Every schoolboy has read how Benjamin Franklin, by means of his famous kite experiment, demonstrated the electrical nature of lightning, and how the same versatile genius invented the lightning rod. It is not proposed to repeat familiar history here. Neither shall we discuss the dubious statements frequently put forth that lightning rods were known before Franklin’s time, nor consider how much credit is due the many philosophers who, at earlier periods, suspected lightning to be a manifestation of electricity. The facts and ideas concerning atmospheric electricity that we have to present in this chapter were, for the most part, quite unknown to Franklin and to many generations of savants after him, and some of them are just now finding their way into the textbooks.

Science still recognizes the existence of two kinds of electricity—positive and negative—which, by combining, neutralize each other’s effects. According to current ideas, however, the more active agent in electrical phenomena is negative electricity, which is believed to consist of (or to provide electrical charges for) exceedingly minute particles called electrons.

Only a few years ago the smallest thing that science had to deal with was the atom, and the lightest of atoms is that of hydrogen. The discovery of electrons marks a new step toward the infinitely little. The mass of the electron—or, in more popular and less exact terms, its weight—is about 1/1800 that of a hydrogen atom. As to its size: Imagine a billiard ball magnified to the size of the earth. Its constituent atoms would be the actual size of billiard balls, but the electrons of which each atom is composed would still be too small to be seen with the naked eye. Now imagine each of these billiard-ball atoms further magnified to the size of a large church. The electrons would then be about as big as one of the periods on this page.

When we say that a body has an electrical charge we mean that it has an excess of positive or negative electricity. An ordinary molecule of an atmospheric gas contains (or perhaps actually consists of) equal amounts of the two kinds of electricity, and is therefore not charged. There are, however, various ways in which an electron may be detached from such a molecule, leaving it positively charged; and again it may receive an extra electron, and thus acquire a negative charge. Under the former circumstances it becomes a positive ion, and under the latter a negative ion. Ions play a very important role as carriers of electricity, because they are impelled to move toward oppositely charged bodies or particles and combine with them. A gas containing ions is said to be ionized; and it is the ionization of the atmosphere that makes it a conductor of electricity.

The number of ions in a given volume of air has been the subject of a great many measurements, both at observatories on land and in the course of scientific expeditions at sea. There are ingenious instruments called “ion counters,” in which air is drawn at a measured rate through the apparatus and its electrical effects are noted. The number of positive ions found in a cubic centimeter of the lower atmosphere varies from a few hundred to a thousand or more, while the number of negative ions in the same space is generally one or two hundred smaller. The ionization is about the same over the ocean as over the land.

There are several ways in which the air may become ionized. The different rays given off by radioactive substances (Alpha, Beta, and Gamma rays) all have the power of driving off electrons from the molecules of gases; i.e., ionizing them. Air is undoubtedly ionized by radioactive matters in the soil (radium and thorium) and especially by the gaseous “emanations” of these substances in the atmosphere, which are also radioactive. It has, however, been a problem to account for ionization over the ocean; because the amount of radioactive matter in sea water is immeasurably small, while the amount of radioactive emanation in sea air is, according to the latest observations, only about 2.5 per cent of that occurring over the land.

The clue to this mystery seems to be found in a special kind of Gamma rays coming from some region far above the surface of the earth. These rays are called the “penetrating radiation,” because they not only are able, like the Gamma rays due to radioactive substances on earth, to pass through the walls of hermetically sealed metal vessels and ionize the air inside, but they also have the power of passing through a great extent of atmosphere without being absorbed. They are estimated to be about ten times as “penetrating” as the Gamma rays coming from known terrestrial substances. The best proof that a radiation of this sort comes from above is that when closed metal vessels are carried up in balloons, there is, above an altitude of about half a mile, a rapid increase in the rate at which ions are produced within them. As to the source of this radiation, one suggestion has been that it comes from strongly radioactive cosmic dust in the upper atmosphere. A hypothesis that seems more plausible at present attributes it to the bombardment of the atmosphere by electrons shot off from the sun.

The knowledge of ions in the atmosphere is one of the recent acquisitions of science. On the other hand, it has been known for some generations that the earth has normally a negative charge as compared with the air, or the air a positive charge as compared with the earth. Thus between the earth and any point in the air (except, as we now know, at great altitudes) there is a difference of what is called “potential,” of such a character that negative electricity will follow any conductor provided for it away from the earth. Variations of potential with altitude have long been measured by means of instruments called “collectors,” which gather, so to speak, a sample of the electrical charge of the air at any point and enable it to be compared with that of the earth. The difference of potential is measured in volts per meter of vertical distance. Thus we get the “potential gradient,” which averages about 150 volts per meter in the lowest part of the atmosphere. It is subject to great irregular variations—especially during thunderstorms—and also to somewhat regular rises and falls during each day, and to an annual fluctuation, being much greater in winter than in summer.

It has also been known for a good many years that the air is a conductor of electricity—though a poor one—and, therefore, does not insulate the earth. Dr. W.F.G. Swann has expressed the extremely small conductive capacity of the air for electricity in the statement that a column of it one inch long offers as much resistance to the passage of an electrical current as a copper cable, of the same cross section, thirty thousand million million miles long!

A fact more recently learned, from observations in balloons, is that the potential gradient falls off rapidly at high levels, and becomes practically zero at an altitude of about six miles. From this fact it is concluded that the lower six miles of the atmosphere contains a charge of positive electricity just equal to the negative charge at the earth’s surface. In other words, the lower atmosphere is not only positive with respect to the earth, but in an absolute sense it contains an excess of positive electricity.

Thus we have a negatively charged earth surrounded by a layer of positively charged air. Since air is a conductor, it is not easy to see why the opposite charges of the earth and the atmosphere do not combine and neutralize each other. An interchange is, in fact, always going on between them; negative ions flow upward from the earth and positive ions flow in the opposite direction. This “earth-air current” is, however, exceedingly small. Moreover, the opposite charges of the earth and air remain from year to year in spite of it.

How does the earth retain its negative charge and the air its positive charge? No other question relating to atmospheric electricity has, in recent years, been so much debated as this. Discussion centers, as a rule, upon the negative charge of the earth; for there are certain reasons for assuming that, when once this is explained, the positive charge of the atmosphere will require no special explanation.

One hypothesis is that the earth is bombarded by positive and negative corpuscles from the sun, and that the negative corpuscles have such penetrating power that they are able to reach the earth, while the positive corpuscles are caught by the atmosphere. Another hypothesis (Swann’s) is that the same “penetrating radiation” that, as we have seen, helps to ionize the lower atmosphere has the effect of driving downward a stream of electrons detached from the air molecules, thus maintaining a constant supply of negative electricity to the earth. The question is not yet settled.

It is now time to turn from these somewhat abstruse matters to a subject of universal interest; viz., lightning. As recently as a few decades ago, though there was already a copious literature on the subject of lightning, very little was really known about it. Even its superficial features were strikingly misunderstood until the advent of photographic methods of investigation. Thus until the middle of the nineteenth century sharply zigzag lightning flashes were represented in scientific books as they still are in conventional art. That so-called zigzag lightning is really sinuous was first asserted by James Nasmyth, in 1856, and his contention was soon afterward confirmed by photography. The camera has revealed a large number of other interesting things about lightning.

Everybody has noticed an appearance of flickering in lightning flashes that are of sensible duration. Several early investigators, such as Arago, Dove, and O.N. Rood, had reached the conclusion that such flashes are multiple, consisting of several successive discharges along an identical path. Rough measurements of the intervals of time between these discharges were made with various forms of rotating disk. Far more accurate information is now obtained on this subject by the use of a camera mounted on a vertical axis and swung in a wide arc, at a fixed rate, by clockwork. The perfection of this device is due, in part, to A. Larsen, in America, but especially to Dr. B. Walter, of Hamburg, whose achievements in the photography of lightning far surpass those of any other investigator.

It is obvious that if a discharge of lightning is not instantaneous, but has a sensible duration, the rotary movement of the camera, arranged as just mentioned, will spread out the image of the flash, on the photographic plate, into a more or less broad band or ribbon. Most photographs of ribbonlike streaks of lightning made with ordinary cameras are, in fact, due to accidental movements of the apparatus during exposure—such as an involuntary start of the operator, in case the camera is held in the hands—though a certain amount of spreading of the image is sometimes caused by what photographers call “halation.” Pictures taken with the revolving camera show that some flashes are practically instantaneous while others may last as long as half a second or more. Those of the latter class nearly always show several parallel streams of light, more or less distinctly separated by darker spaces. Each of these bright streams represents a separate discharge along the common path. As the speed with which the camera turns is known, it is possible to determine the intervals of time between the discharges of a multiple flash. These intervals may vary from a few thousandths to one or two-tenths of a second, while the duration of each of the consecutive discharges is probably not more than two or three hundred-thousandths of a second in most cases. Sometimes the path of the lightning flash is shifted by the wind while the picture is being taken. In one case this shift was estimated at 36 feet.

Photography is also applied to determining the distance of a lightning flash and hence the dimensions of any of its features. For this purpose a stereoscopic method is used, two cameras being mounted side by side and exposed at the same time. Sometimes one of the cameras is made to revolve, while the other remains stationary. The stationary camera will then show the relative positions of the flashes occurring during exposure, while the moving camera will indicate the times at which they occurred.

Streaks of “black lightning” and black borders of the white flashes, both often seen in photographs, are a trick of the camera and are due to what is called the “Clayden effect.” Some kinds of plates are much more susceptible to this effect than others. When a flash of lightning has registered its impression on such a plate, and, before the shutter is closed, another flash occurs, the general illumination of the field by light reflected from clouds, etc., often “reverses” the original image, and consequently it prints black.

“Sheet lightning” presents the appearance of a diffuse glow over the sky. When lightning of this character is seen playing about the horizon on summer evenings, in the absence of an audible thunderstorm, it is often called “heat lightning.” Most sheet lightning is probably a mere reflection of ordinary streak lightning below the horizon or hidden by clouds. Some authorities believe, however, that diffuse, silent discharges actually occur in the clouds. Balloonists claim to have encountered such discharges near at hand. An analogous phenomenon is the glowing of so-called “incandescent” or self-luminous clouds, to which several observers have called attention. A remarkable phenomenon of somewhat similar character has been reported by Dr. Knoche, late director of the Chilean meteorological service, who states that it occurs on a vast scale along the crest of the Andes during the warm season. The mountains seem to act as gigantic lightning rods, giving rise to more or less continuous diffuse discharges between themselves and the clouds, with occasional outbursts simulating the beams of a great searchlight. These displays are visible hundreds of miles out at sea. Something akin to this so-called “Andes lightning” has occasionally been reported from other mountainous regions, including the mountains of Virginia and North Carolina.

“Beaded” lightning and “rocket” lightning are as rare as they are interesting. The former resembles a string of glowing beads, while the latter is a form of streak lightning that shoots up into the air at about the apparent speed of a skyrocket.

“Ball lightning” takes the form of a fiery mass (not always globular), which generally moves quite deliberately through the air or along the ground, and in many cases disappears with a violent detonation. It occurs inside of buildings, as well as out of doors.

In order that a discharge of electricity may break through the resistance of the air along paths as long as those commonly observed, enormous differences of potential must exist in the atmosphere during thunderstorms. How such conditions arise has been the subject of an immense amount of speculation. The explanation now generally accepted was proposed in the year 1909 by the English physicist and meteorologist, Dr. George Simpson. This hypothesis is based upon the fact, well attested by laboratory experiments, that the breaking up of drops of water involves a separation of positive from negative electricity; in other words, the production of both positive and negative ions. In this process the drops become positively charged; i.e., they retain a greater number of positive than of negative ions, the latter being set free in the air. About three times as many negative as positive ions are thus released.

Now a thunderstorm is accompanied by strong upward movements of the air; so strong that small drops cannot fall to the ground, while large drops, which would be heavy enough to fall through such rising currents if they could retain their integrity, are broken up by the blast of air and carried aloft, where they tend to accumulate, recombine, and fall again. This process may be repeated many times, so that the positive charge of the drops is continually increasing, and at the same time negative ions are being set free and carried by the ascending air to the upper part of the clouds. Here they unite with the cloud particles and give them a strong negative charge. Thus eventually there is formed a heavily charged positive layer of cloud between a heavily charged negative layer above and the negatively charged earth beneath. When the differences of potential thus brought about become great enough, disruptive discharges of electricity will occur, and these may be either between the upper and lower layers of cloud or between the clouds and the earth, or, sometimes, between two different clouds. Probably much the most frequent lightning flashes are those that occur within a single thundercloud and do not reach the earth. However, it will often happen that the negatively charged upper layer of cloud is either carried very high or drifted away by the wind, and then the discharges that occur will be chiefly between cloud and earth. Such conditions are likely to prevail in the case of cyclonic thunderstorms, in which there is often great difference in the direction and force of the winds at different levels. On the other hand, heat thunderstorms usually occur when the general winds are light at all levels, and it is probable that such storms are relatively free from cloud-to-earth discharges. We seem to have here an explanation of the paradox that tropical thunderstorms, which are nearly always of the noncyclonic type, though notoriously violent, are generally harmless.

It must not be inferred from what has been said above that the mystery of the lightning flash is now fully resolved. This is far from being the case. It is not at all clear how an electrical discharge can break down the resistance of the air along a path a mile or more in length, as commonly happens in the thunderstorm. It was formerly stated, on good authority, that the difference of potential required to produce such a flash would amount to upward of 5,000,000,000 volts. Certain facts have lately been adduced to show that such great differences of potential need not be assumed. Moving-camera photographs of the sparks produced by electric machines show that such sparks begin with small brush discharges which gradually ionize the air and thus build up a conductive path for the complete discharge. Something of this kind may occur in the atmosphere. Streaks of air already strongly ionized and more or less continuous sheets of rain would also help to provide conductors for a discharge. If lightning does build up its path somewhat gradually, the process might, in certain cases, be so slow as to account for the deliberate movement of rocket lightning, and also, perhaps, furnish a clue to the hitherto unsolved mystery of ball lightning. Humphreys has tentatively suggested that all genuine cases of ball lightning are “stalled thunderbolts”; i. e., lightning discharges that have come to a halt, or nearly so, in their progress through the air.

As to the visibility of lightning Humphreys says, in his “Physics of the Air”:

“Just how a lightning discharge renders the atmosphere through which it passes luminous is not definitely known. It must and does make the air path very hot, but no one has yet succeeded, by any amount of ordinary heating, in rendering either oxygen or nitrogen luminous. Hence it seems well-nigh certain that the light of lightning flashes owes its origin to something other than high temperature, probably to internal atomic disturbances induced by the swiftly moving electrons of the discharge, and to ionic recombination.”

A few attempts have been made to measure the strength of current in a lightning discharge. Many substances become magnetized when an electric discharge occurs in their vicinity, and it has been pointed out by F. Pockels that when basalt rock is magnetized in this way the amount of magnetism is an indication of the greatest strength of current to which it has been exposed. Pockels examined specimens of basalt from the top of Mount Cimone, in the Apennines, where lightning strokes are common, and found many of them more or less magnetized. He also exposed blocks of basalt close to a branch of a lightning rod in the same region. He thus obtained values for the strength of current in lightning discharges ranging from 11,000 to 20,000 amperes. Humphreys, from the crushing effect of a lightning stroke upon a hollow lightning rod, has computed the strength of current in the case examined to be about 90,000 amperes.

The effects of lightning are so various that it would take a book to describe them all. Its audible effects are discussed in our chapter on atmospheric acoustics. Its chemical effects consist chiefly in the production of oxides of nitrogen, ozone, and probably ammonia from the constituents of the atmosphere along the path of the discharge, and these substances, either directly or after further combinations in the atmosphere, contribute to the fertility of the soil. Lightning sometimes bores holes several feet deep in sandy ground and fuses the material along its path, forming the glassy tubes known as fulgurites. Similar holes are bored in solid rock.

The destructive effects of lightning are due chiefly to the heat generated by the passage of an electric current through a poor conductor. When moisture is present in the object struck, its sudden conversion into steam produces the explosive effects seen in the shattering of trees, the ripping of clothes from the human body, etc. There is almost no end to the curious pranks played by lightning—some disastrous, some comical, and some benevolent, as when persons crippled with rheumatism, after having been knocked down and temporarily stunned by a stroke of lightning, have found themselves completely cured of their malady! A well-known book by Camille Flammarion, translated into English under the title “Thunder and Lightning,” is almost wholly devoted to these eccentricities of the lightning stroke.

There is a very common belief that lightning sometimes impresses a photographic image of trees or other objects of the landscape upon the human body. The ramifying pink marks, known as “lightning prints,” often found on the skin of persons who have been struck by lightning, are, however, in no sense photographic, but are merely the lesions due to the passage through the tissues of a branching electric discharge.

A few practical suggestions in regard to danger from lightning are offered by Humphreys, as follows:

“Generally it is safer to be indoors than out during a thunderstorm, and greatly so if the house has a well-grounded metallic roof or properly installed system of lightning rods. If outdoors it is far better to be in a valley than on the ridge of a hill, and it is always dangerous to take shelter under an isolated tree—the taller the tree, other things being equal, the greater the danger. An exceptionally tall tree is dangerous even in a forest. Some varieties of trees appear to be more frequently struck, in proportion to their numbers and exposure, than others, but no tree is immune. In general, however, the trees most likely to be struck are those that have either an extensive root system, like the locust, or deep tap roots, like the pine, for the very obvious reason that they are the best grounded and therefore offer, on the whole, the least electrical resistance.

“If one has to be outdoors and exposed to a violent thunderstorm, it is advisable, so far as danger from lightning is concerned, to get soaking wet, because wet clothes are much better conductors, and dry ones poorer, than the human body. In extreme cases it might even be advisable to lie flat on the wet ground. In case of severe shock, resuscitation should be attempted through persistent artificial respiration and prevention from chill.

“The contour of the land is an important factor in determining the relative danger from lightning because the chance of a discharge between cloud and earth, the only kind that is dangerous, varies somewhat inversely as the distance between them. Hence thunderstorms are more dangerous in mountainous regions, at least in the higher portions, than over a level country. Clearly, too, for any given region the lower the cloud the greater the danger. Hence a high degree of humidity is favorable to a dangerous storm, partly because the clouds will form at a lower level and partly because the precipitation, and probably therefore the electricity generated, will be abundant. Hence, too, a winter thunderstorm, because of its generally lower clouds, is likely to be more dangerous than an equally heavy summer one.”

It is estimated that the total property loss due to lightning in the United States is about $8,000,000 a year, and the number of persons struck about 1,500, of whom one-third are killed. Nine-tenths of these accidents occur in rural localities.

Lightning rods neither prevent lightning stroke nor do they, as is sometimes alleged, attract lightning to buildings. They merely provide good conductors along which a stroke of lightning may reach the earth without doing damage, and, within very moderate limits, determine the path of discharge. While there are many unsettled points regarding the theory of lightning rods and details of construction, their general utility is strikingly indicated by statistics showing the comparative amount of damage done by lightning to rodded and unrodded buildings. According to the United States Bureau of Standards, information gathered in this country shows that “taking rods as they come in the general run of installations, they reduce the fire hazard from lightning by 80 to 90 per cent in the case of houses, and by as much as 99 per cent in the case of barns.” The same bureau, in its valuable publication, “Protection of Life and Property Against Lightning” (Washington, 1915), supplies the answers to a multitude of questions that are constantly asked about lightning rods.

Buildings with metal roofs and frames connected with the ground are generally well protected from lightning (except as to nonmetallic chimneys) without rods.

During actual thunderstorms, and also at other times when there are high potential gradients in the atmosphere, luminous electric discharges of a more or less continuous character are sometimes observed to occur in the shape of small jets and flames, chiefly from pointed objects, including lightning rods, the masts and spars of vessels, the angles of roofs, etc. These are identical in character with the “brush” discharges, or incomplete sparks, produced by electric machines. They are accompanied by a hissing or crackling sound. Their luminosity is comparatively feeble, and for this reason they are much more often observed by night than by day. They are especially common during snowstorms.

This phenomenon is known as St. Elmo’s fire or corposants (not to mention a score of other names, ancient and modern). As seen at sea, corposants sometimes take the form of one or two starlike objects at the trucks of the masts or the ends of yard arms, but occasionally the spars, rigging and other parts of the ship are lighted up with a great number of stationary or moving flames, producing a weird spectacle. The finest examples of corposants are, however, observed on high mountains, and the phenomenon has been carefully studied at certain mountain observatories, such as those on Ben Nevis and the Sonnblick. Of its occurrence on Ben Nevis, Angus Rankin writes: “The most frequent manner in which it makes its appearance is as caps of light on the tips of the lightning rod, but occasionally it appears as jets of flame projecting from all objects on the top of the tower and from the cowl of the kitchen chimney, which rises from the roof at some distance from the tower. These jets are at times from 4 to 6 inches in length, and make a peculiar hissing sound. During a very brilliant display, the observer’s hair, hat, pencil, etc., are aglow with the ‘fire,’ but, except for a slight tingling sensation in the head and hands, he suffers no inconvenience from it. On such occasions, if a stick be raised above the head, jets of electric light will be seen at its upper end. The only drawback to observing it with advantage is the unpleasant character of the weather in which it appears, namely blinding showers of snow and hail, and squally winds, causing a good deal of snowdrift.” Rankin records that it was sometimes heard in the daytime, when its light was invisible. On the Sonnblick a display of St. Elmo’s fire has been observed to last as long as eight hours.

No luminous electrical phenomenon is more beautiful or, at first sight, more mysterious than the aurora, popularly known, in the northern hemisphere, as the “northern lights.” This phenomenon is due to the passage of electrical discharges through the rarefied gases of the upper atmosphere, and it now appears to be settled beyond controversy that the discharges are caused by corpuscles or radiations of some kind emitted from the sun.

Most accounts of the aurora describe the typical appearances that it assumes as seen from a single place on the surface of the earth, but say little, if anything, about the form of the phenomenon as a whole or about its position in space. We shall follow a different plan here, and ask the reader, first of all, to imagine himself viewing the aurora from a point some thousands of miles away from our planet.

The solar emission above mentioned, when sufficiently intense, produces in the upper atmosphere a glow like that seen in a vacuum tube when an electrical discharge passes through it. From our vantage point in outer space we shall notice that this glow is not spread over the whole globe, but forms two rings, which encircle the polar regions of both hemispheres, though neither the geographic nor the magnetic poles lie at their centers. The rings do not extend down into the lower atmosphere, but hang about 60 miles above the earth’s surface.

The reason for this segregation of the aurora in high latitudes is that the earth is a great magnet, and magnets have the power of deflecting an electrical discharge in their vicinity. An appearance much resembling the two auroral zones of the earth was produced, on a small scale, by the late Professor Kr. Birkeland of Christiania, who magnetized a metal globe and allowed an electrical discharge to play upon it in a vacuum. The surface of the globe was coated with a phosphorescent substance, which glowed under the discharge in two rings, corresponding roughly to those of the aurora. In both cases the discharge follows what are called the magnetic “lines of force.” Our earth, like other magnets, is enveloped and penetrated by such lines. At any point on the earth the direction of the neighboring lines of force is shown by the dipping needle, which assumes a position parallel to them. At a magnetic pole the needle points straight up and down, and everywhere in high latitudes it has a position not very much inclined to the vertical, while in low latitudes it is more or less horizontal.

If, now, for the sake of simplicity we confine our attention to the northern hemisphere, and imagine ourselves maintaining our watch for months and years together, we shall discover that much of the time there is no ring to be seen; at other times there may be a small or partial ring; and occasionally there is a very broad, conspicuous ring, spreading so far south that it overlies the northern part of the United States and most of Europe. Evidently the emission from the sun that causes the auroral discharge varies greatly in strength, and this is in accordance with what we know about solar activities in general.

Next let us take a closer look at the ring, whether from outer space or from the earth’s surface. We shall find that it is made up, at least in part, of a multitude of luminous beams directed out into space and undergoing rapid changes in position and form. These beams, which really mark out the streams of the discharge in the upper air, follow the lines of force. In high latitudes they are nearly vertical with respect to the underlying surface of the earth. Even in the United States (when the aurora extends so far south) they are much more nearly vertical than horizontal. A dipping needle will show, at any place, just how they should stand.

From any distant point on the earth’s surface either north or south of it, the visible portion of the auroral ring presents the appearance of an arch across the horizon. Arctic explorers, far within the Arctic Circle, see this arch to the south of them. In our latitudes it spans the northern horizon. Separate beams or streamers may be distinguishable or not, according to the brightness of the discharge or its distance from the point of observation. Combinations of beams constitute so-called “draperies.”

Occasionally, at times of great solar activity, part of the ring actually overlies our Northern States, and the aurora then becomes a magnificent spectacle in this part of the world. The whole sky may be filled with the shifting streamers, along which travel rapid pulsations of light, so that the phenomenon then suggests strongly what it really is—a vast electrical discharge passing down through the atmosphere from outer space. When the observer is thus surrounded by the beams, they seem, on account of perspective, to converge toward a point south of the zenith, where they form a beautiful corona or crown. The position of this crown depends upon the slant of the beams, which, as already explained, follow the lines of force.

A brilliant aurora is always accompanied by disturbances of the magnetic needle, which moves about erratically, so that compasses can no longer be depended upon. At the same time there are strong “earth currents,” which interfere with the operation of telegraph lines.

Observations with the spectroscope seem to show that the light of the aurora is chiefly due to glowing nitrogen, though the most prominent line in the auroral spectrum has sometimes been referred to an unknown atmospheric gas. The various colors seen in bright auroras, including reds, greens, and yellows, are believed by some authorities to depend upon the varying speed of the electrical discharge. Experiments with vacuum tubes show that nitrogen, especially, gives great changes of color with changes in the velocity of the discharge. Another interesting revelation of the spectroscope is that there is apparently, a faint auroral illumination in the sky at all times and in all parts of the world, the so-called “permanent aurora.”

Photography has been used with great success in studying the aurora, especially by the Norwegian physicists StÖrmer, Vegard, and Krogness. Simultaneous photographs of a single detail are taken from two points several miles apart against a background of stars. The apparent position of the auroral detail among the stars will differ in the two pictures, and a comparison of them makes it possible to determine the actual position of the aurora in space. A slow-moving cinematograph has also been used to obtain series of pictures. The measurements of these observers show that the base of the aurora is, generally between 60 and 70 miles above the earth with a strong tendency to assume a definite location at an altitude of about 61 or 67 miles. Its upper limits are not well defined, but it has been photographed up to an altitude of more than 300 miles. Earlier observers reported seeing the aurora at altitudes of only a few miles, and even down to the earth’s surface, but recent authorities are inclined to discredit these observations.

One more phenomenon of atmospheric electricity requires brief mention, viz., the electric waves that produce the erratic disturbances known to wireless telegraph operators as “strays” or “static.” As heard in the receiver of a wireless outfit the noise of strays has been described as “like hailstones beating against a sheet of tin,” or “short hisses from a steam pipe,” or “periodic discharges of coal down a chute.” Another characteristic sound is a sharp “click.” The study of strays has been carried out on a world-wide scale by a committee of the British Association for the Advancement of Science, but their nature is not yet fully understood. Some strays are undoubtedly due to near or distant discharges of lightning, and special forms of wireless apparatus, known as “thunderstorm recorders” or “ceraunographs,” have been used to give notice of the approach of thunderstorms. On the other hand, strays seem frequently to have no connection with thunderstorms, and their principal origin is now sought in electrical disturbances in the upper atmosphere, perhaps similar to those which cause the aurora, and, as in the case of the aurora, having their ultimate source in the sun.