An address to the graduating class of the McKinley Manual Training School, Washington, D. C., February 1, 1917, revised for the National Geographic Magazine.
What a glorious thing it is to be young and have a future before you. To the graduates, especially, of a scientific technical school like the McKinley Manual Training School the outlook for the future looks bright and promising.
When I was a young man the institutions of learning, the higher schools and colleges, paid a great deal more attention to the teaching of Latin and Greek than to the study of science; they made scholars rather than scientists.
The war has changed all that, and the man of science will be appreciated in the future as he never has been in the past. Knowledge is power; and we now realize that the nation that fosters science becomes so powerful that other nations must, if only in self-defense, adopt the same plan. It is safe to say that scientific men and technical experts are destined in the future to occupy distinguished and honorable positions in all the countries of the world. Your future is assured.
We progress from candles to electricity in one lifetime
I said it was a glorious thing to be young; but it is also a glorious thing to be old and look back upon the progress of the world during one's own lifetime.
Now, I don't mean to insinuate that I am old, by any means! I had in mind an old lady, who is now living in Baltimore, at the age of one hundred and seven—she is now in her one hundred and eighth year—with mental faculties unimpaired. Possessed of a bright and active mind, she is able, from her own personal recollections, to look back upon a whole century of progress of the world.
She was born in England and came over to America when quite young; and it is rather interesting to know what brought the family here. The father was a wholesale candlemaker in London and his business was ruined by the introduction of gas!
Gas as an illuminant is now being replaced by electric lighting; and there are many people in this room who saw the first electric lights.
I, myself, am not so very old yet, but I can remember the days when there were no telephones.
I remember, too, very distinctly when there were no automobiles here. There were thousands of horses, and Washington, in the summer-time, smelled like a stable. There were plenty of flies, and the death rate was high.
Now, it is very interesting and instructive to look back over the various changes that have occurred and trace the evolution of the present from the past. By projecting these lines of advance into the future, you can forecast the future, to a certain extent, and recognize some of the fields of usefulness that are opening up for you.
Here we have one line of advance from candles and oil lamps to gas, and from gas to electricity; and we can recognize many other threads of advance all converging upon electricity. We produce heat and light by electricity. We transmit intelligence by the telegraph and telephone, and we use electricity as a motive power. In fact, we have fairly entered upon an electrical age, and it is obvious that the electrical engineer will be much in demand in the future. Those of you who devote yourselves to electrical subjects will certainly find a place and room to work.
From the “hobby-horse” to the motor-cycle of 130 miles speed
Then there is that other line of advance typified by the substitution of automobiles for horse-drawn vehicles. In line with this is the history of the bicycle. First, we had the old French “hobby-horse,” the ancestor of all our bicycles and motor-cycles. Upon this you rode astride, with your feet touching the ground, and propelled the machine by the action of walking. Then came the old “bone-racker,” in which your feet were applied to pedals attached to a crank-shaft on the front wheel of the machine.
This was superseded by a bicycle with an enormous front wheel, about six feet in height, with a little one behind—a most graceful machine, in which the rider appeared to great advantage. There was none of that slouchy attitude to which we are so accustomed now. The rider presented a graceful and dignified appearance, for he had perforce to sit upright, and even lean a little backward, to avoid the possibility of a header! The large wheel also appeared behind and the small one in front, and a tumble over backward was felt to be less disastrous than a header forward. It was much safer to alight upon your feet behind than to be thrown out forward upon your head.
Then came the “safety bicycle”—a return to the form of the old “hobby-horse,” but not a “bone-racker,” because provided with rubber tires. In this machine the power was transmitted from the feet to the wheels by means of gearing. This is still the form of the modern bicycle; but a gasoline motor has been added to do the work of the feet, giving us the power of going faster than railroad trains, on the common roads of the country, and without any physical exertion at all. I believe the speed record upon race-tracks stands at about 137 miles an hour.
On every hand we see the substitution of machinery and artificial motive power for animal and man power. There will therefore be plenty of openings in the future for young, bright mechanical engineers working in this direction.
There is, however, one obstacle to further advance, in the increasing price of the fuel necessary to work machinery. Coal and oil are going up and are strictly limited in quantity. We can take coal out of a mine, but we can never put it back. We can draw oil from subterranean reservoirs, but we can never refill them again. We are spendthrifts in the matter of fuel and are using our capital for our running expenses.
In relation to coal and oil, the world's annual consumption has become so enormous that we are now actually within measurable distance of the end of the supply. What shall we do when we have no more coal or oil!
Apart from water power (which is strictly limited) and tidal and wave power (which we have not yet learned to utilize), and the employment of the sun's rays directly as a source of power, we have little left, excepting wood, and it takes at least twenty-five years to grow a crop of trees.
There is, however, one other source of fuel supply which may perhaps solve this problem of the future. Alcohol makes a beautiful, clean, and efficient fuel, and, where not intended for consumption by human beings, can be manufactured very cheaply in an indigestible or even poisonous form. Wood alcohol, for example, can be employed as a fuel, and we can make alcohol from sawdust, a waste product of our mills.
Alcohol can also be manufactured from corn stalks, and in fact from almost any vegetable matter capable of fermentation. Our growing crops and even weeds can be used. The waste products of our farms are available for this purpose and even the garbage from our cities. We need never fear the exhaustion of our present fuel supplies so long as we can produce an annual crop of alcohol to any extent desired.
The world will probably depend upon alcohol more and more as time goes on, and a great field of usefulness is opening up for the engineer who will modify our machinery to enable alcohol to be used as the source of power.
Evolution in science has not always been accomplished by a series of gradual changes, each small in itself, but cumulative in effect. There have also been sudden “mutations” followed by advances of knowledge by leaps and bounds in a new direction, and the establishment of new and useful arts never before even dreamed of by man.
Although Clerk-Maxwell and others had long ago enunciated the theory that light and electricity were vibratory movements of the so-called “ether” or luminiferous medium of space, differing chiefly in frequency from one another, the world was not prepared for the experiments of Hertz, who demonstrated the reality of the conception and actually measured the wave-length of electrical discharges. Still less was it prepared for the discovery that brick walls and other apparently opaque objects were as transparent to the Hertzian waves as glass is to light. These experiments formed the basis for numerous other startling discoveries and practical applications for the benefit of man.
Flesh proved to be transparent to the Roentgen rays, and the world was fairly startled by the first X-ray photographs of the bones in the living human hand. Now physicians and surgeons use X-ray lamps to enable them to see bullets and other objects imbedded in flesh, and have even devised means of observing the beating of the heart and the movements of other internal organs without pain to their patients.
Other developments of the Hertzian waves have resulted in the creation of the new art of wireless telegraphy. Most of us, I think, can remember the first S.O.S. signals sent out by a ship in distress and the instant response from distant vessels equipped with the Marconi apparatus. Then came the rush of vessels to the scene of disaster and the rescue of the passengers and crew.
Developments of wireless telegraphy are proceeding with great rapidity, and no man can predict what startling discoveries and applications may appear in the near future. Here may be an opening for some of you, and I know of no more promising field of exploration to recommend to your notice.
Honolulu eavesdrops while Washington talks to Paris
Already privacy of communication has been secured by wireless transmitters and receivers “tuned,” so to speak, to respond to electrical vibrations of certain frequencies alone. They are sensitive only to electrical impulses of definite wavelength. The principle of sympathetic vibration operating tuned wireless receivers has also been applied to the control of machinery from a distance and the steering of boats without a man on board. The possibilities of development in this direction are practically illimitable, and we shall probably be able to perform at a distance by wireless almost any mechanical operation that can be done at hand.
Still more recently wireless telegraphy has given birth to another new art, and wireless telephony has appeared. Only a short time ago a man in Arlington, Va., at the wireless station there, talked by word of mouth to a man on the Eiffel Tower in Paris, France. Not only that, but a man in Honolulu overheard the conversation! The distance from Honolulu to the Eiffel Tower must be 8,000 miles at least—one-third the distance around the globe—and this achievement surely foreshadows the time when we may be able to talk with a man in any part of the world by telephone and without wires.
Our most cherished theories upset by a woman
The above illustrations exhibit what we might call “mutations” of science; but the greatest of all these mutations was the discovery that opened the twentieth century, and I may add for the encouragement of our young lady graduates that it was made by a woman. I allude to the discovery of radium by Madame Curie of Paris.
Radium has recently upset our most cherished theories of matter and force. The whole subject of chemistry has to be rewritten and our ideas of the constitution of matter entirely changed. Here is a substance which emits light and heat and electricity continuously without any apparent source of supply. It emits light in the dark, and in a cool room maintains itself constantly at a higher temperature than its environment.
It emits the Roentgen rays without any electrical machinery to produce them, and we have now discovered emanating from that substance several different kinds of rays of the unknown or X-ray variety; and we now recognize the Alpha, Beta, and Gamma rays as distinct varieties, having different properties.
Though radium behaves like an elementary substance, it is found in process of time to disintegrate into other elementary substances quite different from the original radium itself. Helium is one of its products, and, after several transmutations, it apparently turns into lead!
Our forefathers believed firmly in the transmutation of metals, one into the other, and vainly sought a means of transmuting the baser metals into gold. Radium shows that there is some foundation for the transmutation theory, and that at least some of the so-called elements originate by a process of evolution from other elements quite distinct from themselves. Where this line of development is going to lead is a problem indeed, and radium still remains the great puzzle of the twentieth century.
I cannot hope to bring to your attention all of the problems that are awaiting solution, but I think it may be interesting to you to hear of a few upon which I myself have been working. What interests me will probably interest you, and perhaps some of you may carry out the experiments to a further point than I have done.
You know that although I am a lover of Washington, yet, when the summertime comes, I go just as far away from Washington as I can in the direction of the North Pole. I have a summer place in Cape Breton Island, Nova Scotia, where I can always be sure of cool, fresh breezes, while you poor people are broiling here in Washington.
A good many of the people on Cape Breton Island are fishermen, who make their living on the Banks of Newfoundland; and one of the men employed upon my place had two uncles who were fishermen on the Banks. One day they left their vessel in a dory to look after their nets, and while they were gone a fog came up and they were unable to find their way back. The dory drifted about in the ocean for many days and was then picked up with their dead bodies on board; they had perished from exposure and thirst.
Now it is not a very unusual thing on the Banks of Newfoundland for fishermen to be separated from their vessels by fog. Every year dories are picked up at sea, and the occupants are often found to be suffering terribly from thirst. They have found “water, water, everywhere, but not a drop to drink.” Now, it seemed to me that it was really a reflection upon the intelligence of man that people should die of thirst in the midst of water.
There is the salt water of the sea, and all you have to do is to separate the salt from the water and drink the water. That is one problem.
Condensing the water vapor in the human breath
But there is also the fog which prevents you from reaching your vessel, and what is fog but fresh water in the form of cloud. Therefore all you have to do is to condense the fog and drink it. That is another problem.
But there is still another alternative. Water vapor exists in your breath. Why not condense your breath and drink it? This problem is easily solved; just breathe into an empty tumbler and at once you have a condensation of moisture on the inside. If you have the patience to continue the process for a few minutes, you will soon find clear water at the bottom of the tumbler.
I took a bucket of cool salt water from the sea, put it down in the bottom of a boat between my knees, and then put into it a large empty bottle the size of a beer bottle, which floated in the water with the neck of the bottle resting on the edge of the bucket. Then I took a long glass tube, over a meter in length, and put one end into the bottle and the other end in my mouth. I sat back comfortably in a chair with the tube between my lips and inhaled through the nostrils and blew down through the tube. This process was so easily performed that I found I could read a book while it was going on.
I therefore continued the experiment for over two hours, and then I found a considerable amount of water in the bottle, quite enough for a moderate drink. It might not be very much for us, but if you were dying of thirst on the open sea you would be glad enough to get what was there. I tasted the water and found it quite fresh, although I must confess it did not have a very palatable taste; in fact, the water condensed from my breath had a taste of—of tobacco! But I don't suppose that would have mattered much to a man who was dying of thirst.
I have also made experiments to condense drinking water from fog. A large pickle jar was provided and two long glass tubes were let down through the cork. The jar was then submerged at the wharf, with the two pipes sticking up above the surface. The experiment was then made to pump fog down through one of the pipes, the other serving as a vent. This was accomplished by means of a pair of bellows provided with a spiral spring between the handles to keep them apart. This apparatus was fastened on top of the wharf. A heavy log of wood was floated upon the water below, connected by means of a string with the upper handle of the bellows.
The waves moved this log up and down and worked the bellows. The nozzle was connected to one of the pipes leading to the submerged empty jar and at once the bellows began to pump the fog into the jar. It continued pumping all night, and I let it go on pumping all of the next day, because there was to be a meeting of men on my place the next evening, and I thought it would be interesting to open the jar at the men's meeting. With great ceremony the jar was removed to the warehouse and was found to be nearly full of beautiful clear water. A British naval officer was present and offered to be the first to taste the water condensed from fog. He took a good mouthful of it, while the men gathered around in great excitement and shouted, “Fresh or salt?”
He did not reply, but made a face. He then rushed for the window, spat the water out, and exclaimed, “Salt!” Now, this failure did not by any means prove that the process was wrong, but simply showed that it might be advisable in the future, if you use a cork, to employ one that fits tightly and does not leak. The one I used had a hole in it, I found out afterward.
An involuntary experiment relating to the condensation of fresh water from the sea was made in Cape Breton. A man fell overboard and was rescued, with his clothes wringing wet with sea-water. There was a cold wind blowing and he took refuge in a little cabin on the boat covered with a tarpaulin awning. In a little time he began to steam. The heat of his body warmed the sea-water in his clothes, and there actually arose a cloud of steam which condensed on the cold tarpaulin and ran down the sides. It was fresh water, and if it had been collected in a jar there would have been quite enough for a drink.
On large ocean steamers all the drinking water used is condensed from the sea; and we somehow or other have the idea that it is necessary to boil the seawater, or at least have it very hot, and then condense it by means of ice or something very cold. Now, that is not necessary at all. Just think of this: All the fresh water upon the globe comes from the sea, and we do not boil the sea. Water vapor is given off by the sea everywhere and at all temperatures; it is even evaporated from ice and snow. Of course, the warmer the sea-water is, the greater is the amount of water vapor thrown out; but water vapor is everywhere present, and the main point in condensation is that it is removed from the surface by the action of the wind and carried to cooler places, where condensation occurs in the form of cloud or rain. No great amount of heat is required to produce evaporation and no great amount of cold is necessary to effect condensation.
Such considerations as these may lead to some cheap industrial process for the manufacture of fresh water from the sea. All that is necessary is a current of air over your salt water to remove the water vapor collected there, and then the carrying of this confined current into a cool reservoir where the water may condense.
The thermos-bottle idea applied to a water tank
As little or no artificial heating is required, a great saving can be effected in the matter of fuel. It is extraordinary how wasteful we are in our means of producing heat and in retaining it after it has been produced. It is safe to say that a great deal more heat goes up the chimney than we utilize from a fire. Then when we cook our dinner or boil water, we allow the heat to escape by radiation and the things soon cool.
A cosy for our teapot, a fireless cooker for our dinner, and a thermos bottle for our heated liquids show how much heat may be conserved by simply taking precautions to prevent radiation. Our hot-water boilers are not protected by coverings of asbestos paper or other insulating material, so that the water gets too cool for a warm bath very soon after the fire is put out.
I have made experiments to ascertain whether some of the heat wasted by radiation could not be conserved by insulating materials, with rather astonishing results. A large tank of zinc was made which would hold a great deal of water. This was inclosed in a box very much larger than itself, leaving a space of about three or four inches all around, which was filled with wool. I then found that hot water put into that tank cooled almost as slowly as if it had been a thermos bottle.
I then attempted to save and utilize some of the heat given off by a student's lamp. A couple of pipes were led out of this insulated tank and placed in a hood over the lamp. Thus a circulation of water was effected. The water heated by the lamp found its way up into the tank and produced a sensible rise of temperature there. Next day when the lamp was again lighted it was found that the water in the tank still felt slightly warm. It had not lost all of the heat it had received at the former heating. When the lamp was again put out, the temperature of the tank was considerably higher than on the former occasion.
This process of heating was continued for a number of days, and it became obvious that a cumulative effect was produced, until at last the water in the tank became too hot to hold the hand in, and it was determined to see how long it would hold its heat. The temperature was observed from time to time, and more than a week after the lamp had been put out the water was still so warm that I used it for a bath.
Since then this insulated tank has been taken up to the attic of my house in Nova Scotia and has been installed there as a permanent feature. I have the habit of working at night and like to take a warm bath somewhere about 2 o'clock in the morning. Unfortunately the heating arrangements in the house have given out long before that hour and only cold water comes from the kitchen boilers. I connected the insulated tank with an iron pipe let down my study chimney in the hope of saving and utilizing some portion of the heat that escaped up the chimney every time the fire was lighted.
I have had this apparatus in use for over a year, and find that at any time of the day or night I am always sure of a warm bath from the heat that used to be wasted in going up the chimney. In this case there was only one straight pipe, so that the amount of heat recovered bears only a small proportion to that still wasted. A coil of pipe in the chimney or special apparatus there would, of course, be much more efficient.
I think that all the hot water required for the use of a household, and even for warming a house, could be obtained without special expenditure for fuel by utilization of the waste heat produced from the kitchen fire and the heat given off by the illuminants employed.
Of course, water can only be heated to the boiling temperature; but there are many liquids that can be heated to a very much higher temperature than this without boiling. I took a tumbler of olive oil and heated it by means of a thin iron wire connected with a voltaic battery. I placed in the tumbler of oil a test-tube filled with water. In a short time the water was boiling, but the oil remained perfectly quiescent. If you store up hot oil instead of water you will have at your command a source of heat able to do all your cooking, and even produce steam power to work machinery.
We have plenty of heat going to waste in Washington during the summer-time, for the sun's rays are very powerful, and we do not use the roofs of our buildings except to keep off the rain. What wide expanses of roof are available in all our large cities for the utilization of the sun's rays! Simple pipes laid up on the roof and containing oil or some other liquid would soon become heated by the sun's rays. The hot oil could be carried into an insulated tank and stored. You could thus not only conserve and utilize the heat that falls upon the tops of your houses, but effect some cooling of the houses themselves by the abstraction of this heat.
I was once obliged, very much against my will, I can assure you, to remain in Washington right in the midst of the summer, and the thought kept constantly recurring to my mind, If man has the intelligence to heat his house in the winter-time, why does he not cool it in the summer? We go up to the Arctic regions and heat our houses and live. We go down to the Tropics and die. In India the white children have to be sent home to England in order to live, and all on account of the heat. The problem of cooling houses is one that I would recommend to your notice, not only on account of your own comfort, but on account of the public health as well.
Now, I have found one radical defect in the construction of our houses that absolutely precludes the possibility of cooling them to any great degree. You will readily understand the difficulty when you remember that cold air is specifically heavier than warm air. You can take a bucket of cold air, for example, and carry it about in the summer-time and not spill a drop; but if you make a hole in the bottom of your bucket, then, of course, the cold air will all run out.
Now, if you look at the typical tropical houses, you will find that they are all open on the ground floor. Supposing it were possible to turn on a veritable Niagara of cold air into a tropical house, it wouldn't stay there five minutes. It would all come pouring out through the open places below and through the windows and doors. If you want to find your leakage places, just fill your house with water and see where the water squirts out!
I began to think that it might be possible to apply the bucket principle to at least one room in my Washington home, and thus secure a place of retreat in the summer-time. It seemed to be advisable to close up all openings near the bottom of the room to prevent the escape of cold air and open the windows at the top to let out the heated air of the room.
Now, it so happens that I have in the basement of my house a swimming tank, and it occurred to me that since this tank holds water, it should certainly hold cold air; so I turned the water out to study the situation. The tank seemed to be damp and the sides felt wet and slimy.
I reflected, however, that the condensation of moisture resulted from the fact that the sides of the tank were cooler than the air admitted. Water vapor will not condense on anything that is warmer than itself, and it occurred to me that if I introduced air that was very much colder than I wanted to use, then it would be warming up in the tank and becoming dryer all the time. It would not deposit moisture on the sides and would actually absorb the moisture there.
I therefore provided a refrigerator, in which were placed large blocks of ice covered with salt. This was placed in another room at a higher elevation than the tank, and a pipe covered with asbestos paper was employed to lead the cold air into the tank.
The first effect was the drying of the walls, and then I felt the level of the cold air gradually rising. At last it came over my head. The tank was full, and I found myself immersed in cool air. I felt so cool and comfortable that it seemed difficult to believe that Washington stood sizzling outside. I climbed up the ladder in the swimming tank until my head was above the surface, and then found myself breathing a hot, damp, muggy atmosphere. I therefore speedily retreated into the tank, where I was perfectly cool and comfortable.
Guided by this experience, I tried another experiment in my house. I put the refrigerator in the attic and led the cold air downward through a pipe covered with asbestos into one of the rooms of the house. The doors were kept shut and the windows were opened at the top. The temperature in that room was perfectly comfortable, about 65 degrees.
At that time the papers were speaking of some ice plant that had been installed in the White House and congratulated the President upon a temperature of only 80 degrees when the thermometer showed 100 degrees outside. At this very time I enjoyed in my house a temperature of 65 degrees (the ideal temperature), with a delicious feeling of freshness in the air. Even when the air had risen to the same temperature as the rest of the house, as measured by a thermometer, the room still felt cool, because the air was drier, thus promoting perspiration that cooled the skin.
In this connection I may say that there is a very interesting cooling plant in Paris, France, run by the Societé de l'Air Comprimé. Very many of the cafés and restaurants in Paris have cold rooms for the storage of perishable provisions, and these rooms are cooled by compressed air supplied by this company.
The plant consists of large pipes laid down under the streets of Paris, with small branch pipes leading into the cafés and restaurants. At a central station steam-engines pump air into the pipes and keep up a continuous pressure of from four to five atmospheres. As there are several hundred kilometers of these pipes under the streets of Paris, they form a huge reservoir of compressed air at the ground temperature.
In the cooling room of a café they simply turn a little cock and admit the compressed air into the room. A gas meter measures the amount of air admitted and charges are made accordingly.
The compressed air, by its expansion, produces great cold, and the cooling effect is still further increased by allowing the air to do work during the process of expansion. Dumb-waiters, elevators, and even sewing-machines are thus run very economically in connection with the system by means of compressed-air engines.
Now, it appears to me that this process might very easily be developed into a plan for the cooling of a whole city. You would simply have to turn a cock in your room to admit the fresh air; and if you then take precautions to prevent the cold air from running away by having your room tight at the bottom and open at the top, you could keep your room cool in the hottest summer weather.
I must confess that there is one other subject upon which I would like to say a few words before closing.
One of the great evils attending our civilization is the extreme congestion of the population into the larger cities, and one of the great problems of the future is how to spread the population more equally over the land.
The congestion is caused by difficulties of transportation; for, of course, it costs much more to send a person to a distant place than to one near at hand.
But did you ever think of this: that it also costs more to send a letter to a distant place than to one near at hand, and yet a two-cent stamp will carry your letter anywhere within the limits of the United States, and even beyond.
Could postage stamps be used in transportation of persons?
So many more letters are sent to places near at hand than to the remoter parts of the country that an average rate of postage very slightly in excess of the cost for short distances pays for the deficit on the longer routes. Now, the thought that I would like to put into your minds is this: Why could not the postage stamp principle be applied to the transportation of persons and goods? Why should it not be possible to charge an average rate for transportation instead of a rate increasing with the distance traveled?
We have already begun to apply this principle in municipalities. We no longer charge by distance in our large cities, and a five-cent fare will carry you anywhere you want to go within the limits of the municipality involved. As a consequence we find in these cities the poorer people abandoning tenement houses and going out into the country to live, where their children have room to grow. This relief of congestion pervades all classes of the community, and you see homes springing up everywhere in the suburbs of our great cities.
The benefits resulting from a uniform rate of transportation increase in geometrical proportion to the distance traveled, and the possible radius of travel should therefore be extended to the greatest practicable degree.
It may well be doubted whether it will ever be possible to buy a ticket for anywhere in the United States at an average rate; but it might be practicable to apply the principle to some at least of the smaller States. A citizen of Rhode Island, for example, might for a very small amount be enabled to travel anywhere within the limits of that State.
It would certainly be advisable to reduce our charges for transportation to the minimum amount possible. This can be done, first, by adopting the principle of an average rate, and, secondly, by reducing the actual cost of the transportation itself.
Will aërial locomotion solve the road question?
Now, it is noteworthy that the main element of cost resides not so much in the vehicles and locomotives employed as in the cost of the roads on which they have to run; it is this element that increases with the distance.
The railroads, for example, have to expend millions of dollars in the construction of railroad tracks; and what would the automobile be worth without a good road on which to travel? Water transportation is much cheaper than railroad transportation, chiefly because we do not have to build roads in the sea for our ships.
I will conclude with this thought: that a possible solution of the problem over land may lie in the development of aërial locomotion. However much money we may invest in the construction of huge aërial machines carrying many passengers, we don't have to build a road.