hour. cial extent greater in proportion. It is true that a favoring wind at the
Needless to say, the consumption of same speed would accelerate its motion, fuel must be increased in proportion to and enable it to reach its destination the power to be expended. The Royal more quickly. But it is needless to deMail airship will therefore have to con- scribe the practical drawbacks of so sume several times as much coal as uncertain a system of transportation. the engine of the Flying Scotchman if When we look carefully into the matshe is to carry the same burden. What ter, we see that these are by no means the multiplier may be admits of at the only drawbacks inherent to the least an approximate estimate, but it general use of the airship. In addition may be feared that the most careful to her being carried out of her course mathematical computation would show at the rate of twenty or thirty miles a disparity so extravagant as to deaden an hour by a wind blowing across her interest in the subject.
line of motion at this not unusual This view may appear in conflict speed, comes the difficulty, we might with the principle already mentioned, say the impossibility, of finding her that increased economy will be gained destination or effecting a landing in by increasing the size of the airship. foggy weather. To appreciate these But we must remember that the econ- drawbacks it must be remembered that omy is measured by the ratio of cargo they do not arise merely from imperor other weight carried to fuel con- fections in the present development of sumed. It must always cost more to the airship, but are inherent in any run a large ship than to run a small form of aerial vehicle, no matter to one. Economy is gained only when what degree it may be perfected. Unwe increase the dimensions of the air- less the science of the future discovers ship so that she will carry more cargo some form of action between material than the ocean steamer or the railway masses, of the practical attainment of train. The projector of an airship who which the science of to-day gives not would successfully compete with the even a hint, any method of ae al transsteamship in ocean traffic must not per portation must be subjected not only mit his modesty to suggest beginning to the drawbacks we have mentioned, with dimensions less than a length of but to a number of others which we half a mile and a diameter of 600 feet. refrain from setting forth merely beHis ship might then be able to carry cause the items are all on the debit some 10,000 tons of cargo or 15,000 pas- side. sengers, and it would be only through But let us also in fairness see what
is to be placed on the credit side. and the best devices used to diminish First and almost alone among these friction and insure the application of all must be in the reader's mind the fact the power available in the fuel to the that steam transportation on land re- purpose of driving. We have allowed quires the building of railways, which no practical questions of construction are so expensive that the capital in- to interfere with success. We have vested in them probably exceeds that shown what would be the more than inrested in all other forms of trans- colossal dimensions of an airship that portation. Moreover, there are large could successfully compete with the areas of the earth's surface not yet ac- ocean steamship of to-day, without incessible by rail, among which are the quiring into the practicability of buildtwo Poles and the higher mountains. ing her or the problem of managing All such regions, the mountains ex- her in an ocean storm. May we not cepted, we may suppose to be attain- say, as the outcome of these reflections, able by the perfected airship of the that the efforts at aerial navigation now future.
being made are simply most ingenious The more carefully we analyze these attempts to substitute, as a support of possible advantages, the more we shall moving bodies, the thin air for the solid find them to diminish in importance. ground? And is it not evident, on careEvery part of the earth's surface on ful consideration, that the ground afwhich men now live in large numbers, fords a much better base than air ever and in which important industries are can? Resting upon it we feel safe and prosecuted, can be now reached by rail- know where we are. In the air we are ways, or will be so reached in time. carried about by every wind that True, this will involve a constantly in- blows. Any use that we can make of creasing investment of capital. But the air for the purpose of transportathe interest on this investment will tion, even when our machinery attains be a trifle in comparison with the cost ideal perfection, will be uncertain, danand drawbacks incident to the general gerous, expensive, and inefficient, as introduction of the best system of aerial compared with transportation on the transportation that is even ideally pos
earth and ocean. The glamor which sible in the present state of our knowl. surrounds the idea of flying through edge.
the air is the result of ancestral notions,
implanted in the minds of our race beLet us stop a moment to see the fore steam transportation had attained framework of the reasoning on which its present development. Exceptional our conclusions are based. We have cases there may be in which the airnot taken either the airship or the flyer ship will serve a purpose, but they are of to-day as the measure of what is few and unimportant. possible in the future. We have not The attitude of the writer is not that dwelt upon the great ratio of failure to of advocate conducting a success or of labor cost to results in against aerial navigation and leaving it the trials hitherto made. The vehicle to the other side to present its own we have had in mind, and of which we views. He cheerfully admits the poshave shown the shortcomings, is an sibility of exceptional cases in which ideal one to be realized, if possible, in the airship may be a more effective the future-a vehicle in which every means of attaining an end than any part shall be so nicely adjusted that other yet at our command. The most the maximum of efficiency shall be promising result now in sight is the reached with the least possible weight, reaching of the Poles. It may be