the dominion of the air-第48章
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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 airthe 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。
CHAPTER XXII。 THE STORY OF THE SPENCERS。
It has been in the hands of the Spencers that the parachute； as also many other practical details of aeronautics； has been perfected； and some due sketch of the career of this family of eminent aeronauts must be no longer delayed。
Charles Green had stood godfather to the youngest son of his friend and colleague； Mr。 Edward Spencer； and in later years； as though to vindicate the fact； this same son took up the science of aeronautics at the point where his father had left it。 We find his name in the records of the Patent Office of 1868 as the inventor of a manumetric flying machine； and there are acc
