SOME DISCOVERIES CONCERNING OUR PLANET
When was the Earth created?—How we can tell the age of the Earth—How the Oceans became
salt—The birth of the Moon—The inside of the Earth—Eruption of Krakatoa—What
we learn from earthquakes—The thickness of the Earth's crust—The evidence of meteorites
 WE know that our planet has not existed for all time, but had a definite beginning; it was created. Is
it possible to discover when it was created? Our forefathers thought they could reckon back to the
time of the creation, and their estimate was that it took place in the year 4004 B.C. Unfortunately,
they had such confidence in that estimate that they placed that particular date opposite the first
chapter of Genesis in our printed Bibles. It is even more unfortunate that the printers continue to
put this date down as the actual date of the creation, for we have discovered that the age of the
Earth must be reckoned in millions of years. It would be only reasonable to withdraw the erroneous
date from our Bibles, as it was put there merely on the authority of a learned archbishop who was
living at the time the Bible was translated into English. From actual discoveries of the buried
ruins of ancient cities we can trace the history of civilised nations as far back and beyond the
Before considering what we have discovered concerning
 the age of the Earth, it will be of some assistance to picture its early youth. There is no doubt
that at one time it was a great mass of incandescent gas. It was not "burning," because there was no
air surrounding it at that time. However, it was natural that this great fiery mass should cool
down. As it cooled it became smaller and smaller; it passed into the state of a molten liquid and
then a white-hot solid. By this time an envelope of water vapour had formed around the solid globe.
The pressure of this envelope of steam would exert an immense pressure on the surface of the hot
planet, and there is little doubt this pressure played an important part in distorting the surface
of the Earth, depressing it at some places and raising it at others. When things had so far cooled
down that the water vapour could condense upon the Earth, there were formed boiling and steaming
oceans of water lying in the hollows.
The foregoing short description will enable us to appreciate two of the calculations made concerning
the age of the Earth. Knowing that the Earth had cooled from a highly heated body to its present
condition, the late Lord Kelvin set himself the task of calculating how long it had taken the planet
to cool down. The final result was that at least twenty million years had elapsed since the Earth
was a molten globe. Since the discovery of Radium in the Earth's crust it is contended that these
twenty million years must be very substantially increased, and that now the Earth is increasing in
temperature because of these radioactive substances. However, Lord Kelvin's calculation leaves no
doubt that this planet upon which we reside has been in existence for many millions of years.
Professor Joly, of Dublin, has put forward a very
 interesting theory, but one which some geologists do not accept. He maintains that the sea has
become salt very gradually by the rivers washing down sodium compounds from the rocks. He has
calculated how long it would take the rivers to bring down the necessary amount of sodium compounds
to account for the present saltness of the sea. He has calculated that all the rivers of the Earth
could bring down about one hundred and sixty million tons of sodium in each year. He finds that the
total amount of sodium contained in the oceans of the Earth is at least ninety million times more
than these figures, so that the age of the Earth must be at least ninety million years. Some of the
geologists claim one thousand million years as necessary for the formation of the Earth's crust.
Yet another method of calculation has been used in trying to fix the age of our planet. We have no
doubt that our faithful satellite the Moon was at one time a part of this great planet. The Earth
must have been in a very plastic condition when it threw off that portion of itself, which drifted
away and became the Moon. The late Sir George Darwin calculated how long ago the birth of the Moon
must have occurred, and the figures come out about fifty-six million years. Again we are among the
tens of millions, and all those calculations go back only to the time when this world was a molten
mass of, say, five thousand degrees temperature. But when the Earth was a mass of incandescent gas,
its temperature was that of the stars, the hottest of which is about thirty thousand degrees. There
must have been an enormous lapse of time to allow of this great drop of temperature. But if we
consider only the time required to form the crust of the Earth, even then
 we have to count the age of our planet in many millions of years. This will become apparent when we
consider the building up of the Earth's crust in a succeeding chapter.
Some people have held very queer ideas about the inside of the Earth. One of the strangest ideas has
been that the Earth was hollow, and that the inside was the habitation of living beings. The idea
was not that these inhabitants were fiery demons, such as are represented in a pantomime by some
Mephistopheles being shot up from the lower regions. The recorded descriptions tell of a beautifully
mild climate within the Earth, which was reached by an opening at the North and at the South poles
of the Earth. Needless to say that the Arctic and Antarctic explorers did not search for any such
entrances. It is quite apparent that the writers of these absurd notions were not scientists, and
yet we have such writings put down in all seriousness within the last hundred years.
If one desires to know what is inside anything, the best plan is to make a direct investigation and
examine the contents. In the case of the Earth this is impossible; we cannot sink a shaft to any
great depth. When one is going down a deep coal-mine for the first time it seems quite a long
journey, but the deepest mine ever made goes down only about one mile into the Earth. To reach down
to the centre of the planet would mean a journey four thousand times as great. But it would be quite
impossible to continue the journey to any great depth. Even if we were able to overcome the
difficulties of the increasing temperature and the increasing pressure of air, it would take more
than a lifetime to dig a depth of ten miles. Suppose for a moment that it were possible to continue
at the same rate of progress, it would take thirty thousand years
 to reach the centre. But the whole idea is quite ridiculous, so we may give up all hope of ever
discovering the contents of the inner Earth by direct investigation. Our discoveries must be made by
the study of phenomena occurring in the Earth. We can only question and cross-examine these
phenomena as we should do witnesses.
Our conceptions of the inside of the Earth have changed greatly within quite recent years. Some
thirty years ago there occurred a tremendous volcanic eruption in the island of Krakatoa. Although
this volcanic island is situated far away in the East, the eruption threw such immense volumes of
dust particles into the air that for the succeeding three years our sunsets in Great Britain were
more than usually red. A great wave also swept through the oceans of the planet. In a public lecture
dealing with this great eruption, and delivered some years after the occurrence, an eminent
astronomer described the relation of the solid crust of the Earth to be to the molten interior no
more than the solid shell is to the liquid of an egg. Since then we have had to alter this picture
very materially. Not only does the solid crust extend to nearly one quarter of the four thousand
miles, but we have reason to believe that the contents of the Earth are even more solid at the
centre. Indeed, that within the great rock mantle there exists a solid metallic core. We have strong
evidence in this direction from one witness in particular: the phenomena of earthquakes.
When an earthquake occurs at some great distance, we are able to take a record of it here, and at
any other part of the globe where there is a delicate seismograph. One form of such an instrument is
a very delicately poised indicator which will move will every tremor of the Earth.
 The movements of the indicator are recorded by means of a ribbon of photographic paper. There are
now about one hundred different seismological or earthquake stations spread over the planet. On
comparing the records obtained at different stations we can see how long it has taken the tremors to
travel to the different points. The tremors going along the solid crust of the Earth will take a
time proportional to the distance travelled by them. But there are other tremors which go right
through the Earth from the place of the eruption to the distant recording station. If the Earth were
of homogeneous material throughout, then the rate of travel would be proportional to the distance of
this short-cut route through the Earth from point to point. But the earthquake records show that if
the short-cut, from place to place through the Earth, pass through a greater depth than eight or
nine hundred miles, there is a considerable increase in the velocity of the tremors. Indeed, the
tremors pass about twice as fast through the core, and the change of velocity is quite sudden at a
depth of eight or nine hundred miles. The evidence of this witness is that the core of the Earth is
of different materials from its crust. By supposing the centre to be a great metallic core we can
get the different records of earthquakes to agree. Indeed, this seems the only satisfactory way in
which we can account for the evidence, although all geologists are not agreed upon the point.
It will be of interest to inquire if there is no other witness we can call to give evidence on the
question of the metallic core of the Earth. We have discovered the weight of the planet, as we shall
see in the succeeding chapter, and this brings us some further evidence. We know that the weight of
the whole Earth is five and a half times that
 bulk of water. But the rock mantle weighs not more than three and a half times the weight of water,
therefore the centre is heavier. If we allow even one thousand miles for the depth of this rock
mantle, we find that the centre core must be about eight times as heavy as water. It so happens that
iron is about eight times heavier than water, but is there any more reason why we should think of
the Earth's core being iron in preference to copper or gold? Yes, we have several witnesses to call.
The rocks thrown up from a great depth by active volcanoes have been found to be very rich in iron.
Picture the molten Earth cooling, it would be natural for the heavier material, say iron, to fall to
the bottom while the lighter material which forms the rock mantle would float to the surface. We
know that in the blast furnace the stones or slag float to the surface of the molten iron, and are
conveniently drawn off.
Sir Isaac Newton is said to have guessed the density of the Earth to be between five and six times
that of water. It was no blind guess. He argued that when the Earth was cooling from a molten to a
solid state that the lighter materials would float to the surface, and he made his calculations
accordingly. It is remarkable that these figures arrived at some two hundred and fifty years ago are
found to be correct.
There is still another witness to give evidence. When lumps of matter have fallen on to our planet
from stellar space, we find most of meteorites to be very rich in iron. Again, we shall see in a
later chapter how we have discovered the contents of our far-distant Sun, but mean-time it is of
interest to note that iron is one of its constituents.
 We have seen that the Moon was thrown off by the molten earth during its process of cooling. If at
the time of the birth of the Moon the slag were on the surface of the molten metal, we should expect
to find that the Moon is made of the rocky material alone. The weight of the Moon corroborates this,
for its total density is about the same as the rock mantle of the Earth. The most recent
calculations concerning the Earth give a "crustal zone" of about 30 miles in depth, with a mean
density of 2.8. Then a "stony zone" or rock mantle estimated to be from 600 to 900 miles in depth,
and having a density of 3.4.
It may be that there still remains a layer of molten material within the Earth, but we can no longer
picture the centre of the Earth as a fiery mass; we have strong evidence in support of the rock
mantle with a solid iron core.
Before considering what we have discovered concerning the crust of the Earth, it will be of interest
to see how we discovered the weight of the Earth itself.
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