HAMLET, quizzing his two friends concerning their ulterior purpose in visiting him, says: "This most excellent canopy, the air, look you, this brave o'erhanging firmament, this majestical roof fretted with golden fire, why, it appears no other thing to me than a foul and pestilent congregation of vapors.' The erratic young man was justified in calling the clouds vapor congregations, if he had in mind the vapor of water. But in all likelihood neither he nor his creator, the myriad-minded playwright, knew aught about the origin of clouds. Shakespeare not infrequently alludes to the ephemeral beauty of airy crags and castles floating overhead; but exact knowledge of the gases composing air did not come until one hundred and sixty-five years after Shakespeare's death, when that strange, solitary figure in science, Henry Cavendish, separated nitrogen and oxygen.

Even at the close of the eighteenth century no one knew how far up the atmosphere extended, or, in common speech, how high was the sky, meaning by this the outer limit of the earth's atmosphere.

When Cavendish was isolating nitrogen there lived in Paris a certain philosopher-statesman, representing the new American nation. He had in earlier days written much about clouds, vapors, winds, and storms. But Franklin was then an old man, and duties of a political nature occupied most of his time; so we have nothing from

him in these later days worthy of much comment except the expression of his firm belief that man at last had conquered the air. Cavendish had made it plain that hydrogen gas had a definite lifting power in air, hence it was no longer necessary to burn straw, thus making hot air for the inflation of balloons. On August 27, 1783, the professor of experimental philosophy at the University of Paris inflated a large oiled-silk bag with the new lighterthan-air gas. Franklin, who saw the bag rise, wrote to the president of the Royal Society that 'it [the twelve-foot balloon] entered the clouds and seemed scarce bigger than an orange, and then disappeared into the clouds.' Incidentally we may note that some fifty. thousand people beside old Ben saw the experiment and were delighted. In Franklin's words: "The multitude separated, all well satisfied and amusing one another with discourses of the various uses to be made of this new discovery.'

Hot air, however, had its devotees, in that day even as in our own, and before the end of November in that memorable year two men actually ascended in a large balloon which carried a basket grate in which fagots and sheaves of straw were burned. These airmen may very well claim the honor of being the first motor firemen, for each had to pass sheaves of straw into the grate, 'to keep

heads, we could see the Fire which was very considerable.' It may be remarked in passing that life insurance was not then in vogue.

While it is somewhat foreign to the question, 'How high is the sky?' the temptation is strong to conjecture how Shakespeare would have embodied this new knowledge had it come one hundred and seventy years earlier. What immortal lines would have described man's audacity in bringing lightning from the clouds down to earth and the even more astounding feat of rising from earth to journey through the clouds!

II

The first man who carried aloft scientific instruments and, so to speak, tried to 'plumb the air' was a graduate of Harvard College class of 1763. Dr. John Jeffries in 1784 ascended over London, equipped with barometer, thermometer, hygrometer, electrometer, compass, and six small stoppered bottles. These last, filled with water, were emptied when the balloon was highest, then sealed. Samples of air thus obtained were given to the Royal Society and analyzed by Cavendish. The fact was thus determined that up to two miles at least there was no sensible difference between upper air and that at the ground.

A few months later Jeffries, with Blanchard, crossed the English Channel via air. They were four hours going from Dover to Calais in four loops. In the last of these loops they met with disaster and were nearly drowned. As they approached the French coast, all ballast having been thrown out and even their outer clothing cast off, the balloon notwithstanding fell lower and lower until at last the basket was in the water; but as they neared the cliffs a slant of wind carried the balloon upward sufficiently high to clear the hills,

and they were able to make a landing a short distance back from the coast line. A monument marks the spot where the first gliders demonstrated what Professor Langley more than a century later described as the inertia of the air. Making use of this physical property of air, man to-day can not only imitate the buzzard but even outdo that bird by flying upside down.

After a short rest Jeffries went on to Paris and, it is interesting to note, met the great Dr. Franklin at Passy and dined with him a number of times. At one of these dinners Commodore John Paul Jones was present and complimented his fellow countryman upon his courage in venturing thus to cross the Channel. Little could either of them imagine that in years to come airships larger far than any frigate of the line then in existence would cross and recross the Atlantic. Nor could they picture in the mind's eye heavierthan-air machines hustling across the Channel, as they do to-day in half an hour, even against head winds.

The ascension over London, however, was in many respects more important than the flight across the Channel, for it gave the first information of the fall in temperature with elevation. Starting with 50° F., the aeronauts soon experienced freezing conditions, and, at their highest level, a temperature of 20° F., or twelve degrees below freezing. These figures agree quite well with modern values of the 'lapse rate,' as it is called, or the vertical gradient of temperature, to which we shall refer later.

Near the end of the eighteenth century there was much enthusiasm shown in connection with ballooning, and even so cautious a man as Franklin freely prophesied the complete conquest of the air by man. These high hopes were not fulfilled. For a century, indeed, the balloon remained just what it was, a

gas bag. There were several ascensions that were noteworthy, but the one which attracted most attention occurred on September 5, 1862, when Glaisher and Coxwell thrilled the world with a dramatic ascent in which both men became unconscious. The exact height is uncertain, but it was approximately 11 kilometres-6.6 miles. This was not equaled until June 30, 1901, when Berson and Süring ascended to a height of 10.5 kilometres, both men being unconscious for perhaps fifteen minutes.

III

Meanwhile this most excellent canopy, the air, was being bored into from below, if one may so express it, by inquisitive scientists. Not content with climbing mountains and flying kites larger and more complicated than Franklin ever dreamed of, the aerographers began to send up small free balloons carrying automatically recording thermometers, barometers, and wind registers, known as ballons-sondes -sounding balloons. As soon as the results accumulated, an astounding discovery was made. It seemed as if there must be some mistake, some error of registering, for the fall in temperature or lapse rate ended at a rather definite height, 10,000 metres. Above this the temperature remained stationary or grew warmer. It seemed unbelievable, but record after record confirmed the result; and the leading aerographer of that day, M. Léon Teisserenc de Bort, announced that this most excellent canopy, our atmosphere, was a double air sphere - that is, two concentric shells. Or we may think of it as a blanket with an under lining. This lining lies close to the earth, but is not of uniform thickness, being five miles thick or high-in temperate latitudes, but thinning toward the poles and getting thicker over the equator.

This lining is called the 'troposphere' or region of turning, while the upper part of the blanket is called the 'stratosphere' or layer region; and here, although the temperature is very low, some fifty degrees below freezing on the Centigrade scale, the region, no matter how high we go, is isothermal. This means that there is no marked difference in temperature from six miles up to twenty - unless indeed it grows somewhat warmer. A certain Oxford professor deduces from the study of meteors that at an elevation of fifty miles it is actually much warmer than on a summer day at the surface. This, however, is to be taken with reservation.

Teisserenc de Bort's discovery of the dual nature of our atmosphere materially modified all previous conceptions of its structure. A close friend and side partner of the enthusiastic Frenchman was Lawrence Rotch, a graduate of the Massachusetts Institute of Technology, class of 1884, founder of the Blue Hill Observatory, an institution working along much the same lines as Teisserenc de Bort's observatory at Trappes.

Sounding balloons found a practical application in the late unpleasantness between the leading civilized nations of the world, being utilized to advantage in warning airmen of the existence of dangerous conditions at flying levels. To-day they are extensively used by all national weather services. Free pilot balloons are also used, and as a result of all this exploration we have a fairly good idea of the structure of the air up to 35 kilometres or 21 miles. Here our knowledge might have rested but for progress in another field.

Early in 1902 an electrical engineer, Professor Kennelly of Harvard, published in the Electrical World and Engineer a paper on long-distance behavior of wireless waves. He announced the probable existence of a reflecting layer

about 80 kilometres above the earth's surface; for, as every radio expert knows, signals fade out at certain distances and come in strong farther away.

A mathematical genius, the late Oliver Heaviside, in an article written for the Encyclopædia Britannica, December 1902, unaware of Kennelly's paper, also announced the probable existence of an ionizing layer at great heights, which would explain the behavior of radio signals. The layer is commonly called the Heaviside layer, although more justly it should be termed the Kennelly-Heaviside layer.

So we have come by slow marches to the top, or very near the top, of the canopy concerning which our young Hamlet spoke so disparagingly. But that was long ago; and neither Shakespeare nor any of the rovers of the Elizabethan age dreamed that some day ships midway between the cliffs of Devon and the far-away Bermoothes would carry on conversation with folks at home. How loud their laughter would have been at the mere suggestion that voices on the far-spread open sea could be heard distinctly at firesides a thousand leagues away! But so it is.

IV

To return to high levels in the air: the Radio Research Board has now no problem more pressing for solution than the determination of the part played by this stratum in the propagation of electric waves. A ship leaving the sending station will receive signals especially if short wave lengths, say 10 metres, are employed in sendinguntil the ship is about 150 kilometres or 90 miles out. Then the waves die down, being probably flattened or attenuated. When, however, the ship is 1500 kilometres 900 miles out, the signals are heard again. On the vessel used by the Board in its experi

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ments the signals were once more heard at 5000 kilometres or 3000 miles; and they can be picked up at 10,000 kilometres, or one fourth of the distance around the world. It is agreed by the investigators that a layer of thin air, very thin indeed, 90 kilometres above the earth, carries the waves and also deflects them. It has been actually shown that after sunset waves travel downward. Methods for measuring the angles and intensities have been devised and used with such success that probably in a few months radio engineers will announce important discoveries concerning the upper air.

Two other lines of attack on the confines of our atmosphere are open. Trails of meteors offer evidence of pressure and temperature at a height of eighty miles; and auroral displays that have baffled explanation so long now seem about to yield definite information of conditions at a height of one hundred miles or more. The seat of these phenomena is far, far above cloud land. Judged by these heights, the clouds are very near neighbors of man. Indeed, venturesome man has several times vaulted over the highest clouds, the cirri. M. Callizo, on October 10, 1924, reached 12,066 metres, -39,586 feet, while the high cirri measured at Blue Hill are as a rule below 10,000 metres. An American airman, Lieutenant Macready, reached an altitude of 11,797 metres - 38,704 feet - on January 29, 1926.

An astonishing thing about these trips above the clouds is the speed and ease with which they are made. The ubiquitous flea has long been hailed as the champion jumper, considering his size and weight, but man now hops up in the air seven or eight miles and returns to the starting point in an hour. A year ago it seemed that positive knowledge of the nature of the aurora was in our hands. Vegard, bombarding

nitrogen at extremely low temperature with cathode rays, obtained in the spectrum the characteristic green auroral line. Since then McLennan and Shrum at Toronto have definitely obtained the line by using admixtures of helium under like conditions. There is no positive evidence of free hydrogen in our atmosphere. Later experiments may, however, indicate the presence of oxygen at the auroral level. These studies make it plain that our atmosphere extends much farther than we thought. In fact, we may answer the question, 'How high is the sky?' At least one hundred miles!

V

Let us now approach our atmosphere from above. An observer on a near-by planet, studying our most excellent canopy, would see a bluish haze rotating with the same angular velocity as the earth, an elongated ellipsoid of revolution, bulging considerably at the poles. At the outer limit there would be no heat; the temperature would be the absolute zero. Far down in the very dregs would appear a shallow sediment where the temperature would average a few degrees above freezing. Mankind lives in that sediment and actually breathes it, a mechanical mixture of nitrogen, oxygen, carbon dioxide, helium, argon, krypton, niton, zenon, neon, and — last, but most important -vapor of water. Fortunately there are no chemical combinations, and the elements of our atmosphere get along quite peaceably together.

Our planetary observer with a big telescope and high-power eyepiece might discern small black specks. These if we anticipate the future by a few months—would be squadrons of airplanes, darkening the streets below. Our extraterrestrial gazer might even make out infinitesimally small dots

that is, on the

at the very bottom ground. These would be the 'chariots' that 'rage in the streets,' as a prophet of the Old Testament would describe them were he alive.

If he knew as much as our allsearching physicists on earth know, he would call cities atoms, and the inhabitants electrons. The designation of nations would be molecules. He would discover that there were far-flung whirlpools of air, commonly known as storms. He might somewhat hastily conclude that the affairs of men were determined by these vortices at the bottom of the bluish haze. He might tell his fellows that undoubtedly affairs at the bottom of the sky were controlled by wind. Would he be far wrong? Constitutions, political platforms, even commencement orations are they not essentially air in motion? The very term 'democracy' carries the fine flavor of air in motion, for the demos, or crowd, like a wind vane, is blown about by every strange wind of. doctrine.

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Turning from the dregs and turmoil of the winds, the planetary observer would record that the heights were serene. Here no tumults, no congregations of pestilent vapors, but even, as a great playwright put it in the speech of Hamlet, an overhanging firmament, a majestical roof fretted with golden fire. And there on the majestical roof, keeping company with the aurora, electrical waves started heavenward from earth-born lips may dance their way onward. Perhaps the whispered tones of a maiden's 'Yes,' - a maiden of the days when the Autocrat wrote, - uttered within range of a microphone, may reach those uttermost skies. Traveling thence many, many miles, they may bend again earthward, and, caught by dull glowing grids, reclothed as audible speech, be heard by millions far across the seas.