of all comets observed have come within the earth's orbit and no
known comet has its perihelion beyond the orbit of Jupiter. It is, of
course, possible that there may be a number of comets that never come
within the orbit of Jupiter, but it is very unlikely that any such
comet would ever be discovered. The majority of comets are simply
small, fuzzy points of light that are only visible telescopically and
the greater the perihelion distance of the comet the less likely is
it to be seen with the naked eye.
Since comets as well as planets obey Kepler's first law, known as
the law of areas, and sweep over equal areas in equal times, it is
evident that when a comet is at perihelion, or nearest to the sun, it
is moving at maximum speed and when it is at aphelion, or farthest
from the sun, it is moving at minimum speed. Moreover, its speed at
these two points in its orbit varies tremendously since the orbits of
comets are ellipses of very high eccentricity. The speed with which
the planets are traveling is, on the other hand, remarkably uniform
since their orbits are nearly circular.
The leisurely speed with which a comet travels through the frigid
outer regions of the solar system is gradually accelerated as the
comet draws nearer and nearer to the sun until it has acquired near
the time of perihelion passage a velocity that occasionally exceeds
two hundred miles a second. Here, also, the great increase in light
and heat and the strong magnetic field of the sun produce complex
changes in the gaseous and meteoric substances of which the comet is
composed until the characteristic tail and peculiar cometary features
are fully developed. As the comet again recedes from the sun after
perihelion passage its speed slackens once more. It soon parts with
its tail and other spectacular features and fades rapidly from view
even in the largest telescopes.
XXIII
METEORITES
Meteorites, bolides or fireballs, as they are variously called, are
stones that fall to the earth from the heavens. They furnish the one
tangible evidence that we possess, aside from that furnished by the
spectroscope, as to the composition of other bodies in space and
it is a significant fact that no unknown elements have ever been
found in meteorites, though the forms in which they appear are so
characteristic that they make these stones readily distinguishable
from stones of terrestrial origin.
The origin of meteorites is not definitely known, but the evidence is
very strong in favor of the theory that they are the larger fragments
of disintegrated comets of which meteors and shooting stars are the
smaller; the distinction between the two being simply that the latter
class includes all bodies that are completely consumed by friction
with the earth's atmosphere and, therefore, only reach the surface in
the form of meteoric dust.
According to other theories meteorites may be fragments of shattered
worlds that have chanced to come too near to a larger body and have
been disrupted, or they may possibly be the larger fragments of the
disintegrated comets of which the meteoric swarms are the smaller.
Interplanetary space is not altogether a void. Our own planet
sweeps up in the course of a single day, it has been estimated,
approximately twenty million shooting stars or meteors of sufficient
size to be visible to the naked eye, while the estimate for the
telescopic particles runs up to four hundred million.
Meteorites on the other hand are comparatively rare. On the average
it has been estimated about one hundred meteorites strike the earth
in the course of a year, of which number only two or three are
actually seen. According to _Bulletin 94, U. S. National Museum_,
approximately six hundred and fifty falls and finds of meteorites
have been reported, representatives of which appear in museums and
private collections.
Meteorites, as well as shooting stars and meteors, frequently appear
in showers. In such instances the fall usually consists of several
hundred or thousand individual stones and the area over which they
fall is several square miles in extent and roughly ellipsoidal
in shape. One of the most remarkable of such falls [...] hundred
thousand stones, varying in weight from fifteen pounds to a small
fraction of an ounce, fell near Pultusk, Poland. Another remarkable
fall of meteorites occurred at L'Aigle, France, in 1803. Between two
thousand and three thousand stones fell over an ellipsoidal area of
six and two-tenths miles in greatest diameter, the aggregate weight
of the stones being not less than seventy-five pounds.
This fall of stones is of particular interest since it took place at
a time when men were still very doubtful as to whether or not stones
actually fell to earth from the heavens.
After this fall had occurred in a most populous district of France
in broad daylight and attended by violent explosions that lasted for
five or six minutes and were heard for a distance of seventy-five
miles, no reasonable doubt could longer be held as to the actuality
of such phenomena.
Meteorites are without exception of an igneous nature, that is,
they are rocks that have solidified from a molten condition. They
can be classified into three groups, Aerolites or Stony Meteorites,
Siderolites or Stony-iron Meteorites, and Siderites or Iron
Meteorites.
More iron meteorites seem to have fallen in Mexico and Greenland than
in any other part of the world--at least of its land surface.
Yet strange to say, of all the meteorites that have been seen to
fall only nine belong to the group of Siderites or Iron Meteorites,
though the three largest meteorites known, Peary's meteorite from
Cape York, Greenland, weighing 37-1/2 tons, the meteorite lying on
the plain near Bacubirito, Mexico, weighing about 20 tons, and the
Willamette, Oregon, meteorite, weighing 15-1/2 tons all belong to
this group. Moreover, all the Canyon Diablo meteorites, which are
strewn concentrically around Coon Mountain crater in northern Arizona
to a distance of about five miles, are members of this same group.
Coon Mountain or Meteor crater itself is a perfectly round hole,
about six hundred feet deep and over four thousand feet in diameter
and was formed, it is believed, by the impact of a huge meteorite
which has never been found. It is believed that the Canyon Diablo
meteorites, of which there are nearly four hundred individuals in
the U. S. National Museum alone, were all members of this same fall.
It is possible that these meteorites of the Canyon Diablo district,
with the huge meteorite that produced the crater itself, formed the
nucleus of a comet that struck the earth not more than five thousand
years ago, according to the geological evidence.
All iron meteorites or siderites (from the Greek sideros, iron) are
composed chiefly of alloys of nickel and iron. The percentage of
nickel in these iron meteorites is very small, usually from five to
ten per cent., while the iron forms about ninety or ninety-five per
cent. of the whole. Cobalt is also present in practically all iron
meteorites in small quantities of 1 per cent. or less. Usually small
quantities of iron sulphide and phosphide as well as graphite or
some other form of carbon appear in the iron meteorites and in some
instances black and white diamonds have been found, as in some of the
Canyon Diablo irons.
A very interesting and beautiful feature of many iron meteorites
is the Widmanstätten figures which appear when a section of such a
stone is polished and treated by means of a weak acid. These figures
are due to the unequal solubility of the three different alloys of
nickel and iron of which the stones are composed. The irons giving
the Widmanstätten figures are known as octahedral irons. Other irons
known as hexahedral irons give figures of a different type known as
Neumann figures when the polished section is treated with weak acid,
while other irons are so homogeneous in their composition that they
show no figures at all.
Aerolites or Stony Meteorites occur more abundantly than iron or
stony-iron types, and they are classified into many divisions and
subdivisions according to their composition. In these stones appear
certain compounds that are commonly met with in terrestrial igneous
rocks. The mineral that is most abundant in the stony meteorites,
composing sometimes nearly seventy-five per cent. of the stone,
is a magnesium and iron silicate known as olivine, which is also
usually present in terrestrial rocks of an igneous nature. Certain
compounds found in the stony meteorites are rarely if ever found in
terrestrial rocks, however, and these serve to distinguish the stony
meteorites readily from stones of terrestrial origin. The alloys of
iron and nickel, for instance, that occur in minor quantities in the
stony meteorites and make up usually about ninety-five per cent.
of the mass of the iron meteorites, are never found in terrestrial
rocks. Although about thirty of the terrestrial elements are to be
found in meteorites, the forms and compounds in which they appear
are so characteristic and on the whole so different from those
occurring in terrestrial rocks, that the analyst has no difficulty
in distinguishing between the two. There are, for instance certain
formations known as chondrules, peculiar spherical and oval shapes,
varying in size from minute particles to objects the size of walnuts,
appearing in many varieties of stony meteorites that are never found
in terrestrial rocks, and that are one of the most puzzling features
associated with the origin and nature of these stones. Sometimes
the chondrules are so loosely embedded in the stone that they fall
away when it is broken. In some instances almost the entire stone is
made up of these chondrules. According to one theory the chondrules
were originally molten drops, like fiery rain, and their internal
structure, which is greatly varied, depends upon their conditions of
cooling.
Stony meteorites, in which these chondrules are to be found,
are spoken of as chondrites. There are white and gray and black
chondrites and crystalline and carbonaceous chondrites, according to
the nature of the chondrules found in the stones.
Stony meteorites also contain minute quantities of iron and nickel
alloys in the form of drops or stringers.
Upon entering the earth's atmosphere stony meteorites become coated
with a thin black crust which is a glass formed by the fusion of its
surface materials by the heat generated during its passage through
the atmosphere.
In many of the stony meteorites there also appear fine thread-like
veins which are due to the fracturing of the stone prior to its
entrance into the atmosphere. The material filling these veins is
coal black in color, opaque and of an unknown composition.
Many meteorites show signs of collisions and encounters with other
meteorites outside of the atmosphere as would be expected as they
travel in swarms and groups. Sometimes the entire meteorite is
composed of fragments of two or more distinct stones cemented
together. Such a stone is spoken of as a _breccia_.
In the third class of meteorites to which we now come, known as the
stony-iron meteorites, there is a network or sponge of nickel-iron
alloy, the interstices of which are filled with stony material.
When this network or sponge is continuous the meteorite is spoken of
as a stony-iron pallasite. When the network of metal is more or less
disconnected the meteorite is a meso-siderite.
If meteorites are heated in a vacuum, the conditions existing in
interplanetary space being thus produced to a certain extent, they
give forth their occluded gases and it has been found that these
gases give spectra identical with the spectra of certain comets.
Meteoric irons give forth hydrogen as their characteristic gas while
the gases occluded in the stony meteorites are chiefly the oxides
of carbon, carbon monoxide and carbon dioxide. It has been found
that the amount of gases contained in a large meteorite or shower of
meteorites is sufficient to form the tail of a comet. These facts all
tend to strengthen the belief that meteorites are indeed cometary
fragments.
In view of the fact that some geologists believe meteorites may be
fragments of other worlds, it is of interest to know that so far no
fossil-bearing meteorites have been found, and if meteorites are
fragments of a shattered world, such worlds must have been reduced to
a molten condition at the time of the catastrophe.
The rapid passage of the meteorite through the air leaves a partial
vacuum in its trail into which rush the molecules of air from all
sides, producing the characteristic noises that accompany the passage
of a meteorite, which have been variously compared to the rattle of
artillery, the distant booming of cannons or the rumble of thunder.
There may be, also, explosions of inflammable gases occluded in the
crevices of the meteorite which will shatter it into fragments or
the meteorite may be shattered by the resistance and pressure of the
atmosphere or as a result of the extremes of temperature existing
between the interior and its surface. Many meteorites have actually
been seen to burst into fragments in the air with a loud report.
There is practically no foundation for the belief that germs of life
have been brought to our planet on such igneous rocks. No microscopic
examinations of meteorites have yielded any results that could be
interpreted in favor of such a view.
Falls of meteorites are accompanied in nearly every instance by
terrific explosions and sharp reports that can be heard for many
miles around, often causing the ground to shake as in an earthquake.
The meteorite itself has been described as resembling a ball of
fire or the headlight of a locomotive, and is followed frequently
by a trail of light or a cloud of smoke. At the time it enters our
atmosphere a meteorite is moving with planetary velocity ranging from
two to forty-five miles per second. Its interior is intensely cold,
approaching in temperature the absolute zero of interplanetary space,
and it is, therefore, far more brittle than it would be at ordinary
temperatures. As it ploughs its way into the earth's atmosphere its
surface temperature is soon raised by friction to at least 3,000° or
4,000° C., which is sufficient to fuse all surface materials into the
characteristic black crust, with which stony meteorites are coated.
Meteorites are usually first seen at an altitude of fifty or sixty
miles. Although they are moving with a velocity comparable to that of
the planets, when they enter the earth's atmosphere, this velocity is
so rapidly reduced by friction with the atmosphere that they usually
drop to the surface of the earth with a velocity about equal to that
of ordinary falling objects.
The flight of a meteorite often extends over a path several hundred
miles in length and the meteorite may be seen by many observers in
several different States and yet finally fall in some unknown spot
and never be found.
The evidence gathered regarding the actual fall of meteorites is
often contradictory. Some stones are too hot to handle for hours
after they fall, others are merely warm, while still others have been
picked up cool or even intensely cold. Meteorites have been seen to
fall upon dried grass and upon straw without producing even charring
effects. The evidence regarding the depths to which meteorites
penetrate the ground is quite as conflicting. The largest of all
the stony meteorites which fell at Krnyahinya, Hungary, weighed 647
pounds and buried itself to a depth of eleven feet. Yet Peary's Cape
York iron meteorite, weighing 37-1/2 tons, was only partially covered
and showed no signs of abrasions of surface resulting from the fall.
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.