In the early nineties the air ship was engaging the attention of many inventors, and was making important strides in the hands of Mr. Maxim. This unrivalled mechanician, in stating the case, premises that a motive power has to be discovered which can develop at least as much power in proportion to its weight as a bird is able to develop. He asserts that a heavy bird, with relatively small wings - such as a goose - carries about 150 lb. to the horse power, while the albatross or the vulture, possessed of proportionately greater winged surface, can carry about 250 lbs. per horse power.

Professor Langley, of Washington, working contemporaneously, but independently of Mr. Maxim, had tried exhaustive experiments on a rotating arm (characteristically designated by Mr. Maxim a "merry-go-round"), thirty feet long, applying screw propellers. He used, for the most part, small planes, carrying loads of only two or three pounds, and, under these circumstances, the weight carried was at the rate of 250 lbs. per horse power. His important statements with regard to these trials are that one-horse power will transport a larger weight at twenty miles an hour than at ten, and a still larger at forty miles than at twenty, and so on; that "the sustaining pressure of the air on a plane moving at a small angle of inclination to a horizontal path is many times greater than would result from the formula implicitly given by Newton, while, whereas in land or marine transport increased speed is maintained only by a disproportionate expenditure of power within the limits of experiment, in aerial horizontal transport the higher speeds are more economical of power than the lower ones."

This Mr. Maxim is evidently ready to endorse, stating, in his own words, that birds obtain the greater part of their support by moving forward with sufficient velocity so as to be constantly resting on new air, the inertia of which has not been disturbed. Mr. Maxim's trials were on a scale comparable with all his mechanical achievements. He employed for his experiments a rotating arm, sweeping out a circle, the circumference of which was 200 feet. To the end of this arm he attached a cigar-shaped apparatus, driven by a screw, and arranged in such a manner that aero-planes could be attached to it at any angle. These planes were on a large scale, carrying weights of from 20 lbs. to 100 lbs. With this contrivance he found that, whatever push the screw communicated to the aero-plane, "the plane would lift in a vertical direction from ten to fifteen times as much as the horizontal push that it received from the screw, and which depended upon the angle at which the plane was set, and the speed at which the apparatus was travelling through the air." Next, having determined by experiment the power required to perform artificial flight, Mr. Maxim applied himself to designing the requisite motor. "I constructed," he states, "two sets of compound engines of tempered steel, all the parts being made very light and strong, and a steam generator of peculiar construction, the greater part of the heating surface consisting of small and thin copper tubes. For fuel I employed naphtha."

This Mr. Maxim wrote in 1892, adding that he was then experimenting with a large machine, having a spread of over 100 feet. Labour, skill, and money were lavishly devoted henceforward to the great task undertaken, and it was not long before the giant flying machine, the outcome of so much patient experimenting, was completed and put to a practical trial. Its weight was 7,500 lbs. The screw propellers were nearly 18 feet in diameter, each with two blades, while the engines were capable of being run up to 360 horse power. The entire machine was mounted on an inner railway track of 9 feet and an outer of 35 feet gauge, while above there was a reversed rail along which the machine would begin to run so soon as with increase of speed it commenced to lift itself off the inner track.

In one of the latest experiments it was found that when a speed of 42 miles an hour was attained all the wheels were running on the upper track, and revolving in the opposite direction from those on the lower track. However, after running about 1,000 feet, an axle tree doubled up, and immediately afterwards the upper track broke away, and the machine, becoming liberated, floated in the air, "giving those on board a sensation of being in a boat."

The experiment proved conclusively to the inventor that a machine could be made on a large scale, in which the lifting effect should be considerably greater than the weight of the machine, and this, too, when a steam engine was the motor. When, therefore, in the years shortly following, the steam engine was for the purposes of aerial locomotion superseded by the lighter and more suitable petrol engine, the construction of a navigable air ship became vastly more practicable. Still, in Sir H. Maxim's opinion, lately expressed, "those who seek to navigate the air by machines lighter than the air have come, practically, to the end of their tether," while, on the other hand, "those who seek to navigate the air with machines heavier than the air have not even made a start as yet, and the possibilities before them are very great indeed."

As to the assertion that the aerial navigators last mentioned "have not even made a start as yet," we can only say that Sir H. Maxim speaks with far too much modesty. His own colossal labours in the direction of that mode of aerial flight, which he considers to be alone feasible, are of the first importance and value, and, as far as they have gone, exhaustive. Had his experiments been simply confined to his classical investigations of the proper form of the screw propeller his name would still have been handed down as a true pioneer in aeronautics. His work, however, covers far wider ground, and he has, in a variety of ways, furnished practical and reliable data, which must always be an indispensable guide to every future worker in the same field.

Professor Langley, in attacking the same problem, first studied the principle and behaviour of a well-known toy - the model invented by Penaud, which, driven by the tension of india-rubber, sustains itself in the air for a few seconds. He constructed over thirty modifications of this model, and spent many months in trying from these to as certain what he terms the "laws of balancing leading to horizontal flight." His best endeavours at first, however, showed that he needed three or four feet of sustaining surface to a pound of weight, whereas he calculated that a bird could soar with a surface of less than half a foot to the pound. He next proceeded to steam-driven models in which for a time he found an insuperable difficulty in keeping down the weight, which, in practice, always exceeded his calculation; and it was not till the end of 1893 that he felt himself prepared for a fair trial. At this time he had prepared a model weighing between nine and ten pounds, and he needed only a suitable launching apparatus to be used over water. The model would, like a bird, require an initial velocity imparted to it, and the discovery of a suitable apparatus gave him great trouble. For the rest the facilities for launching were supplied by a houseboat moored on the Potomac. Foiled again and again by many difficulties, it was not till after repeated failures and the lapse of many months, when, as the Professor himself puts it, hope was low, that success finally came. It was in the early part of 1896 that a successful flight was accomplished in the presence of Dr. Bell, of telephone fame, and the following is a brief epitome of the account that this accomplished scientist contributed to the columns of Nature: -

"The flying machine, built, apparently, almost entirely of metal, was driven by an engine said to weigh, with fuel and water, about 25 lbs., the supporting surface from tip to tip being 12 or 14 feet. Starting from a platform about 20 feet high, the machine rose at first directly in the face of the wind, moving with great steadiness, and subsequently wheeling in large curves until steam was exhausted, when, from a height of 80 or 100 feet, it shortly settled down. The experiment was then repeated with similar results. Its motion was so steady that a glass of water might have remained unspilled. The actual length of flight each time, which lasted for a minute and a half, exceeded half a mile, while the velocity was between twenty and twenty-five miles an hour in a course that was constantly taking it 'up hill.' A yet more successful flight was subsequently made."

But flight of another nature was being courageously attempted at this time. Otto Lilienthal, of Berlin, in imitation of the motion of birds, constructed a flying apparatus which he operated himself, and with which he could float down from considerable elevations. "The feat," he warns tyros, "requires practice. In the beginning the height should be moderate, and the wings not too large, or the wind will soon show that it is not to be trifled with." The inventor commenced with all due caution, making his first attempt over a grass plot from a spring board one metre high, and subsequently increasing this height to two and a half metres, from which elevation he could safely cross the entire grass plot. Later he launched himself from the lower ridges of a hill 250 feet high, when he sailed to a distance of over 250 yards, and this time he writes enthusiastically of his self-taught accomplishment: -

"To those who, from a modest beginning and with gradually increased extent and elevation of flight have gained full control over the apparatus, it is not in the least dangerous to cross deep and broad ravines. It is a difficult task to convey to one who has never enjoyed aerial flight a clear perception of the exhilarating pleasure of this elastic motion. The elevation above the ground loses its terrors, because we have learned by experience what sure dependence may be placed upon the buoyancy of the air."

As a commentary to the above we extract the following: - "We have to record the death of Otto Lilienthal, whose soaring machine, during a gliding flight, suddenly tilted over at a height of about 60 feet, by which mishap he met an untimely death on August 9th, 1896." Mr. O. Chanute, C.E. of Chicago, took up the study of gliding flight at the point where Lilienthal left it, and, later, Professor Fitzgerald and others. Besides that invented by Penaud, other aero-plane models demanding mention had been produced by Tatin, Moy, Stringfellow, and Lawrence Hargrave, of Australia, the subsequent inventor of the well-known cellular kite. These models, for the most part, aim at the mechanical solution of the problem connected with the soaring flight of a bird.

The theoretical solution of the same problem had been attacked by Professor Langley in a masterly monograph, entitled "The Internal Work of the Wind." By painstaking experiment with delicate instruments, specially constructed, the Professor shows that wind in general, so far from being, as was commonly assumed, mere air put in motion with an approximately uniform velocity in the same strata, is, in reality, variable and irregular in its movements beyond anything which had been anticipated, being made up, in fact, of a succession of brief pulsations in different directions, and of great complexity. These pulsations, he argues, if of sufficient amplitude and frequency, would be capable, by reason of their own "internal work," of sustaining or even raising a suitably curved surface which was being carried along by the main mean air stream. This would account for the phenomenon of "soaring." Lord Rayleigh, discussing the same problem, premises that when a bird is soaring the air cannot be moving uniformly and horizontally. Then comes the natural question, Is it moving in ascending currents? Lord Rayleigh has frequently noticed such currents, particularly above a cliff facing the wind. Again, to quote another eminent authority, Major Baden-Powell, on an occasion when flying one of his own kites, found it getting to so high an angle that it presently rose absolutely overhead, with the string perpendicular. He then took up a heavy piece of wood, which, when tied to the string, began to rise in the air. He satisfied himself that this curious result was solely due to a strong uptake of the air.

But, again, Lord Rayleigh, lending support to Professor Langley's argument, points out that the apparent cause of soaring may be the non-uniformity of the wind. The upper currents are generally stronger than the lower, and it is mechanically possible for a bird, taking advantage of two adjacent air streams, different in velocity, to maintain itself in air without effort on its own part.

Lord Rayleigh, proceeding to give his views on artificial flight, declares the main problem of the flying machine to be the problem of the aerial plane. He states the case thus: - "Supposing a plane surface to be falling vertically at the rate of four miles an hour, and also moving horizontally at the rate of twenty miles an hour, it might have been supposed that the horizontal motion would make no difference to the pressure on its under surface which the falling plane must experience. We are told, however, that in actual trial the horizontal motion much increases the pressure under the falling plane, and it is this fact on which the possibility of natural and artificial flight depends.

Ere this opinion had been stated by Lord Rayleigh in his discourse on "Flight," at the Royal Institution, there were already at work upon the aero-plane a small army of inventors, of whom it will be only possible in a future chapter to mention some. Due reference, however, should here be made to Mr. W. F. Wenham, of Boston, U.S.A., who had been at work on artificial flight for many years, and to whose labours in determining whether man's power is sufficient to raise his own weight Lord Rayleigh paid a high tribute. As far back as 1866 Mr. Wenham had published a paper on aerial locomotion, in which he shows that any imitation by man of the far-extended wings of a bird might be impracticable, the alternative being to arrange the necessary length of wing as a series of aero-planes, a conception far in advance of many theorists of his time.

But there had been developments in aerostation in other lines, and it is time to turn from the somewhat tedious technicalities of mechanical flight and the theory or practice of soaring, to another important means for traversing the air - the parachute. This aerial machine, long laid aside, was to lend its aid to the navigation of the air with a reliability never before realised. Professor Baldwin, as he was termed, an American aeronaut, arrived in England in the summer of 1888, and commenced giving a series of exhibitions from the Alexandra Palace with a parachute of his own invention, which, in actual performance, seems to have been the most perfect instrument of the kind up to that time devised. It was said to be about 18 feet in diameter, whereas that of Garnerin, already mentioned, had a diameter of some 30 feet, and was distinctly top-heavy, owing to its being thus inadequately ballasted; for it was calculated that its enormous size would have served for the safe descent, not of one man, but of four or five. Baldwin's parachute, on the contrary, was reckoned to give safe descent to 250 lbs., which would include weight of man and apparatus, and reduce the ultimate fall to one not exceeding 8 feet. The parachute was attached to the ring of a small balloon of 12,000 cubic feet, and the Professor ascended, sitting on a mere sling of rope, which did duty for a car.

Mr. Thomas Moy, who investigated the mechanics of the contrivance, estimated that after a drop of 16 feet, the upward pressure, amounting to over 2 lb. per square foot, would act on a surface of not less than 254 square feet. There was, at the time, much foolish comment on the great distance which the parachute fell before it opened, a complete delusion due to the fact that observers failed to see that at the moment of separation the balloon itself sprang upward.