Decembee 1, 1894.] 



KNOWLEDGE. 



275 



wind, in a journey of ninety miles. The whole of the gas 

 had then to he wasted, the bag and apparatus paclfed up, 

 and all returned to Paris by railway. That was a long 

 way ofl' the accomplishment of aerial navigation, and we 

 are not likely to get much nearer by means of propelled 

 gas bags. In spite of the many failures in tliis direction, 

 many people look upon it as the only feasible plan, and the 

 United States Patent Examiners have refused Mr. Maxim 

 a patent because he does not describe and claim a gas bag 

 in his combination. 



Just as one class of experimenters have inscribed 

 " Finality Jack " upon the gas bag, so another and larger 

 class have come to the conclusion that the screw is the 

 best and only available propeller ; among the rest, Mr. 

 Maxim and Lord Kelvin. 



This mistake has arisen from its purtial success in water, 

 in the propulsion of steam ships. 1 say " partial " success 

 because there is such a thing as " slip." This slip of the 

 screw is bad enough in water ; it is ruinously wasteful in 

 attempts at mechanical flight. It is the delusive estimate 

 of the efficiency of the screw propeller in air which caused 

 someone to express himself as follows, in a daily newspaper ; 

 " Inventors are constantly, to the amusement of practical 

 engineers and mechanicians, inventing forms of flying 

 machines, oblivious to the fact that every engineer of 

 experience could produce at a few hours' notice the neces- 

 sary form of machine, when the, at present, undiscovered 

 and necessary materials are found." I withhold the name. 

 This man evidently had settled it in his mind that all the 

 apparatus required consisted of an aeroplane, screws, and a 

 lir/ht motor ; the error into which Henson fell in 18i2, and 

 Maxim fifty years after. 



The following extract from a scientific journal is still 



more amusing; — "It is true that Mr. has recourse 



to a whirling propeller, which is about as foreign to a 

 natural device as anything could well be. The neglect of 

 rotary motion as a means of propulsion (except in the 

 transference of wave motion) is one of Nature's most 

 unaccountable proceedings. If it were not for the beautiful 

 efficiency of her nerve and muscle motors, perhaps she 

 would be driven to the continuous efl'ort of the screw or 

 wheel rather than the flappings and reciprocations of wings 

 and legs she now adopts." 



The assurance of this critic of Nature's methods will be 

 obvious, if we substitute the Creator of Nature for " she." 



One gentleman became so enamoured of the aerial screw, 

 some few years ago, that he actually proposed to drive 

 ships with aerial screws instead of marine screws. It is 

 surprising that he did not propose to drive railway trains 

 by the same means. 



Now what is slip .' Suppose a boat, when travelling in 

 water at eight feet per second has a resistance of 62'5 

 pounds. Fit this boat with a small steam engine and a 

 drum. Let this drum wind up a rope, the other end of 

 which is made fast to some stationary object. Disregarding 

 the weight of the rope, we may assume it to be five miles 

 long. With a strain upon the rope of sixty-two and a half 

 pounds exerted by the engine, the boat would cover the 

 distance of five miles in an hour. There would be no slip. 

 Now, instead of making the rope fast to a fixed object, let it 

 be attached to a floating board, having a vertical immersed 

 surface of one square foot. A strain or pull upon this 

 surface of sixty-two and a half pounds would produce a 

 speed of eight feet per second of both the boat and the 

 board — that is to say, the board would "come home" 

 towards the boat at the same speed that the boat would 

 travel, thus winding in the rope at sixteen feet per second, 

 and the rope would be all wound on the barrel when the 

 boat had travelled but two and a half miles. In this case, 



half the power would be wasted in slip. For certain well- 

 known reasons, the screw propeller is more efficient than 

 a mere flat board would be, but the slip of the screw in 

 water generally causes a loss of twenty to twenty-five 

 per cent. 



A screw bolt driven into a nut has no slip. A marine 

 screw propeller, working in a watei- nut, produces slip and 

 consequent waste of power. The air nut, however, is 

 excessively yielding, and the slip is far too great for the 

 propulsion of a successful aerial machine. 



In the sixth annual report of the Aeronautical Society 

 of Great Britain (1871) will be found a calculation by 

 Mr. Wenham of the slip, with vertical screws in the air, 

 in which he estimated that twenty-two horse-power would 

 be required to raise a weight of two hundred pounds. My 

 experiments have given better results than this, but even 

 five horse-power to two hundred pounds indicates great 

 loss by slip. 



Comparmg Henson's device of 1842 with Maxim's of 

 1892, one sees very little advance in the latter. Henson 

 placed the axles of his screws in line with his aeroplane, 

 and Maxim places his lower down. Of the two, Henson's 

 appears the most logical arrangement. Henson and 

 Stringfellow used cop-tube boilers, to prevent priming. 

 Maxim's first boiler consisted of a crowd of tubes, which I 

 know from experience would get uncomfortably hot ; and 

 he afterwards reduced the number considerably, and 

 brought his boiler to a less complicated state, something 

 like my 1877 patent. Mr. Henson's machine failed for 

 want of propulsive power, due to slip. When started it 

 slid backwards and injured its tail. Mr. Maxim's has 

 failed for the same cause. 



But it may be thought that I used common screws in 

 1875 ; this would be a mistake. My 1871 patent expressly 

 describes feathering screws — that is, the pitch of the blades 

 was very much greater on the descending side than on the 

 ascending side, whereby a lifting force was obtained on both 

 sides of the axle ; one derived from the downward action, 

 and the other from the forward motion. 



My experiment with this machine was very much 

 handicapped by having to sustain half the weight of two 

 guide ropes, each one hundred and fifty feet long, and the 

 reduction of power from the forward lamps extinguishing 

 the lamps at the rear of the boiler. The machine was 

 afterwards destroyed in a stiff gale, while moving it from 

 the Rotunda in the Crystal Palace grounds to a new shed 

 near the Penge entrance. I afterwards fitted two hori- 

 zontal screws, twelve feet diameter, to the three-horse 

 engine ; but these only gave an upward thrust of one 

 hundred and twenty pounds, thus again proving the 

 importance of loss by slip. 



I will conclude by suggesting that Mr. Maxim should 

 take his machine to (say) Salisbury Plain ; lay down his 

 piece of railway without the top rails ; lay down a rope on 

 the ground, the longer the better ; unship his screws and 

 mount a drum, as shown in the sketch, passing the rope 

 two or three times round the drum, and driving the drum 

 by his engines. By this arrangement he will avoid slip, 

 and, if his engines give out anything like three hundred 

 horse-power, his machine will rise a short distance from 

 the ground, as shown. The weight of rope looped up from 

 the ground will limit the height, and the two small guide 

 pulleys, fore and aft, will secure horizontal stability. 



