Popular Science Monthly/Volume 56/March 1900/Early Experiments in Air Flight
|EARLY EXPERIMENTS IN AIR FLIGHT|
MAN has sought in all times and at all places to find means of leaving the earth's surface, in imitation of the birds, and rising into the air. Ancient legendary lore furnishes many stories, like those of Dædalus and his son Icarus, of attempts of this sort. In the fourth century b. c., Archytas of Tarentum, a learned Pythagorean, who has been credited with the invention of the screw, the pulley, and the kite, according to Aulus Gellius, constructed a wooden dove which could rise and sustain itself in the air by some mechanism the arrangement of which is not known. Credible accounts exist of an English Benedictine monk, Oliver of Malmesbury, in the eleventh century, having tried to fly by precipitating himself from the height of a tower, with the assistance of wings attached to his arms and his feet. It is said that, after having gone along a little way, he fell and broke his legs. He attributed his accident to failure to provide his apparatus with a tail, which would have helped preserve his equilibrium and made the descent a gentler one.
In the sixteenth century, Leonardo da Vinci first demonstrated that a bird, which is heavier than the air, sustains itself, advances in the air, "by rendering the fluid denser where it passes than where it does not pass." In order to fly it has to fix its point of support on the air; its wing in the descending stroke exerts a pressure from above down, the reaction of which from below up forces the center of gravity of its body to ascend at each instant to the height at which the bird wishes to maintain it. Some sketches that have come down to us prove that Leonardo occupied himself, like Oliver of Malmesbury, with giving man power to fly by the aid of wings suitably fixed to his body. We owe to Leonardo also the invention of the parachute, which he described in the following terms: "If a man had a pavilion, each side of which was fifteen braces wide and twelve braces high, he might cast himself from any height whatever, without fear of danger." It may be said, too, of Leonardo da Vinci, that he was the first to suggest the idea of the screw propeller. "If," he said, "this instrument in the form of a screw is properly made—that is, made of linen cloth, the interstices of which have been filled with starch—and if we turn it rapidly, such a screw will make a bearing nut for itself through the air and rise. This can be proved by moving a broad, thin rule rapidly through the air, when it will be found that the arm is forced to follow in the direction of the edge of the board. The frame for the cloth of which I have been speaking should be made of long, stout reeds. A model of it might be made in paper, with, for its axis, a thin strip of iron which we twist forcibly. When the strip is left free it will turn the screw."
In 1680 Borelli published some studies of a remarkably correct character on the flight of birds. According to his view, the wing acts upon the air in the phase of beating down, in the manner of an inclined plane, so as, by virtue of the resistance opposed by the air, to push the body of the animal upward at first and then onward. The action of the ascending wing was compared to that of a kite, and it would consequently continue to sustain the body of the bird while waiting the following stroke. But Borelli never thought of turning his observations to advantage, so as to supply man with the means of flying. Attention was much engaged in 1742 with the attempt of the Marquis de Bacqueville, substantially repeating that of Oliver of Malmesbury, which was terminated by a similar accident. Mention should also be made of Paucton, who in 1768 drafted a plan for a screw machine. In 1784 Launoy and Bienvenu exhibited and operated, before the Academy of Sciences in Paris, a screw which was moved by a strong spring. Before this, however, Joseph and Stephen Montgolfier had filled the world with the noise of their discovery of the air balloon, and the ingenious machine of these aëronauts failed to receive the attention it deserved.
It has been known since the days of Archimedes that every body partly or wholly submerged in a liquid in equilibrium suffers a vertical push upward from the fluid equal to the weight of liquid it displaces.
Let us consider the case of a body entirely plunged in a liquid—water, for example. If its weight exceeds the thrust it suffers it will fall to the bottom of the water under the action of a descensional force equal, at each instant, to the difference between the weight of the body, which is invariable, and the thrust, which is invariable also, and thus constant in direction and also in amount. If the weight of the body is less than the thrust, the latter overcomes it, and, contrary to the usual laws of weight, the body will rise under the action of an ascensional force, which will evidently be likewise constant in amount as well as in direction. A cork held down at the bottom of a vessel of water and then left to itself will supply an example of this ascensional movement.
A third case may be presented—that in which the weight of the body is equal to the thrust of the water. Weight and thrust are then in mutual equilibrium. No force invites the body either to descend or to rise, and it remains balanced in the midst of the liquid, wherever it happens to have been placed. This state of indifferent equilibrium is, however, possible only if the weight of the body remains rigorously constant. The slightest augmentation of the weight immediately causes the body to descend, while the slightest diminution sends it up. From this source arise the difficulties that are met in the construction of submarine boats, when their ascent or descent is obtained by means of air chambers, which are filled with water or emptied of it according to the requirements. The equilibrium of these engines is always precarious, and this explains why none of them, from that of Van Drebbel in 1620 to the experiments of Goubet in 1895, have given really practical results in the matter of stability of immersion.
When Galileo, following Aristotle, had demonstrated the ponderability of the air, and Torricelli had proved that atmospheric pressure was a result of that property, it was immediately thought that the principle discovered by Archimedes might be extended to the air, and Otto von Guericke gave an experimental demonstration of it by the invention of the baroscope.
From this period it seems, then, that the discovery of aëronautics was possible. If the weight of the volume of air displaced is greater than that of the body, the latter should take an ascensional movement in the atmosphere, as a cork does when plunged into water; and it is evident that for a body to satisfy such conditions we have only to fill a very light envelope with a gas less dense than the ambient air. But the study of gases was still in its infancy in the seventeenth century, and it required the labors of Mortrel d'Élement and Hales, at the beginning of the following century, to teach physicists how to collect and retain them.
The history of the progress of the human mind shows, further, that the pure and simple acceptance of a scientific discovery is not enough to make it produce all the consequences we have a right to expect from it. It must, further, impregnating the mind with itself, pass, we might say, into the condition of an innate idea. Chemistry, in this very matter of the discovery of the weight of the air and of the gases, presents a striking example of the accuracy of our proposition. The ponderability of the air had been accepted by physicists for a long time, while chemists continued to take no account of it, although, as Mendeleef has remarked, no exact idea could be conceived, under such conditions, concerning most chemical phenomena. It is to the glory of Lavoisier that he first took account of this ponderability and of that of all the gases as well. When we reflect that it was not till about 1775, or a hundred and fifty years after Galileo, that this illustrious Frenchman began to set forth those ideas, it is not any wonder that the discovery of aërostats was not made till toward the end of the eighteenth century. Lalande was therefore much in the wrong when he said "it was so simple! why was it not done before?"
It would not be just, however, to refer the discovery of aërostats solely to the efforts of the Montgolfiers. Like all inventors, like Lavoisier himself, these brothers, as Figuier has remarked, had the benefit of a long series of isolated labors, carried on often without special purpose, by which the elements of their invention had been gathered up.
Père Lana, of Brescia, conceived a plan in 1670 for constructing a ship which should sustain itself in the air and move by the aid of sails. Four copper globes, in which a vacuum had been produced in order to render them lighter than the volume of air displaced, were to support the ship while the sails propelled it. The scientific conception of the empty globes was correct, but Père Lana did not think of the enormous collapsing force which the atmospheric pressure would exercise upon them. The idea of a sail which would give his aërial boat a resemblance to a vessel driven by the winds was wholly erroneous.
Sixty-five years later, in 1735, Père Galien, of Avignon, gave a fairly clear expression to the theory of aërostats. Resting on the principle of Archimedes, he maintained that if he could fill a globe made of light cloth with a sufficiently rarefied air the globe would necessarily possess an ascensional force, which would permit it to lift itself up in the air with a ship and all its cargo. He proposed to draw this rarefied air from out of the upper regions of the atmosphere, down from the summits of high mountains, forgetting that the air, when brought down to the level of the ground, would contract in volume and assume the density of the ambient atmosphere.
In the condition of ignorance of the properties of gases that existed in that age, it did not occur, and could not have occurred, to Père Galien to use other gases than air; no more could he have thought of employing heat to rarefy the air, for the first not very precise notions on the decrease in densities of gases by heat only date from Priestley. But when Cavendish, in 1765, had fully studied hydrogen gas, and shown that as it was prepared then it was seven times lighter than air. Black was enabled to suggest that by filling a light bag with hydrogen the bag would be able to raise a certain weight in the air. The labors of Cavendish, Black, and the discoveries of oxygen, nitrogen, and other gases by Priestley, were described by Priestley a few years afterward in the celebrated book on The Different Kinds of Air—a book which Stephen and Joseph Montgolfier had in their possession. The two brothers evidently found the germ of their invention in it.
It is fair to say that the Montgolfiers, who were already known in the learned world by their discoveries in the mechanical sciences, had thought, before they knew of Priestley's book, of a way of imitating Nature by inclosing vapor of water, a gas lighter than air, in a paper bag, which would be lifted up, the vapor contained in the bag being sustained in the air like a cloud. But the vapor condensed, and the weighted balloon shortly fell to the ground. The smoke produced by burning wood inclosed in a bag gave no better results. After seeing Priestley's book, they substituted hydrogen for vapor and for smoke, but the gas passed through the paper bag, and they gave up this attempt.
They then fancied that electricity was one of the causes of the rise of clouds, and sought for a gas that had electrical properties. They thought they could obtain it by burning wet straw and wool together. A box made of silk was filled with this gas, and they had the great satisfaction of seeing it rise to the ceiling of their room, and, in a second experiment, into the air. This was in November, 1782.
Five months previously, Tiberius Cavallo, in England, had repeated Black's experiment of filling a paper sack with hydrogen; but, as the Montgolfiers had found, the hydrogen leaked through the paper. Cavallo had better success with soap bubbles, which held the gas. His experiments stopped here, while the Montgolfiers carried theirs on to practical success.—Translated for the Popular Science Monthly from the Revue Scientifique.