"A S we have seen, all animal and vegetable life has need of air, and air is supplied in inexhaustible volume. It forms around the earth a continuous envelop known as the atmosphere and having a thickness of at least fifteen leagues.
"It is a veritable ocean of air, but an ocean without shores, an ocean holding in its lowest depths all that lives and moves upon this earth of ours. In its upper reaches it extends far beyond the loftiest mountain peaks and occupies regions of space that no bird in its most daring flight has ever visited; and it makes its bed upon the dry land of the continents and upon the waters that encircle them, these latter constituting another ocean far heavier and denser, the abode of the aquatic population of the globe.
"In the daytime we see above our heads a boundless blue vault which we call the sky; but this vault is one in appearance only, owing its seeming existence to the atmosphere. To account for the azure cupola of the sky, note that substances very slightly tinged with color do not show this color until they are seen in masses of great thickness. A pane of glass looks colorless, yet if viewed edgewise it is found to be of a delicate green hue. In the first position there is nothing in the thin pane to arrest the eye; in the second, the considerable body of glass presented reveals the green color.
"Similarly, the water in a bottle, being of small volume, appears colorless; but if we look at a great mass of water, such as a lake or the ocean, it is seen to be tinged with either green or blue. The same holds true of air: colorless and therefore invisible where a volume of only moderate thickness is concerned, it becomes visible and shows its delicate tint of blue if the thickness be considerably increased. Thus it is that the enormously thick layer surrounding the earth assumes the appearance of an azure vault.
"Since it is matter, air must have weight; and in fact it has—very slight weight, it is true, but more than that of many other substances. Suppose we had a hollow cube measuring one meter each way. The air contained in this cube would weigh one kilometer and three hundred grams. A like volume of water weighs one thousand kilograms, or seven hundred and sixty-nine times as much.
"It is in the atmosphere, now higher, now lower, that the clouds float; and it is in the atmosphere that smoke rises and is dissipated. Why do clouds remain at a considerable height, and why does smoke rise? Because they are lighter than air. A piece of wood, if forced down to the bottom of a body of water and then released, immediately comes up again of its own accord; it does so because it is lighter than water. Exactly similar is the behavior of clouds and smoke, which are lighter than air, in the atmospheric ocean in which they are immersed. If there were no atmosphere, smoke would not leave the ground and clouds would trail along the surface of the earth. If there were no atmosphere to offer resistance to the strokes of their wings, birds could not fly.
"To protect us from the cold we have clothing. The terrestrial globe likewise has its thick blanket under which is preserved for some time the heat received from the sun during the day; it has its atmosphere, a cloak of air fifteen leagues thick. Without this protection, which plays a part similar to that of our eider-down bed-coverings in retaining heat, the earth would undergo, every night, a cooling-off that no living creature could withstand.
"With any diminution in the thickness of this envelop of air there is a corresponding decrease in the protection it furnishes, just as is the case in respect to the clothes we wear. Hence it is found that the cold increases rapidly in the upper regions of the atmosphere, because the protecting covering is thinner in proportion to the depth of atmosphere below. Thus we understand why very high mountains are covered with snow the year around, not even excepting summer: their summits, less protected by the atmospheric blanket than the surrounding plains, are subjected to a more rigorous cooling-off at night.
"A considerable mass of cotton may be compressed in the hands until it becomes a small ball. In like manner air is compressible: it becomes denser and occupies less space in proportion to the pressure exerted upon it. With this truth in mind let us consider the atmospheric envelop in its entire thickness. The layer next to the ground bears the weight of all that is above; it sustains the greatest pressure and therefore, volume for volume, is the richest in matter, just as the most compact ball of cotton is the one containing the most of that material. Without entering into further details we see that the atmosphere becomes less and less dense as we ascend, because it has less and less of superincumbent atmosphere to support.
"Living at the bottom of this atmospheric ocean, we breathe its lower layers, the air of which, denser than elsewhere, meets the needs of our lungs. If we ascend three or four thousand meters we find the air thinner and respiration difficult and inadequate. Higher still, we experience something worse than discomfort; we face very serious danger. Finally, at a height not really tremendous in itself strength fails, the mind wanders, and sudden faintness supervenes, followed by death.
"Though life ceases for lack of sufficient air, the atmosphere is still there, its upper layers extending far beyond the elevation here referred to; nevertheless it has not the degree of density required for sustaining life. Only in the lower layers of the atmosphere, then, is the air suitable for breathing; at a greater height all life ceases. Of course no bird mounts to those desert spaces; nor, in fact, would its wings find there a sufficiently resistant atmosphere to admit of flying.
"Freezing cold, prostrating physical discomfort, and at last sudden death—these await the daring adventurer who attempts to scale the vault of heaven. Let us then stay below, in the lowest depths of the atmosphere, the only abode suited to our needs; and if we are seized with a desire to know more in detail what goes on up yonder, let us be content to hear the report of those audacious explorers who have penetrated the upper regions of the atmosphere.
"A balloon ascends to heights unattained by the loftiest mountain peaks. The enormous globe of woven fabric is inflated with a gas called hydrogen, like ordinary air in its invisibility, but far rarer and lighter, weighing only a hundred grams to the cubic meter, or one thirteenth as much as air. Thus rendered buoyant by this tenuous gas, the balloon ascends because its total weight is less than that of a like volume of atmospheric air.
"It carries heavy weights, very heavy weights, it is true, notably the aëronaut, the one who makes the aërial voyage; but it rises in spite of this. How is it to be explained? It is very simple. Recall once more the piece of wood forced to the bottom of a body of water and rising of its own accord as soon as released. A piece of lead would not have acted thus, because lead is heavier than water; but with outside help it will rise readily enough. Attach to it a piece of wood or, better, of cork, making sure that the piece is large enough, and the whole will return to the surface after being forced down into the water and then released. The wood or cork, being lighter than lead, will pull up the latter with it as it rises. Exactly so does the balloon conduct itself: the very light gas inflating it carries aloft, as it ascends, the heavy objects in the car or great wicker basket suspended from the gas-bag.
"When he wishes to come down again the aëronaut opens a valve by pulling a cord hanging within reach of his hand. A little hydrogen escapes and ordinary air takes its place, whereupon the balloon, rendered so much the heavier, begins to descend, slowly or rapidly according to the amount of gas discharged. With this explanation of the principles governing the balloon's ascent and descent let us turn to the narrative of an English scientist, James Glaisher, who in September of the year 1862 attained the greatest elevation yet reached by man.
" 'We left the earth,' he says, 'at one o'clock in the afternoon, in a mild temperature. Ten minutes later we were floating in a dense cloud which shrouded us in impenetrable gloom. After passing out of this layer of fog the balloon rose to a region flooded with light, the brilliant sunshine giving to the sky an extraordinarily vivid tint of blue.
" 'Above our heads we had nothing but the azure of the firmament; under our feet lay a widely extended surface of clouds arranged in imitation of hills, mountain chains, and isolated peaks, all of resplendent whiteness. One might have mistaken it for a mountain scene covered with snow of incomparable purity. As we ascended we caught momentary glimpses of the earth through occasional openings in the clouds.
" 'In twenty-five minutes the balloon had carried us up four thousand, eight hundred meters, which is very nearly the height of Mont Blanc, the loftiest mountain peak of Europe. A like ascent on foot would have cost us several days of extremely wearisome climbing. The temperature had by this time fallen very low, and ice was forming on the balloon. Another upward spurt raised us to the height of eight thousand meters, where the severest of winter cold prevails; and still we continued to ascend.
" 'When we had reached an altitude of eleven thousand meters, or almost a league beyond the highest mountain in the world, my assistant, Coxwell, noticed that the valve cord was entangled in the rigging of the balloon, and he climbed up to disengage it.
" 'Just then my right arm became suddenly paralyzed. I tried to use my left, but it too was paralyzed. They both refused to obey my will. I then attempted to move my body, but with so little success that I seemed to have no longer any body at all. I essayed at least to read the marks on the instruments, but my head fell over, inert, on my shoulder.
" 'I had my back against the edge of the car, and in this position I looked up at Coxwell, who was engaged in disentangling the valve cord. I tried to speak to him, but could not utter a sound. Finally thick darkness came over me, my eyesight having in its turn become paralyzed. Nevertheless I was still perfectly conscious. I thought I needed air and that I should die unless we could very soon manage to descend. With that I lost consciousness just as if I had suddenly fallen asleep.'
"A minute more of this swooning fit and it would have been all over with Glaisher. Coxwell climbed up into the cordage amid long icicles hanging from the balloon. He hardly had time to free the valve cord: the extreme cold had seized him and his hands, benumbed and blue, refused to do their office. He had to descend into the basket by clinging to the ropes with his elbows.
"Seeing Glaisher lying motionless on his back Coxwell at first thought his companion was resting, and he spoke to him, but received no reply. The prostrate man's silence indicated that he had fainted. Coxwell then undertook to succor him, but paralysis and insensibility were fast overcoming the assistant also and he could not drag himself to the dying man's side, whereupon he at last became convinced that they must descend without delay if they were not both to perish in a very few minutes.
"Fortunately the valve cord hung within his reach; but being unable to grasp it with his hands, benumbed as they were with cold, he seized it between his teeth and after a few tugs succeeded in opening the valve. Immediately the balloon began to descend. Before long, in an atmosphere less cold and less rarefied, Glaisher recovered consciousness and gave his attention to the frozen hands of his companion.
"They regained the earth safe and sound, both men, but with no desire for further ascents of a like perilous nature. Far more serious was the issue in the case of three French aëronauts who, some years later, wishing to add to our knowledge of atmospheric conditions, made an equally daring ascent. When their balloon came down again two of the rash explorers were dead, stiffened by the cold and suffocated by the insufficiency of the upper air, while the third, saved as by a miracle, was at his last gasp. Knowledge is sometimes bought very dearly. Science has its heroes and martyrs."