Although, when air is liquefied, the oxygen and nitrogen are condensed simultaneously, the latter has a lower boiling point than the former and therefore passes off more rapidly when the liquid is allowed to evaporate. This fact makes it possible to separate the two substances, by the process known as “fractional distillation,” and hence liquid air plants have been established for the special purpose of manufacturing oxygen and nitrogen, for both of which there is a large and growing commercial demand. Scores of millions of cubic feet of oxygen are used every year in the wonderfully efficient process of welding metals with the oxyacetylene blowpipe, the flame of which has a temperature of about 6,000° Fahrenheit. Most of the supply now comes from liquid air. An equally large amount is used in a recently introduced method of cutting metal. The object to be cut is first heated to incandescence, after which a jet of oxygen is played upon it. The metal actually burns away in the stream, and a clean cut is made like that of a saw. It is interesting to reflect, when we fill our lungs with oxygen in order to keep our bodily machinery in operation, that the same atmospheric gas is applied to the building of motor cars, bicycles, safes, boilers, and battleships. Cartridges made of lampblack, dipped for a few moments in liquid oxygen and then primed with a fulminate cap, constitute an explosive as powerful as dynamite and much cheaper to produce. A small percentage of oxygen added to the air supplied to blast furnaces has been found to effect a great saving of fuel used in the furnace.

The most important industrial demand for nitrogen is for use in “fixation” processes—i. e., for making nitrogen compounds to be used as fertilizers, explosives, etc. Before describing these processes, it may be of interest to mention that some of the “rare” gases of the atmosphere are now obtained on a commercial scale as by-products of the manufacture of oxygen and nitrogen from liquid air. Thus neon, on account of its exceedingly small resistance to the passage of electric discharges, is a promising substance for filling glow lamps; especially as means have been found of correcting the glaring red color of the light which characterized the original neon lamps. Argon is likewise used for filling electric lamps.

The idea of using the unlimited store of atmospheric nitrogen for the benefit of agriculture and the manufacturing industries has been very prominently before the public in recent years, and gained special notoriety during the late war, when great efforts were being made to increase the supply of nitrogenous materials suitable for use in explosives. Nitrogenous matters in the soil are indispensable to the growth of plants, and as long ago as 1898 Sir William Crookes, in an address before the British Association for the Advancement of Science, alarmed the world by pointing out the possibility of a general famine owing to the prospective exhaustion of Chilean nitrates and other sources of nitrogenous fertilizers. Nitrogen also enters on an immense scale into the composition of many industrial products besides explosives. No wonder popular writers have dwelt upon the fact that the atmosphere contains far more nitrogen than mankind needs for every possible purpose—actually something like 20,000,000 tons over every square mile of the earth’s surface.

A widespread misunderstanding, however, prevails as to the problem involved in utilizing this supply of nitrogen. Free (i. e., uncombined) nitrogen is of no use as a fertilizer, and it cannot be readily used in the arts. The process of extracting it from the atmosphere is an easy one, thanks to the liquid air industry. The real difficulty is to make this inert gas enter into chemical combination with other substances, forming useful compounds such as ammonia and nitrates; in other words, to “fix” it.

As we have stated on another page, lightning discharges cause nitrogen and oxygen to combine in the atmosphere, and perhaps also combine nitrogen and hydrogen to form ammonia. There is one other natural process by which atmospheric nitrogen is fixed. Certain species of bacteria are able to extract this gas from the atmosphere and combine it with other materials. Some of these bacteria are independent organisms, while others form colonies of parasites growing on the roots of higher plants, chiefly members of the pea family. In the latter case the bacteria use the nitrogen of the air and carbohydrates drawn from the roots on which they grow to form nitrogenous compounds, which are, in part, transmitted to the host plant.

Unfortunately these natural processes do not suffice to maintain agricultural soils in a high state of fertility. Mineral deposits of combined nitrogen are practically limited to the nitrate fields of Chile, from which more than two million tons of nitrate of soda are exported annually; but this supply cannot last more than a few decades. Combined nitrogen in the form of ammonia is supplied on a large and rapidly growing scale from by-product coke ovens, and another perennial source of nitrogenous matter is found in animal and vegetable refuse of all kinds, including fish scrap and slaughter-house refuse, garbage, sewage, manure, etc. Since, however, the demands of agriculture and the manufacturing industries greatly exceed the total amount of combined nitrogen obtainable from all these sources, the ingenuity of inventors has been spurred to the task of fixing atmospheric nitrogen by artificial methods, and several such methods have now been put in operation commercially. Their combined product at present constitutes nearly one-third of the total nitrogen supply of the world.

It is not proposed here to describe these methods in detail, but it may be mentioned that one of them, known as the “arc process,” imitates the action of lightning in combining the nitrogen and oxygen that occur naturally in the air, while the others utilize nitrogen that has been previously separated from the air by the liquid air process. The arc process requires, for commercial success, a large supply of cheap electrical power, and it is at present almost confined to Norway and Sweden, where electricity is obtained from waterfalls. In this process air is blown through a huge electric flame, spread out by a powerful electromagnet. The air yields nitric oxide, which is combined with water to form nitric and nitrous acids, and these substances are combined with others to form marketable products. The most widely used fixation process, and the one which the United States Government proposed to employ in the large plants that were in course of construction in this country at the close of the war, is known as the “cyanamide process.” This process requires, as a part of its raw materials, large supplies of limestone and coke, from which calcium carbide is made in an electrical furnace. The calcium carbide, at red heat, absorbs nitrogen, forming an intermediate product from which, by further processes, are made ammonia and nitric acid. A third method of fixing atmospheric nitrogen, which has been applied on a vast scale in Germany and is now coming into use in other countries, is commonly called the “Haber process.” In this process nitrogen is combined with hydrogen, obtained from water, to form ammonia, the combination being facilitated by the presence of what chemists call a “catalyzer,” i. e., a substance that enables other substances to combine without itself undergoing any change. Several different catalyzers have been used in the Haber process.

Two or three other methods of nitrogen fixation are beginning to assume commercial importance.

While the power of the wind holds an important place among the resources of the atmosphere, it cannot be said that the utilization of this resource has undergone developments in modern times at all comparable with the striking inventions and discoveries we have just been recording, if we except the use of the wind in aeronautics. Atmospheric resources used by aeronauts will be discussed in subsequent chapters.

The chief use made of the wind to-day, as in ages past, is to propel sailing ships, and its use for this purpose is, of course, of less importance, in a relative sense, than it was before the introduction of steam. The importance of windmills has also greatly declined. This fact was strikingly brought out some years ago when the United States Bureau of Statistics collected, through American consuls abroad, detailed information concerning the use of the windmills in foreign countries. In most parts of Europe windmills are rapidly disappearing. In Holland, for example, the traditional home of the windmill, the perpetual task of draining the polders is now performed by steam pumps, and the total number of windmills is estimated to be only about one-tenth what it was centuries ago. Our own country is probably the only one in which the use of windmills is increasing. The modern American windmill, with its disklike assemblage of numerous light sails, and ingenious contrivances for veering, reefing, etc., is a much more efficient contrivance than the old-fashioned windmill; but its utility, like that of other windmills, is limited by the irregular force of the winds.

For years the hope has been entertained that the windmill would eventually become a common means of generating electricity, but this hope has not yet been realized, though isolated installations of this character are in successful use.

CHAPTER III
THE ATMOSPHERE AS A HIGHWAY

In a subsequent chapter, dealing with Aeronautical Meteorology, we shall touch briefly upon the mechanical principles that underlie aerial navigation, by way of preface to a more detailed description of the conditions of wind and weather encountered by aircraft, and of the services that the meteorologist is rendering to the aeronaut.

The history of aeronautics may be divided into two periods, with the year 1914 as the dividing line between them. Before the great war the many brilliant minds that were trying to solve the problems of aerial navigation received comparatively little help or encouragement from humanity at large. The airship and the aeroplane were both accomplished facts, but most people looked upon them as ticklish contrivances of very little practical value. From the year 1909 onward aviation occupied an immense share of public attention; liberal prizes for aerial feats were offered; new records for speed, altitude, and endurance were made from day to day; but to the public, and perhaps to most of the aviators themselves, all this meant merely that a new and thrilling sport had been created, rather than a new art of boundless utility. Very few business men felt inclined to invest money in the development of aircraft, and the governments of the leading nations, with a single exception, were incredibly blind to the importance of building air fleets for use in war. The exception was Germany, which not only gave strong support to Count Zeppelin in the building of his dirigibles, but developed military aviation to such an extent that she entered the war with about 800 aeroplanes and a thousand trained pilots.

With the outbreak of the war the budding art burst into vigorous bloom. Unlimited funds were now available for experimenting and building. Thousands of flyers invaded the air, and the battle zone was a testing ground on a vast scale, where one improvement was hardly introduced before it was replaced by another. Some of the best engineering talent of the world was diverted from many and various fields to the one task of supplying the demands of the military aeronauts for more speed, more power, more reliable motors, better materials and appliances. Thus the war not only perfected aeronautics—especially aviation—as an art, but practically created it as an industry. At the close of hostilities the world found itself in possession of a vast fleet of aircraft, a multitude of aircraft factories, and a great army of trained aeronauts. For a time people asked—and perhaps some still ask—“What shall we do with them?”

There are many answers to this question, and new ones are coming to light every day. In the aggregate they mean that a new era has dawned in human affairs—the era in which the sky has been annexed to the world in which man lives. Henceforth we shall have more elbow room. We shall no longer be imprisoned in Flatland, but set free in Spaceland. It is impossible to foresee all the implications of this fact, but those that are already apparent suffice to fill us with enthusiasm.

Some of the most vexed problems of the present day will soon be solved by aerial navigation. Take that of our overcrowded cities. Everybody knows how first the trolley car and then the automobile helped to relieve the congestion of towns by making it feasible for people to live many miles from the scenes of their daily work, but at the same time seriously swelled the traffic of the streets in business quarters. Aircraft will bring far greater improvements in this respect, without corresponding disadvantages. In a few years it will probably be no inconvenience to live fifty or a hundred miles from one’s place of business. Aeroplanes, built for carrying several passengers in perfect comfort, already fly at speeds of from 120 to 150 miles an hour, and are almost independent of weather. Much greater speeds will doubtless be common in the future. Automobiles, all running on the same level, have almost reached the limit of space available in our busiest streets, and, under such conditions, they have nearly lost the advantage of speed they once possessed over the obsolete horse-drawn vehicle. There can never be such crowding in the air. When a great volume of aerial traffic is concentrated toward the centers of towns, people will fly their vehicles at various prescribed levels, and probably “park” them on many-storied landing stages. New methods of landing will undoubtedly be invented. The device known as the “helicopter,” which has made progress toward the practical stage during the past year, points out the possibilities in this direction. In the helicopter the propeller blades revolve around a vertical shaft, thus permitting the vehicle to rise or descend vertically. A prize of $100,000 has recently been offered by M. Michelin, the well-known French patron of aviation, for the perfection of this device, which may soon revolutionize the design of flying machines.

Mr. Holt Thomas, the Englishman whose foresight and enthusiasm have done so much to hasten the arrival of practical commercial aeronautics, believes that in the near future the main airways of the world will be served by airships rather than by aeroplanes. For long journeys the airship has the advantage that it can carry an ample supply of fuel without encroaching too much upon the space available for passengers and cargo. It is, therefore, especially suitable for transoceanic journeys. Hitherto airships, when not in flight, have been housed in enormous hangars, involving heavy cost of installation and their landing has required the services of hundreds of men—an operation that will probably seem laughable in its crudity to the next generation. The airship of the future will probably never go into a hangar at all except for occasional overhauling, as an ordinary ship goes into drydock. Hence only a few of these costly structures will be needed. While in service the airship will, on reaching an air port, moor herself at the bow to a great steel tower, and swing with the wind as a marine vessel swings at her anchor. At the top of the tower there will be a landing stage for passengers and freight, connected by lifts with the ground below. From the main air ports, thus equipped, will radiate minor air routes, served by aeroplanes, and, in some cases, by flying boats.

Such landing places for airships were predicted by Kipling in his “With the Night Mail”—but the author’s vista was of the year 2000! We are not traveling so slowly as that. Consider what it means that the world heard with bated breath of Blériot’s flight over the English Channel in 1909; and just ten years later men had flown over the Atlantic Ocean.

We have been writing of the future; but we need not look ahead for illustrations of the practical value of aerial navigation. Useful feats already accomplished are so astonishing in their variety that they make one cautious about assigning a limit to the possible applications of the new art. It has happened, for example, that a man who had booked passage on a trans-Pacific steamer missed his boat at Seattle; whereupon he hired an aeroplane, at a cost of $75, and overtook the steamer on her way down Puget Sound, thus saving some weeks of delay in waiting for the next one. Another man, who produces honey on a large scale, found that spray-poisoned orchards were playing havoc with his bees. He traveled in an aeroplane over the surrounding country, selecting stands for his hives at safe distances from such orchards, and he estimates that this precaution saved him $10,000 in a single year. In August, 1919, a flying boat deposited a bag of mail on the White Star liner Adriatic two hours after the ship had left New York.

Several aerial mail routes are now in operation on both sides of the Atlantic. The first regular service of this character in America was begun May 15, 1918, between New York and Washington, and during the first year carried 7,720,840 letters, with few accidents and no fatalities. The first year of service cost the Government $137,900, and the sale of aeroplane mail stamps during the same period yielded a revenue of $159,700. Out of 1,261 possible trips on this route, 1,206 were undertaken, and only fifty-five were abandoned on account of unfavorable weather. During 1919 the Post Office Department not only established other aerial routes, but relegated the aerial mail service to the ranks of the commonplace by reducing the postage on letters carried by aeroplane to the ordinary first-class rate of two cents an ounce.

In Europe lines of fast aeroplanes carrying mails, passengers, and freight daily over regular routes are becoming part of the established order of things. The operators of a line between London and Paris, which was inaugurated in November, 1919, are now planning to establish an hourly service. Some of these lines have been equipped with wireless telephony, so that the pilots can keep in constant communication with numerous stations of the company along the route, and also with one another. They are thus able to obtain, among other things, current information about the prevalence of fog or other atmospheric conditions at points ahead of them. Presumably the passengers who patronize the aeroplane express will also, eventually, enjoy the use of the wireless telephone en route. In connection with the new air routes suitable landing grounds, for regular or emergency use, are being laid out at short intervals; the ideal aimed at, for the present, being the so-called “ten-mile chain”; i. e., a series of emergency landing grounds about ten miles apart. From ordinary flying levels a pilot on such a route can always glide to one of these grounds in case his motor fails. The landing grounds will be utilized, under certain restrictions, for grazing cattle and for agricultural purposes, to help cover the cost of rental and maintenance. During 1919 the British Government established a chain of landing grounds in Africa, all the way from Cairo to the Cape.

One of the developments of the war was the use of aeroplanes for photographic mapping. The aeroplane flies over a long tract of ground, and the camera, exposed vertically, takes pictures automatically at fixed intervals. The pictures thus taken are carefully joined together in a single strip. A second tract, parallel with the first, is photographed in the same manner, and so on, until the whole area has been covered. Eventually all the pictures are assembled to form a so-called “mosaic.” This process is highly successful for mapping a flat country, but presents difficulties when there are hills and mountains. Some sort of stereoscopic process will probably be perfected for depicting accurately differences in level and producing a “contoured” map. Although aeronautical mapping does not yet replace old-fashioned methods, it already has several obvious uses. It is especially suitable for the revision of existing maps. Thus the plan of a city can be quickly brought up to date by this process. In the United States the Geological Survey has been engaged for many years in producing large-scale topographic maps of all parts of the country. This work proceeds slowly, and some of the maps are ten or fifteen years old. The contours and other natural features on such a map are still correct, but changes in the region due to the work of man are often extensive. Revision of these features can easily be made by the method above described.

For the preliminary mapping of a new country, by photography or by hand, the aeroplane offers the means of saving an immense amount of time and effort. The surveyor no longer needs to cut tracks through the jungle or scale mountains. No region is very difficult of access to the aviator. The summit of Mount Everest, the highest mountain in the world, is actually a mile lower than the greatest altitude attained by an aeroplane. Aviation has become an important feature of exploring expeditions. Captain Amundsen, the polar explorer, qualified as an air pilot before he embarked on his drift across the North Polar basin, and took aeroplanes with him on that journey. In India the Survey Department has organized a regular aerial photographic and reconnoissance service, and has lately photographed the high waters of the River Sutlej in order to obtain data for a big electrification project. Photographs of the Nile country have also been made for hydrological purposes. British aviators in Mesopotamia have mapped the flood boundaries of the Tigris and provided data for estimating crop areas. In the Philippines an engineer recently made a long aeroplane flight to determine which of three general routes was most suitable for a new railway. Many months of time and thousands of dollars were thus saved, as it was only necessary to send out one party of locating engineers instead of three after the selection had been made.

Recently the aerial surveyor has become the rival of the hydrographer in mapping shoals, channels, submerged rocks, and other features beneath the water. If the water is clear and suitable atmospheric conditions prevail, objects submerged to a considerable depth may be distinctly seen from an aeroplane flying far above the surface. It was on account of this fact that Allied aviators were able to spot submerged German submarines during the World War. The camera, equipped with proper plates and ray filters, can pierce the water even better than the eye. Thus objects have been photographed at a depth of more than 50 feet. British aviators charted the harbor of Rahbeg, on the coast of Arabia, by the process in 1917. In this country the leading exponent of underwater photography is Dr. Willis T. Lee, of the United States Geological Survey, who has taken scores of photographs showing submerged features of the waters adjacent to Chesapeake Bay. It is likely that rivers like the Mississippi, with ever shifting sand bars, will soon be made safe by monthly or weekly mapping from the air. In earthquake regions, such as southern Italy and Japan, the changing coast lines, shallows and harbors can easily be photographed after each new quake, thus keeping navigation open and protecting the lives of mariners.

Another application of this process of sighting submerged objects from the air is the aerial fish patrol. The plan of using aircraft to locate schools of fish appears to have been first suggested by Professor Joubin, of the Oceanographic Institute of Monaco, and it has been carried out with much success in both Europe and America. Its promoters hope that it will eventually revolutionize the fishing industry and add greatly to the world’s food supply. In the year 1919 seaplanes from the North Island Air Station at San Diego, California, made regular flights at an altitude of about 500 feet over the adjacent waters as an adjunct to the important fisheries in that vicinity. When a school of fish was detected, the aviator dropped low enough to ascertain the species, and if it proved to be of a commercial kind, such as the sardine, the news was flashed by wireless to the fishing fleet. The ocean in the neighborhood of San Diego was divided into numbered squares, shown on charts, and locations were reported by number. In 1920 a daily patrol was maintained by Navy seaplanes over the waters of Chesapeake Bay in behalf of the menhaden fishery. According to an official report, “the experiments fully demonstrated the commercial value of planes in this fishery.” It is believed that aircraft might be used with equal success in connection with the whaling industry.

The United States Forest Service has made considerable use of Army aeroplanes and aviators in patrolling the great forests of the West, where a constant lookout for fires must be kept throughout the summer. There are about 28,000 forest fires in this country every year, and the average area burned over amounts to more than 8,000,000 acres, entailing an average annual loss of $10,000,000 worth of timber. Observations are maintained on mountain peaks and towers, but the aerial watchman commands a much greater range of vision and can readily detect fires in places such as deep canyons where they are, in many cases, hidden from the existing lookout points. When a big fire is in progress, the aviator can quickly ascertain its extent and report the information by wireless to the fire-fighting forces. In case the fire is difficult of access on account of the absence of roads, the fire fighters can be transported to the spot in aeroplanes. It has even been proposed to fight forest fires by dropping bombs filled with fire-extinguishing chemicals. At one time it was thought that aeroplanes might largely replace fixed lookout stations, but experience shows that both systems of observation are desirable. Many foresters favor the use of small dirigible airships in place of aeroplanes, owing to their ability to fly very low, when desired, land in any small clearing, discharge passengers by rope-ladder while hovering over a selected spot, and transport relatively large loads of men and supplies.

Such are a few of the valuable peace-time uses that have already been found for the aerial vehicles that owed their production chiefly to the late war and for the host of pilots trained during the same conflict. Undoubtedly the immediate future holds far more interesting developments in store.

One important practical aspect of aeronautics remains to be mentioned, and that is the question of safety. In their early days the steamboat and the steam railway were both risky contrivances. It is recorded that at one time steamboats were barred from the Thames on account of their dangers. Undoubtedly the tradition of frequent boiler explosions lingered in people’s minds long after it had ceased to be a substantial fact. Aerial navigation—and particularly aviation—has now passed beyond the pioneer stage, but it still bears the dubious reputation that it acquired when it was in its infancy. Aerial travel, under standardized conditions, is no longer unsafe. There are good reasons for regarding it already as safer than automobiling. According to a report of the British Department of Civil Aviation, there were 21,000 commercial flights in Great Britain during the six months from May 1 to October 31, 1919, and 52,000 passengers were carried. The total mileage covered was 303,000. Not a single passenger was killed during this period, and only ten were injured. There were two fatalities among pilots and six pilots were injured.

Commander Read, who made the first transatlantic flight, writes on this subject:

“There are some pilots with whom I would refuse to risk my life. But, given a modern machine with the proper attention paid it, and a skillful but conservative flyer, it is as safe a means of rapid transit as an automobile traveling at less than half the speed. Nowadays there is scarcely ever an accident in an aeroplane of standard type due to the fault of material; they are all due to the inexperience or to the dare-devil stunting proclivities of the pilot—the pilot who ‘takes chances.’”

Aeronautics is now more than an art. It is a rapidly expanding branch of applied science. Aeronautical engineering has become one of the recognized professions. Some of the leading government laboratories of the world, including the National Physical Laboratory in Great Britain and the United States Bureau of Standards, are devoting their attention to aeronautical research. There are also many unofficial “aerodynamical” laboratories for studying, with the aid of wind tunnels and other apparatus, the many problems pertaining to the physics of flight and the principles of aeroplane designing.

Aeronautical questions have begun to figure conspicuously in jurisprudence. Legislators, as somebody has said, are busy making vertical laws to supplement the old-fashioned horizontal ones. In international law, especially, aerial navigation has given rise to thorny problems and it is already the subject of elaborate international agreements.

The physiological effects of flight and altitude have added a new chapter to the science of medicine. Seasickness has been the crux of the ship’s doctor; will “air sickness” prove equally baffling? What are the therapeutic possibilities of flying? Will physicians advise their patients to seek a “change of air” vertically instead of horizontally?

The atmosphere, once monopolized by the birds, has become the abode of man. That is one excellent reason why everybody should acquire a knowledge of meteorology—the science of the air.

CHAPTER IV
DUST AND SMOKE IN THE ATMOSPHERE

While there are many agencies that help to charge the atmosphere with dust, the most important of them all is the wind. Let us see what happens when the wind blows over the surface of a dusty road, for example. If the air flowed in a smooth horizontal stream over such a surface, its friction would drag the dust along on the ground, but would not lift it. Such surface drifting, due to the horizontal component of the wind’s motion, does, of course, occur, and its effects are strikingly visible in the shifting dunes that often form over a broad surface of sand or snow. All winds near the earth’s surface are, however, full of waves and eddies, and in many cases, as over a stretch of strongly heated soil, there are strong updrafts, sometimes extending to a great height in the atmosphere. All kinds of dust are heavier than air, and, contrary to popular belief, never truly “float” in the atmosphere. Dust may enter the atmosphere at high levels, through the disintegration of meteors, or it may be spouted up by volcanoes, but dust blown up from the earth’s surface rises only because the air is rising with it; and, in still air, all dust sinks more or less rapidly toward the ground. The rate of its fall depends upon its specific gravity, and upon the size and shape of the dust particles. Other things being equal, the finest particles fall most slowly. Exceedingly fine dust, even without upward air movements to support it, requires months or even years to fall to the ground from the higher levels of the atmosphere.

Upward movements in the air suffice to carry millions of tons of dust aloft every year, and horizontal air currents carry the same dust far and wide over the earth. The transportation of soil by the wind leads to some results of remarkable interest, practical as well as scientific. In the first place, far-reaching changes in topography are brought about by this process. Thus in China vast areas are covered to a depth of hundreds or even thousands of feet with a fine yellowish earth, called “loess,” which is believed to have been blown thither by the winds from the deserts of Central Asia. Less extensive deposits of this wind-borne material are found in many other parts of the world, including the Mississippi Valley. Another effect of wind transportation is the mixing of soils. There is a constant interchange of soil material between different regions, so that the composition of the soil on a particular farm, for instance, is not the same now that it was a few years ago or that it will be a few years hence. Lastly, the presence of dust in the atmosphere, whether derived from the soil or otherwise, has various interesting and important effects upon the heat and light we receive from the sun and modifies, in numerous ways, the conditions of human life upon our planet.

Several cases in which enormous quantities of solid matter have been carried to great distances by the wind have formed the subject of elaborate investigations on the part of meteorologists. Thus, during the three days, March 8–10, 1901, heavy dust storms occurred in the deserts of southern Algeria, and the sequel of these storms was carefully studied by Hellmann and Meinardus. A widespread cyclonic storm, central over Tunis at the time, sucked up the dust, which was carried northward by the winds at high altitudes. Deposits from this dust cloud occurred over an area extending as far as 2,500 miles from the place of origin. Reports collected from hundreds of observers indicated that 1,800,000 tons of dust fell over the continent of Europe, and one-third of this fell north of the Alps. As much more is believed to have fallen over the Mediterranean, while on the African coast itself the deposit is supposed to have amounted to 150,000,000 tons. In March, 1918, a shower of dust discolored falling snow at various places in the United States over an area of at least 100,000 square miles, extending in an east-west direction from Dubuque, Iowa, to Chelsea, Vt. Reports of this shower were collected by Messrs. E. R. Miller and A. N. Winchell, who estimate that the amount of dust could not have been less than a million tons, and may have been several hundred million. The dust is believed to have been blown up from the arid regions of the far southwestern United States and to have been transported a thousand miles or more.

Off the west coast of Africa, between the Canaries and the Cape Verde Islands, haze due to dust blown up from the Sahara Desert is frequently encountered by vessels, especially during the first four months of the year. This haze probably gave rise to the ancient legend of a Sea of Darkness—the Mare Tenebrosum—one of the mysterious terrors of the ocean reported by the navigators who first sailed toward the New World.

Extensive deposits of atmospheric dust have attracted attention from the earliest times. Ehrenberg, in 1849, collected records of 349 such cases, and published a map showing their distribution, which embraces the greater part of the world. Atmospheric dust is always brought down in greater or less quantities by rain. When it consists of fine powdery sand, the rain sometimes acquires a brownish or reddish tinge, staining objects on which it falls and constituting the “showers of blood” that have been regarded as prodigies from remote antiquity. Homer describes such a shower, and many similar occurrences are recorded by the Roman historians. Italy, owing to its proximity to the African coast, is often visited by these showers, which still strike superstitious terror into the hearts of the peasantry.

The millions of meteors that enter the earth’s atmosphere every day contribute their quota of dust, though the total amount is small compared with that of the material lifted from the earth. Fine ferruginous particles are often seen on the snowy summits of high mountains and the polar ice fields, and both their appearance and their composition indicate that they are derived from meteors.

Forest fires, burning peat beds, and other conflagrations on a large scale discharge quantities of dust into the atmosphere. Cinders from the great Chicago fire spread over a large part of the globe. They are said to have reached the Azores some forty days after the beginning of the catastrophe. In Europe, the once common practice of burning the moors to prepare them for cultivation gave rise to huge volumes of smoke, which was carried by the wind hundreds and even thousands of miles. The stronghold of this old custom—which still survives to some extent—was East Friesland, in northwestern Germany, and the characteristic haze to which it gave rise, known as “moor smoke” (German, Moorrauch), was sometimes observed as far away as Spain, Italy, and Greece.

The famous “dark days” that figure in both ancient and modern history, though in a few cases probably due to eclipses of the sun, have generally been the result of an abnormal accumulation of smoke or dust in the air; sometimes arising from volcanic eruptions, but more often from burning forests, moors, or prairies. Forest fires are the principal cause of dark days in the United States. Probably the most celebrated of such days was May 19, 1780, when, in consequence of great forest fires along Lake Champlain and down to the vicinity of Ticonderoga, darkness like that of night prevailed in New England. All but the most necessary business was suspended, the schools were dismissed, and the greater part of the population flocked to church to prepare for the end of the world, which was believed to be at hand. The great Idaho fire of August, 1910, was responsible for dark days over a larger area than in any other case on record in this country. Artificial light was required in the daytime over a broad belt, extending from Idaho to northern Vermont, but smoke was observed far beyond this area. The British ship Dunfermline reported that on the Pacific Ocean, 500 miles west of San Francisco, the smell of smoke was noticed and haze prevailed for ten days. When smoke in the air forms a rather thin layer, through which the sunlight penetrates feebly, we sometimes get an effect similar to the golden glow of sunset, a yellow or coppery tinge being cast over the landscape. Such was the cause of the “yellow day” still remembered in New England—September 6, 1881—attributed to the burning of the immense peat bogs of the Labrador barrens.

Another occasional cause of atmospheric dustiness is the eruption of volcanoes, especially those of an explosive character, which carry fine dust to heights at which it cannot be washed out of the atmosphere by rain. The remarkable dry fog of 1783—the most famous in history—which covered the greater part of Europe and North America for three or four months—was undoubtedly due to the violent eruptions of that year in Iceland and Japan. Its connection with the Iceland eruption was suggested even by contemporary writers. The outbreak of Krakatoa, in the East Indies, in 1883, spread a veil of dust over the greater part of the globe. For two or three years its presence in the air was the cause of striking optical phenomena, including gorgeous sunset glows. The story is told of an American fire brigade which, deceived by one of these brilliant sunsets, set out to extinguish what was mistaken for a great fire in a neighboring village. A large species of corona around the sun, known as “Bishop’s ring,” because it was first observed by the Rev. Sereno Bishop of Honolulu, appeared shortly after the eruption and reached its maximum intensity the following year. This was due to the diffraction of light by the exceedingly fine dust from the volcano, and the same phenomenon has been seen after other great explosive eruptions; e. g., that of Mont Pelée, in 1902. Some authorities believe that the finest particles of dust from the Krakatoa eruption were carried to an altitude of over fifty miles above the earth, and remained suspended at very high levels for several years, constituting the strange “noctilucent clouds,” seen on summer nights from 1885 onward. These clouds glowed with a silvery luster, attributed to reflected sunlight.

A persistent veil of volcanic dust in the upper air is thought to exercise marked effects upon terrestrial temperatures, and prolonged periods of intense vulcanism have been regarded as the cause, or one of the causes, of the recurrent ice ages of which geology furnishes the record. This explanation of ice ages was advanced by P. and F. Sarasin, in 1901, and was first put upon a scientific basis by Dr. W. J. Humphreys in 1913; but the idea that volcanic dust might be the cause of cold seasons was suggested by Benjamin Franklin as early as 1784. Franklin’s speculations on this subject were prompted by the cold winter of 1783–1784, which followed the extraordinary fog of 1783, already mentioned. Humphreys has published a list of all the great volcanic outbreaks recorded since 1750, and has shown that each of them registered itself in the temperatures of the earth and also, since accurate measurements began to be made of solar radiation, in these instrumental records. Thus, the intensely cold winters of 1783–1785 followed the tremendous eruptions of Asama, Japan, and Skaptar Jökull, Iceland, in 1783; the famous “year without a summer” (1816) was the sequel of the gigantic outbreak of Tomboro, in the Sunda Islands, in 1815, which is said to have hurled thirty-six cubic miles of solid matter into the atmosphere; and definite periods of low temperatures and reduced sunshine were observed after the eruptions of Mont Pelée, in 1902, and Mount Katmai, Alaska, in 1912.

The effect of a volcanic dust veil in lowering temperatures on earth is attributed chiefly to the fact that, while the fine grains of dust are able to reflect back into space the short waves of radiation coming from the sun, they do not bar the passage of the long heat waves radiated outward from the earth. According to Humphreys’s calculations, such a veil is about thirtyfold more effective in shutting solar radiation out than in keeping terrestrial radiation in. This process is just the reverse of the familiar effect of the greenhouse; where the glass lets in the short waves of solar radiation but does not readily let out the long waves of earth radiation.

A small contingent of atmospheric dust consists of common salt (sodium chloride) due to the evaporation of spray from the ocean. This substance is frequently found in rain, as well as in samples of air, not only near the seashore, but even in the interior of continents and on high mountains. According to Du Bois the amount of sodium chloride annually deposited on the dunes of Holland is at least 6,000,000 kilograms (more than 6,600 tons).

One of the striking phenomena of arid regions is the dust whirlwind; exemplified in the “devils” of India and South Africa, the “twisters” of Texas, etc. E. E. Free, in his treatise on “The Movement of Soil Material by the Wind” (U. S. Bureau of Soils, Bulletin 68), says of these whirls:

“They may be seen nearly every hot day, sometimes running rapidly over the surface; sometimes remaining nearly, if not quite, stationary, but never losing their rapid rotation. They usually last only a few minutes, but occasionally persist much longer. One observed by Pictet lasted for over five hours. They are largest and last longest on the flat, bare plains of the desert, and are usually seen in a calm or when only a light breeze is blowing, although their occurrence in windy weather is not unknown. These whirls have been noticed by many travelers in desert and steppe regions and have been carefully observed by Baddeley in India, and by Pictet in Egypt. They are frequent in China and on the pampas of South America, and occasionally occur during the dry season even in the humid regions. One of the most interesting phenomena in connection with the dust whirls is the occurrence of systems of several whirls, each revolving rapidly about its own center and also moving about a common center in a more or less perfect circle a few rods in diameter.”

The little whirls often seen on dusty roads are a miniature variety of the same phenomenon.

One very important class of dust particles in the atmosphere consists of organic matter, living or dead, including the pollen of plants and the countless myriads of microorganisms, as well as a variety of other products of the animal and vegetable kingdoms. An abundance of pollen in the air accounts for the occasional fall of yellow rain, described as “sulphur rain,” “golden showers,” etc. The promptness with which a piece of stale bread becomes moldy in a damp atmosphere is one of many proofs of the omnipresence in the atmosphere of the microscopic spores of fungi, ready to propagate their species with amazing rapidity as soon as they light upon a suitable nutrient medium. Last, but not least, bacteria, the most minute of all known organisms—so small that thousands or millions of them clustered together would make a mass not larger than the head of a pin—swarm in the air, as they do in water, the soil, and the bodies of animals. Fortunately, while certain species of bacteria carry disease and death with them, the great majority are harmless to mankind.