Artificial and Natural Flight

When the aeroplane a (Fig. 28) was placed in a horizontal position, and the apparatus carefully balanced, it showed at a wind velocity of 40 miles an hour, a lift of 1·56 lbs., and a drift of 0·08 lb.; at an angle of 1 in 20, a lift of 3·62 lbs. and a drift of 0·21 lb.; at an angle of 1 in 16, a lift of 4·09 lbs. with a drift of 0·26 lb.; at an angle of 1 in 14, a lift of 4·5 lbs. and a drift of 0·33 lb.; at an angle of 1 in 12, a lift of 5 lbs. and a drift of 0·43 lb.; at an angle of 1 in 10, a lift of 5·75 lbs. and a drift of 0·60 lb.; at an angle of 1 in 8, a lift of 6·75 lbs. and a drift of 0·86 lb. The blast was then increased to a velocity of 47·33 miles an hour, when it was found that the lift at an angle of 1 in 16 was 5 lbs. and the drift 0·33 lb. It will be observed that this aeroplane was only 8 inches wide, while the others were 12 inches or more. They were all rather more than 3 feet long, but the width of the blast to which they were subjected was exactly 3 feet, and they were placed as near to the end of the trunk as possible.

The next experiments were made with the view of ascertaining what effect would be produced when objects were placed near to each other (see Fig. 29). Two bars of wood 2 inches thick, and shaped as shown in the drawing, were placed on the machine and subjected to a blast of 41 miles per hour; the drift at various distances from center to center was as follows:—

It will be seen by this that the various members constituting the frame of a flying machine should not be placed in close proximity to each other.

A bar of wood similar in shape to d (Fig. 25), but being 9 inches wide instead of 12 inches, was mounted in a wind blast of 41 miles an hour, with the front edge 3·31 inches above the rear edge, and this showed a lift of 7·08 lbs. and a drift of 3·23 lbs. When the angle was reduced to 2·31 inches, it gave a lift of 4·53 lbs. with a drift of 0·78 lb., and with the angle reduced to 1·31 inches, the lift was 3·37 lbs. and the drift 0·5 lb. It will, therefore, be seen that even objects rounded on both sides give a very fair lift, and in designing the framework of machines advantage should be taken of this knowledge. The bar of wood c (Fig. 25) was next experimented with. With the sharp edge to the wind, and with the front edge 2 inches higher than the rear edge, the lift was 2·54 lbs. and the drift 0·76 lb. By turning it about so that the wind struck the thick edge, the lift was 4·45 lbs. and the drift 0·47 lb. This seemed rather remarkable, but, as it actually occurred, I mention it for other people to speculate upon. It, however, indicates that we should take advantage of all these peculiarities of the air in constructing the framework of a machine, which in itself is extremely important, as I find that a very large percentage of the energy derived from the engines is consumed in forcing the framework through the air. It is quite true that a certain amount of this energy may be recovered by the screw, provided that the screw runs in the path occupied by the framework. Still, it is much better that the framework should be so constructed as to offer the least possible resistance to the air, and, as far as possible, all should be made to give a lifting effect.

Having ascertained the lifting effect of wooden aeroplanes of various forms and at varying velocities of the wind, and, also, the resistance offered by various bodies driven through the air, I next turned my attention to the question of condensation. I wished to recover as much water as possible from my exhaust steam. I had already experimented with Mr. Horatio Philipps’ sustainers, and I found that their lifting effect was remarkable. A curious thing about these aeroplanes was that they gave an appreciable lift when the front edge was rather lower than the rear. I therefore determined to take advantage of this peculiar phenomenon, and to make my condenser tubes as far as possible in the shape of Mr. Philipps’ sustainers. Fig. 30 shows a section of one of these tubes, in which a, a is the top surface, b a soldered joint, and c the steam space. These were mounted on a frame as shown at a (Fig. 31). I had already found that bodies placed near to each other offered an increased resistance to the air, but by placing these sustainers in the manner shown this was avoided, as the air had sufficient space to pass through without being either driven forward or compressed. It was found by experiment that the arrangement of tubes or sustainers, shown at d, d (Fig. 31), was very efficient as a condenser, but it gave a very heavy drift and no lifting effect at all; whereas, on the other hand, the arrangement shown at a was equally efficient, and, at the same time, gave a decided lifting effect. When twelve of these tubes or sustainers were placed at an angle of 1 in 12, the lifting effect was 12·63 lbs. and the drift 2·06 lbs. It was found, however, that a good deal of the drift was due to the wind getting at the framework that was used for holding the sustainers in position. With a wind velocity of 40 miles an hour and a temperature of 62° F., 2·25 lbs. of water were condensed in five minutes, and, while running, the back edge of the sustainers was quite cool. At another trial of the same arrangement under the same conditions, the lift was 11 lbs. and the drift 2·63 lbs. It is quite possible on this occasion that the metal was so extremely thin that the angles were not always maintained; consequently, that no two readings would be alike. It was found at this point that the belt was slipping, and a larger pulley was put on the driving shaft of the screws; and under these conditions, with a wind of 49 miles per hour and an angle of 1 in 8, the lifting effect ran up to 14·87 lbs. with a drift of 2·44 lbs., and the condenser delivered 2·87 lbs. of water from dry steam in five minutes. The weight of metal in this condenser was extremely small, the thickness being only about ¹⁄₅₀₀ of an inch. This condenser delivered the weight of the sustainers in water every five minutes. They should, however, have been twice as heavy. Cylinder oil was now introduced with the steam in order to ascertain what effect this would have. After seven minutes’ steaming, 2·25 lbs. of water were condensed in five minutes. It will be seen from these experiments that an atmospheric condenser, if properly constructed, is fairly efficient. Roughly speaking, it requires 2,400 times as much air in volume as of water to use as a cooling agent. With the steam engine condenser only a relatively small amount of water is admitted, and this is found to be sufficient; but in an atmospheric condenser working in the atmosphere, it must be as open as possible, so that no air which has struck one heated surface can ever come in contact with another.

CHAPTER V.
EXPERIMENTS WITH APPARATUS ATTACHED TO A ROTATING ARM.

From what information I have at hand, it appears that Prof. Langley made his first experiments with a small apparatus, using aeroplanes only a few inches in dimensions which travelled round a circle perhaps 12 feet in diameter. With this little apparatus, he was able to show that the lifting effect of aeroplanes was a great deal more than had previously been supposed. After having made these first experiments, he seems to have come to the conclusion that Newton’s law was erroneous. Shortly after Langley had made these experiments on what he called a whirling table, which, however, was not a very appropriate name, I made an apparatus myself, but very much larger than that employed by Prof. Langley. I reckoned the size of my aeroplanes in feet, where he had reckoned his in inches. The circumference of the circle around which my aeroplanes were driven was exactly 200 feet, and shortly after this Langley constructed another apparatus, the same dimensions as my own. From an engraving which I have before me, it appears that he constructed an extremely large wooden scale beam supported by numerous braces, but free to be tilted in a vertical direction after the manner of all other scale beams. As this apparatus was of great weight and offered enormous resistance to the air, I do not understand how it was possible to obtain any very correct readings, especially as it was in the open and subject to every varying current of air.

In constructing my apparatus, which is shown in the photographs, and also in a side elevation (Fig. 32), I aimed at making the apparatus very light and strong, avoiding as far as possible atmospheric resistance. In the drawing, a, is a thick seamless steel pipe 6 inches diameter; b, is a cast-iron pedestal firmly bolted to d, and connected to a large cast-iron spider embedded in hydraulic cement; by this means great rigidity and stiffness were obtained. n, n was formed of strong Georgia pine planks 2 inches thick, and strongly bolted together. The two members of the long radial arm h, h, were made of Honduras mahogany, an extremely strong wood, and had their edges tapered off as shown at y, y. The power was transmitted from a small steam engine provided with a sensitive governor through the shaft f, f. In the base c, of the casting b, was placed a pair of tempered steel bevel gears, giving to the vertical shaft a high velocity. From a pulley on the top of this shaft, the belt i, was run through the arms h, h, as shown in section y, y. This gave a rapid rotation to the screw shaft in a very simple manner. The operation of the machine was as follows:—the aeroplane g, to be tested was secured to a sort of weighing apparatus which is shown in detail (Fig. 36), and the screw attached to the shaft. Upon starting the engine, a very rapid rotation was given to the screw which caused the radial arm to travel at a high velocity, the whole weight resting on a ball bearing at w. The radial arms and all of their attachments were balanced by a cigar-shaped lead weight s, which was secured to a sliding bar so as to make it easily adjustable. The thrust of the screw caused the screw shaft to travel longitudinally, and this was opposed by a spring connected by a very thin and light wire to the pointer of the index o. As the apparatus rotated rather slowly on account of its great diameter, it was quite possible to observe the lift while the machine was running at its highest speed. The aeroplanes were mounted after the manner of the tray of a grocer’s scales (see Fig. 36), and the lift of the aeroplane was determined by what it would lift at r—that is, while the machine was running at a given speed, iron or lead weights were placed in the pail r, until the lift of the aeroplane was exactly balanced, and then, in order to ascertain exactly what the lift was, the aeroplane was placed under what might be called a small crane, and a cord, running over a pulley, attached. The amount of weight necessary to lift the plane into the same position that it occupied while running was taken as its true lift. In order to facilitate experiments the gauge p, was provided. This gauge consisted of a large glass tube and the index p, with a quantity of red water at q. The centrifugal force of rotation caused the red water to rise in the tube. This was easily seen, so that if experiments were being tried, we will say at 50 miles an hour, it was always possible to turn on steam until the red liquid mounted to 50. This device was very simple and effective, and saved a great deal of time. In order to prevent the twisting of the radial arm, a piece of stiff oval steel tube 12 feet long was secured between the arms at j, and on each end of this tube were attached the wires u, u. This not only effectually supported the end of the arm, but at the same time prevented twisting and made everything extremely stiff. Of course, while the machine was running at a high velocity, centrifugal force had to be dealt with, and in order to prevent this from causing friction in the articulated joints of the weighing apparatus (Fig. 36), thin steel wires k, k were provided. As this apparatus was in the open, it was found that the slightest movement of the air greatly interfered with its action. On one occasion when a fabric covered aeroplane, 4 feet long by 3 feet wide, was placed in position, the four corners being held down by the wires v, v, and the apparatus driven at a high velocity, a sudden gust of wind snapped two of the wires, broke the aeroplane, and the flying fragments smashed the screw, and this notwithstanding that each of the four wires was supposed to be strong enough to resist at least four times any possible lifting that the whole aeroplane might be subjected to.

In order to ascertain the force and direction of the wind, I made an extremely simple and effective apparatus which is fully shown (see Fig. 38). Whilst conducting these experiments it occurred to me, when a large aeroplane was used, that after it had been travelling for a considerable time, it would impart to the air in the path of its travel, a downward motion, and that the lifting effect would be greatly reduced on this account. In order to test this, I provided four light brass screws and mounted them, as shown at x, on a hardened polished steel point much above their centre of gravity, so that they balanced themselves. On account of the absence of friction, they were easily rotated, and responded to the least breath of air that might be moving. One morning when there was a dead calm, I placed four of these screws equidistant around the whole circle. Some of them rotated very slowly in one direction and some in another; some alternated, but all their motions were extremely slow. However, upon setting the machine going with a large aeroplane and a powerful screw, I found after a few turns that the air was moving downwards around the whole circle at a velocity of about 2 miles an hour, but as the screw was a considerable distance below the aeroplane, I estimated that the actual downward velocity of the air in which the aeroplane was travelling was about 4 miles an hour. The result of my experiments are clearly shown in an unpublished paper which I wrote at the time, and as it is of considerable historical interest, I have placed it in the appendix, notwithstanding that there may be certain repetitions.

In Fig. 36, a, a is the body of the apparatus, partly of gunmetal and partly of wood. It is provided with a steel shaft to which the screw h, is attached, and also with a cylindrical pulley for taking the belt. The thrust of the screw pushes the shaft inwards and records the lift at o (Fig. 32). The corners of the aeroplane g, g, are attached by wires to the steel plate e. b, b, is a four-arm spider for holding the ends of the parallel bars c, c, and d, d, show vertical steel bars to which all devices to be tested are attached. In testing aeroplanes, weights may be placed at e, sufficient to balance the lifting effect, and then by adding the weight to the upward pull of the aeroplane, the true lift of the aeroplane is obtained. It is also possible to attach an aeroplane at e, that is, the machine was made to test superposed aeroplanes if required. In these experiments, I naturally assumed that the best position for a screw was at the rear and in the path of the greatest resistance, but as some experimenters with navigable balloons placed the screw in front in order to pull the apparatus along instead of to push it, I made experiments to see what the relative difference might be. In order to do this, I wound a large amount of rope one-half inch in diameter around the whole apparatus forward of the screw, converting it into an irregular mass well calculated to offer atmospheric resistance. Upon starting the engine, I was rather surprised to see how little retardation these ropes gave to the apparatus. It appeared to me that nearly all of the energy consumed in driving the ropes through the air was recovered by the screw. I then removed the right-hand screw and replaced it by a left-hand screw of the same pitch and dimensions (Fig. 37a). I then found that the blast of the screw blowing against the tangle of ropes greatly retarded the travel; in fact, with the same number of revolutions per minute, the velocity fell off 60 per cent. I think that these experiments ought to show that there is but one place for the screw, and that is at the stern, and in the direct path of the greatest atmospheric resistance.

Fig. 38 shows an original apparatus which I designed and made for my own use; with ordinary anemometers it is necessary to count the number of turns per minute in order to ascertain the velocity of the wind. I wanted something that would indicate the velocity and the direction of the wind without any figures or formulæ. I therefore made the apparatus shown in the drawing, in which a, a, is a metallic disc 13·54 inches in diameter, giving it an area of exactly 1 square foot. This is attached to the horizontal bar b, and the whole mounted on two bell crank levers as shown. When the wind is not blowing, the long arms of these two levers assume a vertical position, and the spiral spring h, is in exact line with the pivots on which these levers are mounted, and has no effect except to hold the levers in a vertical position. As the spring has very little tension in this position, and as it requires a considerable movement in order to give it tension, the arms c, c, and the bar b, b, are very easily pushed backwards, but as the distance through which they travel increases, the angle of the lever changes and the tension of the spring increases at the same time, so that when the disc is pushed backwards to any considerable distance, a strong resistance is encountered. Had I made this apparatus so that the pressure acted directly on the spiral spring, the spaces on the index indicating low velocities would have been very near together, while those indicating high velocities would have been widely separated, but with this device properly designed, the spacing on the index became regular and even. The index being very large enabled one to read it at a considerable distance, and at the same time, it acted as a tail and kept the apparatus face to the wind. The spaces of the dial were not laid off with a pair of dividers, but each particular division was marked by an actual pull on the bar b, through the agency of a cord and easily running pulley and weight. The markings, however, were not correct, because Haswell’s formula was employed in which the pressure of the wind against the normal plane is considerably greater than with the more recent formula, which is now known to be correct. Haswell’s formula was V² × ·005 = P, and the recent formula P = 0·003 × V², where P = pressure in lbs. per square foot and V = velocity in miles per hour. In my experiments, I also employed a very well made and delicate anemometer by Negretti & Zambra.

CRYSTAL PALACE EXPERIMENTS.

Having fully satisfied myself that aeroplanes flying around a circle 200 feet in circumference had their lifting effect reduced to no insignificant degree by constantly engaging air which had already had imparted to it a downward movement by a previous revolution, I determined to make some experiments where this trouble could not occur, but the opportunity did not present itself until after the large roundabout, erroneously described as “a captive flying machine,” was put up at the Crystal Palace. This presented a fine opportunity for making experiments at an extremely high velocity around a very large circle. I will only refer to a few of these experiments. To a prolongation of one of the long arms, I attached a thin steel wire rope about 60 feet above the platform; I then attached to this wire rope the little device shown (Fig. 39), in which a, is an aeroplane, 5 feet long and 1 foot wide, placed at an inclination of 1 in 20. Great care was used in preparing this aeroplane to see that it was free from blemish, smooth, and without any irregularities. Both edges were sharp and the curvature was about one-eighth of an inch on the underneath side. It was made relatively thick in the middle where it was attached to the bar c, and thinner at the ends. b, shows a lump of lead just heavy enough to balance the bar c, and the tail; d, was a light but strong wooden frame, all the edges being thin and sharp, and covered with a special silk that Mr. Cody had found to be best for such purposes. The wire rope e, was attached to the long arm which I referred to. The great length of the bar c, and the accuracy with which the whole was made and balanced caused the aeroplane to travel straight through the air adjusting itself to all the shifting currents. Upon starting the machine on a very calm day, this apparatus mounted as high as the point of support, sometimes going 10 or more feet higher and sometimes 8 or 10 feet lower. However, as a rule, it carried its own weight at a velocity of 80 miles an hour around a circle 1,000 feet in circumference. Under these conditions, of course, there could be no downward motion of the air as all the air affected would be removed long before it could be struck the second time by the aeroplane. I had no means of ascertaining exactly how much this plane did actually lift, because the air was always moving to some extent, and it was not an easy matter to ascertain whether it was above or below the point of support. I am sure, however, that it was as much as 36 lbs., or rather more than 7 lbs. to the square foot, and this is just what it should have lifted, providing that we consider the results obtained by smaller planes placed in an air blast of 40 miles an hour and at the same angle. When these experiments were finished, I made a very small apparatus having only about 25 square feet of lifting surface, and this carried the weight of a man, in fact several gentlemen came up from London and went round on it themselves. I saw, however, that it was a dangerous practice, because if the wind was blowing at all, the apparatus would mount very much above the point of support while travelling against the wind, only to drop much below the point of support on the other side of the circle where it was travelling with the wind; in fact, on one occasion the apparatus shown (Fig. 39) mounted in a high wind fully 20 feet above the point of support and came down with such a crash on the other side that it broke the wire rope. In connection with this, it is interesting to note that when I erected the first so-called “captive flying machine” on my own grounds at Thurlow Park, I intended that instead of ordinary boats such as were ultimately employed, each particular boat should be fitted with an aeroplane, that the engine should be of 200 H.P., and that the passengers should actually be running on the air, each boat being provided with a powerful electric motor in addition to the motive power that drove the shaft. Had this been carried out as was originally designed, it would have removed the apparatus altogether from the category of vulgar merry-go-rounds, but such was not to be. Unforeseen circumstances were against me. I had some of these boats fitted up with aeroplanes and running on my grounds, and two of the engineers of the London County Council came out to see the apparatus before it was put up for public use. On that occasion the wind was blowing a perfect gale of 40 miles an hour, and as the boats travelled at the rate of 35 miles an hour, they, of course, encountered a wind of 75 miles an hour when passing against the wind, and a minus velocity of 5 miles an hour when travelling with the wind on the other side of the circle. The aeroplanes, although of considerable size, were small in relation to weight. I had neglected to put any weight in the boats, and when three of us were studying the eccentric path through which the boats were travelling, suddenly one of them in passing to the windward, raised very much above the point of support and plunged down with great force on the other side; in fact, the shock was so great that it made everything rattle, but nothing was broken. Nevertheless, the engineers said at once, it would not do to run the boats with those aeroplanes; it was too dangerous. This would not, however, have occurred if the boats had been loaded, or the velocity of the wind had been less. It, however, demonstrated what a tremendous lift may be obtained from a well-made aeroplane passing at a high velocity through the wind at a sharp angle. These aeroplanes were only about 12 feet long and 5 feet wide, having, therefore, 60 square feet of surface. They were, however, strong, well-made, and perfectly smooth, both top and bottom. I would say right here that I am not a success as a showman—previous long years of rubbing up against honest men have disqualified me altogether for such a profession. I was extremely anxious to go on with my experiments. I appreciated fully that I had made a machine that lifted 2,000 lbs. more than its own weight, and I knew for a dead certainty if I took the matter up again, got rid of my boiler and water tank, and used an internal combustion engine, such as I thought I could produce, that mechanical flight would soon be a fait accompli. I had already spent more than £20,000, and was looking about for some means of making the thing self-supporting. I believed that the so-called “captive flying machine” would be very popular, and bring in a lot of money, and it would have done so, if it had been put up as originally designed. I proposed to use my share of the profits for experimental work on real flying machines. That I was not far wrong in believing that such a machine would be a success, is witnessed by the fact that just about the same time, an American inventor thought of the same thing, put up some three or four machines the first year, and the next year about 50. They were highly profitable, and there are fully 140 of them running at the present time in the U.S.A. It is a fact that nothing in the way of side-shows at exhibitions or public resorts has ever had the success of this machine in the U.S.A., and even the little machine at Earl’s Court took £325 10s. in one day and £7,500 in a season. However, this little attempt to make one hand wash the other cost me no less than £10,400, not to mention more than a year of very hard work. This sum would have been amply sufficient to have enabled me to continue my experiments until success was assured.

CHAPTER VI.
HINTS AS TO THE BUILDING OF FLYING MACHINES.

For those who really wish to build a flying machine that will actually fly with very little experimental work, I have given an outline sketch sufficiently explicit to enable a skilful draughtsman to make a working drawing in which Fig. 40 is a front elevation, Fig. 41 a side elevation, and Fig. 42 a plan. Fig. 41, a, a, shows the two forward or main aeroplanes; b, b, the two after aeroplanes, which are smaller and shorter; c, the rudder; d, the forward horizontal rudder; e, the screw; f, the motor; g, the condenser or cooler; h, the steering gear; i, and j, atmospheric buffers; k and l, wheels attached to a lever pivoted to the body of the machine; q, a shield for protecting the screw. It will be observed that the framework is extremely long, and, consequently, the distance between the aeroplanes is very great; but it should be borne in mind that the longer the machine, the less any change of center of lifting effect, as relates to the center of gravity, will be felt. Moreover, it is much easier to manœuvre a machine of great length than one which is very short, because it gives one more time to think and act. If the length was infinitely great the tendency to pitch would be infinitely small. I have shown a steering gear consisting of a lever with a handle n, arranged in such a manner that it moves both the vertical rudder c, and the horizontal rudder d, so that the man who steers the machine has nothing to think of except to point the lever n, p, in the direction that he wishes the machine to go. This lever is mounted on a universal joint at h, and is connected with suitable wires to the two rudders. In order to prevent shock when the machine alights, it is necessary to provide something that is strong and, at the same time, yielding, and able to travel through a considerable distance before the machine comes to a state of rest. In the machines which I have seen on the Continent, a very elaborate apparatus is employed, which is not only very heavy, but also offers a considerable resistance to the forward motion of the machine through the air. It consists of many tubes, very long levers and heavy spiral springs, etc. In the device which I am recommending, all this is dispensed with, and something very much simpler, cheaper, and lighter is substituted. Moreover, with my proposed apparatus a certain amount of lifting effect is produced. The levers k, k, to which the wheels are attached, should be of thin wood, light and strong, and say about a foot wide, strongly pivoted to the frame and held in position by an atmospheric buffer made of strong and thin steel tubing, shown in section (Fig. 51). These pneumatic cylinders may be pumped up to any degree, so as to support the weight of the machine, and then, as it comes down, the compression and escape of air arrest its motion. The condenser g, is placed in such a position that it will act even while the machine is on the ground and the propellers working. In Continental machines, very small screw propellers are used. These screws have probably been made small because the experimenters have found that they encounter a good deal of friction in the atmosphere, but this is caused by imperfect shape and the rib of steel at the back of the blades. In order to use a small screw, experimenters have been forced to use a very quick-running engine which makes it necessary to have the cylinders very short, so, in order to get the necessary power, they are obliged to use no less than eight cylinders. However, by increasing the diameter of the screw and making it of such a form that very little or no atmospheric skin friction is encountered, a much better and cheaper engine of a totally different type may be employed. There is no reason why more than four cylinders should be used, but the stroke of the piston and diameter of the cylinder should be increased. Doubtless Continental experimenters have an idea that, as the engine cannot be provided with a flywheel, it must have a very large number of cylinders in order to give a steady pull completely around the circle, and thus avoid so-called “dead centers”; but, when we consider the enormously high velocity of the periphery of the screw, and also take into consideration that the momentum is in proportion to the square of the velocity, it is quite obvious that there can be no slowing up between strokes even if only one cylinder should be employed working on the four-cycle principle, in which work is only done one stroke in four. Then, again, I find that the weight of these Continental engines can be greatly reduced, providing that they are made with the same degree of refinement that I employed in building my steam engines.

Recently there has been a great deal of discussion in Engineering and other journals regarding the comparative merits of the aeroplane system and the hélicoptère. Some condemn both systems and pin their faith to flapping wings. It has been contended that the screw propeller is extremely wasteful in energy, and that in all Nature neither fish nor bird propels itself by means of a screw. As we do not find a screw in Nature, why then should we employ it in a machine for performing artificial flight?

Why not stick to Nature? In reply to this, I would say that even Nature has her limits, beyond which she cannot go. When a boy was told that everything was possible with God, he asked; “Could God make a two-year old calf in five minutes?” He was told that God certainly could. “But,” said the boy, “would the calf be two years old?” It appears to me that there is nothing in Nature which is more efficient, or gets a better grip on the water than a well-made screw propeller, and no doubt there would have been fish with screw propellers, providing that Dame Nature could have made an animal in two pieces. It is very evident that no living creature could be made in two pieces, and two pieces are necessary if one part is stationary and the other revolves; however, the tails and fins very often approximate to the action of the propeller blades; they turn first to the right and then to the left, producing a sculling effect which is practically the same. This argument might also be used against locomotives. In all Nature, we do not find an animal travelling on wheels, but it is quite possible that a locomotive might be made that would walk on legs at the rate of two or three miles an hour. But locomotives with wheels are able to travel at least three times as fast as the fleetest animal with legs, and to continue doing so for many hours at a time, even when attached to a very heavy load. In order to build a flying machine with flapping wings, to exactly imitate birds, a very complicated system of levers, cams, cranks, etc., would have to be employed, and these of themselves would weigh more than the wings would be able to lift. However, it is quite possible to approach very closely to the motion of a bird’s wings with no reciprocating or vibrating parts, and without flapping at all.