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.
The familiar twinkling or scintillation of stars and, more rarely,
of the planets, is a result of interference of light waves due to
irregular and variable refraction in air that is not uniform in
density, owing to the presence of constantly rising and descending
atmospheric currents of different densities. This also produces
the shimmering or unsteadiness of star images in the telescope,
that interferes so greatly with accurate measurements of angles or
observations of planetary markings.
One may ask why it is, if light from an object, say a star, is bent
from its course and separated into rays of various colors upon
entering the earth's atmosphere, that we do not see the object
drawn out into a band of spectral colors. It is because the angular
separation of the various colors is so slight under ordinary
circumstances that light from one point is blended with light from
a neighboring point of complementary color to produce white light
again. Under certain circumstances, however, beautiful color effects
may be seen in the earth's atmosphere as a result of the refraction
of sunlight.
The blue color of the sky and its brightness is caused by the
scattering of the rays of shortest wave-length, the violet and
blue rays, by the oxygen and nitrogen in the upper atmosphere. The
molecules of these gases interfere with the passage of these rays,
powerfully scattering and dispersing them, and thus increasing the
length of their path through the air and diffusing their color and
brightness in the upper atmosphere, while permitting rays of longer
wave-length, the red and orange, to pass on practically undisturbed.
When an object in the heavens lies close to the horizon, the rays of
light from it have to travel a longer path through the atmosphere
than when the object is overhead, and that too through the densest
part of the atmosphere, which lies close to the earth's surface,
and in which are floating many dust particles and impurities from
the earth's surface. All these particles, as well as the increased
density of the atmosphere, interfere with the free passage of the
rays, especially of shorter wave-lengths. The violet and blue rays
are sifted out and scattered in their long journey through the lower
strata of air, far more than when they come to us from an object
high in the sky. Even the red and yellow rays are more or less
scattered and bent aside--diffracted--by these comparatively large
particles near the surface. The reddish color of the sun, moon and
even of the stars and planets, when seen near the horizon, as well
as the beautiful sunset tints, in which reds and pinks and yellows
predominate, are due to the fact that the rays of longer wave-length
are more successful in penetrating the dense, dust-laden layers of
the lower atmosphere. It is to be free of the dust and impurities as
well as the unsteadiness of the lower atmosphere, that observatories
are built at high altitudes whenever possible.
When there have been unusually violent volcanic eruptions, and great
quantities of finely divided dust have been thrown into the upper
atmosphere, the effect upon the blue and violet rays from the sun is
very great. The volcanic dust particles are so large that instead of
scattering these rays of shorter wave-length, as do the oxygen and
nitrogen in our atmosphere, they reflect them back into space and so
decrease the amount of light and heat received from the sun. For this
reason the general temperature of the earth is lowered by violent
volcanic eruptions. Unusually cold periods, that lasted for months,
followed the terrible eruption of Krakatoa in 1883 and of Katmai in
1912.
At times when much dust is present in the atmosphere, the sky
is a milky white color by day as a result of the reflection of
sunlight from the dust particles. Sunrise and sunset colors are then
particularly gorgeous, with reds predominating. At such times the
blue and violet rays are almost completely shut out, and the red,
orange and yellow rays are powerfully diffracted and scattered by the
dust particles in the air.
The twilight glow that is visible for some time before sunrise or
after sunset is, of course, entirely an atmospheric effect caused by
the reflection of sunlight to our eyes from the upper atmosphere,
upon which the sun shines, while it is, itself, concealed from
our view below the horizon. The atmosphere extends in quantities
sufficient to produce twilight to an elevation of about sixty miles.
When _all_ the rays of which sunlight is composed are reflected in
equal proportions we get the impression of white light. Dust and haze
in the air reflect all rays strongly and give a whitish color to an
otherwise blue sky. Brilliant white clouds appear white, because they
are reflecting all rays equally. Clouds or portions of clouds appear
black when they are in shade or, at times, by contrast with portions
that are more strongly illuminated, or when they are moisture-laden
and near the point of saturation, when they are absorbing more light
than they reflect. At sunrise and sunset, when the light that falls
upon the clouds is richest in red and orange and yellow, clouds
reflect these colors to our eyes, and we see the brilliant sunset
hues which are more intense the more the air is filled with dust and
impurities.
The familiar and beautiful phenomenon of the rainbow is produced
by refraction, reflection and interference of sunlight by drops of
falling water, such as rain or spray. As the ray of sunlight enters
the drop of water, which acts as a tiny glass prism, it is refracted
or bent from its course and spread out into its spectral colors.
Reflection of these rays next takes place (once or twice, as the
case may be) from the inside of the drop and a second refraction of
the reflected ray takes place as it leaves the drop. The smaller the
drops the more brilliant is the rainbow and the richer in color.
The most brilliant rainbows are produced by drops between 0.2 and
0.4 millimeters in diameter. In addition to the primary bow, which
has a red outer border with a radius of 42°, there is the secondary
bow with a radius of about 51° and with colors reversed, the red
being on the inner border; the supernumerary bows which are narrow
bands of red, or green and red, appear parallel to the primary and
secondary bows along the inner side of the primary bow and the outer
side of the secondary bow. No rainbow arches ever appear between the
primary and secondary bows, and it can be shown in fact, that the
illumination between these two bows is at a minimum.
The primary, secondary and supernumerary bows all lie opposite the
sun in the direction of the observer's shadow and the observer must
stand with his back to the sun in order to see them. The primary and
secondary rainbow arches take the form of arcs of circles that have
their common center on the line connecting the sun with the observer
at a point as far below the horizon in angular distance as the sun is
above the horizon. It is, therefore, never possible to see a rainbow
arch of more than a semicircle in extent unless the observer is at an
elevation above the surrounding country, under which circumstances
it might be possible to see a complete circle formed by the rainbow.
The highest and longest arch appears when the sun is on the horizon,
and the greater the altitude of the sun the smaller and lower the
visible arch. As the angular radius of the primary bow is 42° and of
the secondary bow 51° and as the common center of the two circles
is always as far below the horizon as the sun is above, it is never
possible to see either primary or secondary rainbow when the altitude
of the sun is over 51°, or the primary bow when the altitude is over
42°. For this reason rainbows are rarely seen at or near noon in
mid-latitudes, since the sun is usually at an elevation of more than
42° at noon, especially in the summer season, which is also the most
favorable season for rainbows, owing to the great likelihood of rain
and sunshine occurring at the same time.
The light which comes to an observer from the primary bow is once
reflected within the drop, and that which comes from the secondary
bow is twice reflected within the drop. The sharper and brighter
light therefore comes from the primary bow of 42° radius. The space
between the two bows is particularly dark, because it can be shown
that the drops there do not reflect any light at all.
The rainbow colors are rarely pure or arranged in spectral order,
owing to interference of light waves. It is the interference of light
waves from different parts of the same drop that produces the bands
of alternate maximum and minimum brightness, that lie below the
primary bow and beyond the secondary bow. The red or green and red
bands of maximum brightness produced thus by interference, are called
the supernumerary bows, and they are always found parallel to the
primary and secondary bow within the former and above the latter.
The distance of the rainbow from the observer is the distance of
the drops that form it. A rainbow may be formed by clouds several
miles distant or by the aid of the garden hose on our lawn. No two
observers can see exactly the same rainbow because the rainbow arch
encircles the surface of a cone whose vertex is at the observer's
eye and no two such vertices can exactly coincide. Two observers see
rainbows formed by different drops.