Part 11

The Willamette iron meteorite, weighing 16-1/2 tons, lay in a forest when found and was not deeply buried. The Bacubirito iron meteorite, weighing 20 tons, lay in soft soil, barely beneath the level of the surface. On the other hand a fragment of a stony-iron meteorite, weighing 437 pounds, that fell at Estherville, Iowa, buried itself eight feet in stiff clay.

Geologists in charge of the meteoric collections of various museums quite frequently have stones sent to them for analysis that are reputed to be of celestial origin. More often than not such stones are found to be purely terrestrial in their origin. The composition of a meteorite is so characteristic and unique that such a stone can never be mistaken. Finds of bona-fide meteorites are on the whole extremely rare.

It is also a peculiar fact that meteorites are usually observed in the months when ordinary meteors or periodic swarms of meteors are least prevalent, that is in the months of May, June and July.

XXIV

THE EARTH AS A MAGNET

If a small, freely suspended compass needle is moved over a highly magnetized steel sphere, it will be seen that it constantly changes its position both horizontally and vertically so as to lie always along the "lines of force" of the sphere.

There will be one point on the sphere which we will call the _North Magnetic Pole_, where the _north-seeking_ end of the needle will point vertically downward or make a "dip" of 90° with the tangent plane. At the diametrically opposite point on the sphere, called the South Magnetic Pole, the opposite end of the compass, the south-seeking end, will point vertically downward; while at a point midway between the magnetic poles of the sphere the needle will lie parallel to the diameter connecting the two poles and there will be no dip.

The total intensity of the magnetic field surrounding the sphere will be found to be greatest in the vicinity of the magnetic poles and least, midway between the poles.

Now, a freely suspended compass needle carried to all parts of the earth will behave very much in the same manner as the needle moved over the magnetized steel sphere. There are two points on the earth's surface, known as the North and South Magnetic Poles, where the needle points vertically downward and approximately midway between is the _Magnetic Equator_ where the compass needle places itself in a perfectly horizontal position and the "dip" of the needle is zero. In other words, the earth acts as a huge magnet and possesses a magnetic field with lines of force converging towards its poles similar to the lines of force of the steel sphere.

There are, however, some very important differences between the sphere of steel and our earth. The matter of which the earth is composed is not homogeneous. It is believed to possess an iron core of considerable size, it is true, but its outer shell is composed of heterogeneous masses that in certain regions cause very appreciable local deflections of the needle. It is surrounded, moreover, by an atmosphere permeated by electrified particles of matter shot forth from the sun, which we now know is a still greater magnet surrounded by a magnetic field that is of the order of 50 gausses at the poles and about eighty times more powerful than that of the earth.

It is now a well-established fact that the sun's magnetic field exerts a powerful influence over the condition of the earth's magnetic field, and that vast solar disturbances affect very materially the direction and intensity of the lines of force.

It is thus little wonder that this non-homogeneous and rapidly rotating terrestrial globe, surrounded by an electrified atmosphere and subject to the action of a still more powerful magnet, the sun, should not behave in a manner exactly analogous to a uniformly magnetized steel sphere.

The earth's magnetic poles are neither symmetrically placed nor absolutely fixed in position. There is every reason to suspect that they shift about from year to year, and possibly fluctuate irregularly in position in the course of a few days or hours under the influence of disturbing forces. The position of the earth's North Magnetic Pole, last visited by Amundsen in 1903, now lies approximately in Latitude 70° N. Longitude 97° W. The position of the South Magnetic Pole, according to the latest determinations, is, in round numbers, in Latitude 73° S. and Longitude 156° E. of Greenwich. It is evident, therefore, that the magnetic poles of the earth are not symmetrically placed and that they lie fully 30° from the _geographical_ poles. The chord connecting the magnetic poles passes 750 miles from the earth's center, and it is about 1,200 miles from the geographic pole to the nearest magnetic pole. There exist, moreover, in high latitudes local magnetic poles, due possibly to heavy local deposits of ore. One such pole was discovered at Cape Treadwell, near Juneau, Alaska, during Dr. L. A. Bauer's observations there in 1900 and 1907. In the center of the observing tent at this point the needle pointed vertically downward and the compass _reversed_ its direction when carried from one side of the tent to the other.

It is a well-known fact that there are very few points on the earth's surface where the compass needle points either to the true geographical pole or to the magnetic pole, and if it does chance to do so, it is a transient happening. The "variation of the compass" or the declination of the needle, as it is called, is the angle that the compass needle makes with the true north and south line or the meridian. It is an angle of greatest importance to navigators and explorers, for it gives them their bearings, yet it is unfortunately subject to ceaseless variations of a most complicated nature, since it depends on the constantly pulsating and never ceasing magnetic changes that sweep over the surface of the earth and through its crust. It is affected by long period or secular changes, as they are called, progressing more or less regularly in obscure cycles of unknown period. It is subject to a diurnal change that depends on the position of the sun relative to the meridian, and that varies with the seasons and with the hour of the day. It is affected by the sun spot cycle of 11.3 years which has a direct effect upon the intensity of the earth's magnetic field. The intensity of the magnetic field in sun spots is, according to Abbot, sometimes as high as 4,500 gausses or 9,000 times the intensity of the earth's field. At times of maximum spottedness of the sun the intensity of the earth's magnetic field is reduced.

Moreover, when great and rapidly changing spots appear upon the sun, electrified particles are shot forth from the sun with great velocity and in great numbers, and are drawn in towards the magnetic poles of the earth. Meeting the rarefied gases of the earth's upper atmosphere, they illuminate them as electric discharges illuminate a vacuum tube. Some of these electrons are absorbed by gases at high elevations, other descend to lower levels. The most penetrating rays have been known to descend to an altitude of twenty-five miles which is about the lowest limit yet found for auroral displays. It is the passage of these rays through the atmosphere that cause the magnetic disturbances known as _magnetic storms_, that are associated with the appearance of great sun spots and auroral displays. At such times sudden changes take place in the intensity of the earth's magnetic field that cause the compass needle to shiver and tremble and temporarily lose its directive value. These _magnetic storms_ have been known to produce great temporal changes in the intensity of the earth's field. According to Dr. L. A. Bauer, Director of the Department of Terrestrial Magnetism of the Carnegie Institute of Washington, the earth's intensity of magnetization was altered by about one-twentieth or one-thirtieth part by the magnetic storm of September 25, 1909, which was one of the most remarkable on record, and the earth's magnetic condition was below par for fully three months afterwards as a result.

In addition to these various regular and irregular changes in the variation of the compass, or declination of the needle, due to changes in the earth's magnetic field _as a whole_, there are local effects due to restricted regional disturbances of the earth's magnetic field or to local deposits of ore, or to volcanoes or other local causes. The effect of all these disturbances upon the declination of the needle must be determined by continual magnetic surveys of all portions of the earth's surface.

As a whole the earth's magnetic field is more uniform over the oceans than over the land, with all its disturbing topographical features. Yet this advantage is offset largely in navigation by the fact that every steel ship that sails the seas is a _magnet_, with its two magnetic poles and its neutral line where the two opposite magnetic forces are neutralized, as is the case with every magnet. The direction in which a steel ship lies with reference to the earth's magnetic field while it is being built determines the position of the magnetic poles in its hull and the position of its neutral line and this distribution of magnetism over a ship's hull must be taken account of in the installation of its standard compass. Every piece of horizontal and vertical iron aboard ship has an effect upon the variation of the compass and compensation must be made for such disturbing forces. The direction of sailing, the position in which a ship lies at dock, storms encountered at sea, the firing of batteries (on warships) are some of the factors that are operative in producing changes in the variation of the magnetic compass aboard a ship.

Every ship must undergo at frequent intervals magnetic surveys for the purpose of determining its magnetic constants and its "Table of Deviations of the Compass."

The direction in which the compass needle points aboard ship is the _resultant_ of the effect of the earth's magnetic field and the magnetic field of the ship, and both fields are subject to continual and complicated variations from year to year, from day to day, _and even from hour to hour_!

The elements of the earth's magnetic field are determined for any one epoch by long-continued magnetic surveys carried on to a greater or less extent by the various nations of the world, and the results are published in the form of _magnetic charts_ for land and sea, showing the values of the three magnetic elements, declination of the needle, dip or inclination, and horizontal intensity of the earth's field for a definite period. So rapid are even the long-period changes in the earth's magnetic field that a magnetic chart can be relied upon for only a very few years and fresh data for the construction of these charts that are so valuable to navigators and explorers must be gathered continually.

The _Department of Terrestrial Magnetism_ of the Carnegie Institute of Washington is engaged in continual magnetic surveys of the earth by land and sea that are of the highest value not only to navigators but also to scientists interested in solving the great and mysterious problem of the underlying causes of the earth's magnetism.

To give an idea of the extent and scope of the work of this department it may be mentioned that its non-magnetic ship _Carnegie_ made in the period 1909-1918 a total run of 189,176 nautical miles, nearly nine times the earth's circumference, with an average day's run of 119 nautical miles. Magnetic observations were made practically every day at a distance of 100 to 150 miles apart. In this nine-year period five cruises were made. On her first cruise the _Carnegie_ sailed from St. John's, Newfoundland, to Falmouth, England, over practically the same course followed by the famous astronomer, Halley, in the _Paramour Pink_ two centuries earlier to determine the variation of the compass. In her fourth voyage the _Carnegie_ circumnavigated the world in sub-antarctic regions in 118 days--a record time. She has traversed all oceans from 80° North to the parallel of 60° South and has crossed and recrossed her own path and the path of her predecessor, the _Galilee_, many times, thus making it possible to determine for the points of intersection the secular changes in the magnetic elements.

After a thorough overhauling in 1919 and the installation of a four-cylinder gasoline engine, _made of bronze_ throughout, to take the place of the producer-gas engine used on earlier cruises, the _Carnegie_ started on her sixth cruise with a crew of twenty-three officers and men on October 9, 1919. A cruise of 61,500 miles was planned in the South Atlantic, Indian and Pacific Oceans to last approximately two years. Unsurveyed regions in the South Atlantic and Indian Ocean were to be covered and the route was planned so as to obtain a large number of observations of the progressive changes that have taken place in the magnetic elements. This is accomplished as stated above by intersecting former routes and obtaining new values of the element at the points of intersection.

In addition to its ocean magnetic surveys the _Department of Terrestrial Magnetism_ also carries on extensive land surveys in all parts of the globe. In 1919 special expeditions were sent out by the Department to observe the total solar eclipse of May 29th at stations distributed over the entire zone of visibility of the eclipse and immediately outside. At Dr. Bauer's station in Liberia the total phase was visible in a cloudless sky for more than six minutes, which is very close to the maximum length of phase that can possibly be observed. Unmistakable evidence was gathered at all stations of an appreciable variation in the earth's magnetic field during a solar eclipse, which variation is the reverse of that causing the daylight portion of the solar diurnal variation of the needle.

In addition to the magnetic survey work on land and sea which is the chief work of the _Department of Terrestrial Magnetism_, atmospheric-electric observations are carried on continually on land and sea and experiments have been carried on at Langley Field, Va., lately, in the development of methods and instruments for determining the geographical position of airplanes by astronomical observations. There has also been recently formed under this department a _Section of Terrestrial Electricity_.

The cause of the earth's magnetic field is still one of the greatest unsolved problems of astro-physics. The theory that has been advanced by Schuster that all large rotating masses are magnets _as a result of their rotation_ has received considerable attention from astrophysicists, and attempts have been made to prove this experimentally. It has been found that iron globes spun at high velocities in the laboratory do _not_ exhibit magnetic properties. This may mean simply that the magnetic field is too weak to be detected in the case of a comparatively small iron sphere spun for a limited period under laboratory conditions. It must be remembered that the earth has been rotating rapidly on its axis for millions of years and is, compared to terrestrial objects, an extremely large mass. Yet it has been shown that as a whole our earth is an extremely weak magnet, and that if it were made entirely of steel and magnetized as highly as an ordinary steel-bar magnet, the magnetic forces at its surface would be a thousand times greater than they actually are.

If it is true that all rotating bodies are magnets, then all the heavenly bodies, planets, suns and nebulæ are surrounded by magnetic fields. We know nothing to the contrary. In fact, we know this to be true for the earth and sun, and strongly suspect that it is so in the case of the planets Jupiter and Saturn.

When we understand more about the properties of matter, the nature of magnetism, as well as of gravity, may be revealed to us.

XXV

SOME EFFECTS OF THE EARTH'S ATMOSPHERE UPON SUNLIGHT

It is impossible to exaggerate the importance of the atmosphere to all forms of life upon the surface of the earth. If there were no atmosphere there would be no life, because it is through the agency of the water-vapor, carbon-dioxide and oxygen in the atmosphere that all life-processes are maintained.

If there were no atmosphere there would not only be no life upon the earth; there would be also none of the beautiful color effects produced by the passage of sunlight through the atmosphere. There would be no blue skies, no beautiful sunrise and sunset effects, no twilight, no rainbows, no halos, no auroral displays, no clouds, no rains, no rivers nor seas, no winds nor storms. The heavens would be perfectly black except in the direction of the heavenly bodies which would shine as brilliantly by day as by night.

To understand how the atmosphere produces color effects such as blue skies, sunrise and sunset tints, rainbows and halos, as well as the twinkling of the stars, and numerous other phenomena, we must know something of the nature of light itself.

Light moves outward from any source, such as the sun, in all directions radially, or along straight lines (so long as it does not encounter a gravitational field) with the unimaginable velocity of 186,000 miles per second. As it advances it vibrates or oscillates back and forth across its path in all directions at right angles to this path, unless it is plane polarized light, in which case its vibrations are confined to one plane only.

These vibrations or oscillations of light take the form of a wavelike motion, one wave-length being the distance passed over in the time of one vibration, measured from crest to crest or from trough to trough of adjacent waves.

We may consider that a beam or ray of sunlight is made up of a great number of individual rays of different wave-lengths and different colors. The average wave-length of light, the wave-length of the green ray in sunlight, is about one-fifty-thousandth part of an inch, that is, it would take about fifty-thousand wave-lengths of green light to cover a space of one inch. Now, since light makes one vibration in passing over a distance of one wave-length, it makes fifty thousand vibrations, while advancing one inch, and since it advances one hundred and eighty-six thousand miles in one second we can easily figure out that a ray of sunlight of average wave-length makes about six hundred trillion vibrations (600,000,000,000,000) in a single second!

The chief colors of which sunlight or white light is composed are red, orange, yellow, green, blue, indigo and violet, though there are an infinite number of gradations of color which blend into one another, gradually producing the intermediate tints and shades. The colors just mentioned are called the primary colors of the solar spectrum, which can be produced as a band of light of variegated colors, arranged in the order named by passing a ray of ordinary sunlight through a glass prism. The individual rays of different color and wave-length that make up a beam of sunlight, or white light, then separate out in the order of the wave-lengths. The red rays vibrate the most slowly and have the longest wave-length of all the rays of the visible spectrum. About four hundred trillion vibrations of red light reach the eye in one second. Violet rays, on the other hand, vibrate the most rapidly of all the visible rays and have the shortest wave-length. About eight hundred trillion vibrations of violet light reach the eye every second. The wave-lengths of the intermediate colors decrease in length progressively from the red to the violet and, of course, the frequencies of their vibrations increase in the same order. All sunlight is made up of these rays of different colors and different vibration frequencies, and of other rays as well, to which the human eye is not sensitive, and which, therefore, do not appear in the visible spectrum. Among these invisible rays are the infra-red rays which come just below the red of the visible spectrum, and which are of longer wave-length than the red rays, and the ultra-violet rays, which lie beyond the violet rays of the visible spectrum, and are of shorter wave-length than the violet rays.

Now a ray of ordinary sunlight is separated into the rays of various colors, which form the solar spectrum when it passes from a medium of one density obliquely to a medium of another density, as when it passes from air to glass, or from air to water, or from outer space into the earth's atmosphere. Under such circumstances its velocity is slowed down when it passes from a rare to a denser medium, and the waves of different wave-lengths are bent from their former course, or refracted, by different amounts. The red rays, of longest wave-length, are bent from their former course the least, and the violet rays, of shortest wave-length, are bent the most upon passing from a rare to a denser medium. As a result the ray of sunlight is spread out or dispersed into its rays of different wave-length and color upon entering a medium of different density. It is this refraction and dispersion of sunlight that produces many color effects in the earth's atmosphere.

The atmosphere is not of uniform density throughout. At high altitudes it is extremely rare. That is, there is little of it in a given volume. Close to the earth's surface, however, it is comparatively dense. Half of all the atmosphere is within three and one-half miles of the surface and half of the remainder lies within the next three and a half miles. We may consider it as made up, on the whole, of layers of different densities, strongly compressed near the surface.

Imagine a ray of sunlight entering the earth's atmosphere from without. If it comes from a point in the zenith its course is not changed upon entering the atmosphere, because light passing from a certain medium--as space--into a medium of different density, is not bent from its course, or refracted, provided it enters the new medium in a direction perpendicular to the surface. If it enters the atmosphere (which is the new medium of greater density) _obliquely_, refraction, or bending of the ray, takes place, and as the ray advances toward the earth, through layers of increasing densities, it is bent from its former course more and more. As the advancing rays of different colors and wave-lengths in the beam of sunlight are slowed down in the new medium, the red rays are turned from their course the least and the violet rays the most and the entire advancing wave-front of the beam of sunlight is bent down more and more toward the horizon, as it proceeds through the atmosphere. As we on the earth's surface see the ray not along its bent course through the atmosphere, but in the direction in which it finally enters our eyes, the effect of refraction upon a ray of light passing through the atmosphere is to displace the object in the direction of the zenith or increase its distance above the horizon. As a result of refraction we see the sun--or moon--above the western horizon after it has really set, and above the eastern horizon before it has really risen. The oval shape that the sun, or moon, often presents on rising or setting, is due to the fact that the light from the lower limb is passing through denser air than the light from the upper limb, and so is refracted more. As a result the lower limb is lifted proportionately more than the upper limb. This distorts the form of the solar or lunar disk, making it appear oval instead of circular.