THE EARTH’S BEGINNING.
The Earth to be Studied—A great Experiment—The Diamond Drill—A Boring
upwards of a Mile Deep—A Mechanical Feat—The Scientific Importance
of the Work—Increase of Temperature with the Depth—A special Form of
Thermometer—Taking the Temperature in the Boring—The Level of
Constant Temperature—The Rate of Increase of Temperature with the
Depth—One degree Fahrenheit for every Sixty-six Feet in
Depth—Temperatures at Depths above a Mile—Conclusions as to the Heat
at very great Depths—The Heat developed by Tidal Action—This will
not account for the Earth’s Internal Heat—The Earth must be
continually Cooling—Inferences from the incessant loss of Heat from
the Earth—The Earth’s Surface once Red-Hot, or Molten—The Earth must
have originated from a Nebula—The Earth’s Beginning.
IN the last chapter we endeavoured to ascertain what can be learned from
the radiation of the sun with regard to the history of the solar system.
In this chapter we shall not consider any body in the heavens, but the
condition of the earth itself. We have learned something of the history
of the solar system from the celestial bodies; we shall now learn
something about it in another way—from the condition of our globe at
depths far beneath our feet.
It will be convenient to commence by mentioning a remarkable experiment
which was made a few years ago. Though that experiment is of great
scientific interest, yet it was not designed with any scientific object
in view. Not less than £10,000 was expended on the enterprise, and
probably so large a sum has never been expended on a single experiment
of which the sole object was to add to scientific knowledge. In the
present case the immediate object in view was, of course, a commercial
one. There was, it may be presumed, reasonable expectation that the
great initial cost, and a handsome profit as well, would be returned as
the fruits of the enterprise. Whether the great experiment was
successful from the money-making point of view does not now concern us,
but it does concern us to know that the experiment was very successful
in the sense that it incidentally afforded scientific information of the
very highest value.
The experiment in question was made in Germany, at Schladebach, about
fifteen miles from Leipzig. It was undertaken in making a search for
coal. Some enterprising capitalists consulted the geologists as to
whether coal-seams were likely to be found in this locality. They were
assured that coal was there, though it must certainly be a very long way
down, and consequently the pit by which alone the seams could be worked
would have to be unusually deep. The capitalists were not daunted by
this consideration. But, before incurring the great expense of sinking
the shaft, they determined to make a preliminary search and verily the
actual presence of workable seams of useful fuel. They determined to
bore a hole down through the rocks deep enough to reach the coal, if it
could be reached. A boring for coal was, of course, by no means a
novelty; but there was an unprecedented degree of mechanical skill and
scientific acumen shown in this memorable boring near Leipzig. The
result of this enterprise was to make the deepest hole which, with
perhaps a single more recent exception not of so much scientific
interest, has ever been pierced through the crust of the earth. This
boring was merely a preliminary to the operations which would follow if
the experiment were successful in discovering coal. It was accordingly
only necessary to make a hole large enough to allow specimens of the
strata to be brought to the surface.
The instrument employed in sinking a hole of such a phenomenal depth
through solid rock is characteristic of modern enterprise. The boring
tool had a cutting edge of diamonds: for no other cutting implement is
at once hard enough and durable enough to advance steadily, yard by
yard, through the various rocks and minerals that are met with in the
descent through the earth’s crust. We might, perhaps, illustrate the
actual form of the tool as follows: imagine a piece of iron pipe, about
six inches in diameter, cut squarely across, with diamonds inserted
round its circular end, and we have a notion of the diamond drill. If
the drill be made to revolve when held vertically, with the diamonds in
contact with the rocks, the cutting will commence. As the rotation is
continued, the drill advances through the rocks, and a solid core of the
material will occupy the hollow of the pipe. We do not now enter into
any description of the many mechanical details; there are ingenious
contrivances for removing the _débris_ produced by the attrition of the
rocks as the diamonds cut their way, and provision is also made for
carefully raising the valuable core which, as it provides specimens of
the different strata pierced, will show the coal, if coal is ever
reached. There is, of course, an arrangement by which, as the first
length of drill becomes buried, successive lengths can be added, so as
to transmit the motion to the cutting edge and enable the tool to be
raised when necessary; in this manner one length of solid rock after
another is brought up for examination. These cores, when ranged in
series, give to the miner the information he requires as to the
different beds of rock through which the instrument has pierced in its
descent and as to the depths of the beds. A series of cores will
sometimes show astonishing variety in the material through which the
drill has passed. Here the tool will be seen passing through a bed of
hard limestone, and then entering a bed of soft shale; now the tool
bores through dense and hard masses of greenstone, anon it pierces, it
may be, a stratum of white marble; and finally the explorer may hope to
find his expectations realised by the arrival at the surface of a
cylinder of solid coal.
The famous boring to which we are now referring, though very deep, was
not large in diameter. As it descended the comparatively large tool
first employed was replaced by a succession of smaller tools, so that
the hole gradually tapered from the surface to the lowest point. At its
greatest depth the hole was indeed hardly larger than a man’s little
finger. It increased gradually all the way to the surface, where it was
large enough for a man’s arm to enter it easily.
How often do we find that the success which rewards mechanical
enterprise greatly transcends even the most sanguine estimate previously
formed! Without the actual experience which has been acquired, I do not
think anyone could have anticipated the extraordinary facilities which
the diamond drill has given in the operations of a deep boring. This
hole at Schladebach was, indeed, a wonderful success. It pierced deeper
than any previous excavation, deeper than any well, deeper than any coal
pit. From the surface of the ground, where the hole was some six inches
in diameter, down to the lowest point, where it was only as large as a
little finger, the vertical depth was not less than one mile and a
hundred and seventeen yards.
It is worth pondering for a moment on the extraordinary mechanical feat
which this represents. When the greatest depth was reached, the total
length of the series of boring rods from the surface where the machinery
was engaged in rotating the tool down to the cutting diamonds at the
lower end where the penetration was being effected, was as long as from
Piccadilly Circus to the top of Portland Place. If a hole of equal
length had been bored downwards from the top of Ben Nevis, it would have
reached the sea level and gone down 1,200 feet lower still. When the
foreman in charge wished to look at the tool to see whether it was
working satisfactorily, or whether any of the diamonds had got injured
or displaced, it was necessary to raise that tremendous series of rods.
Each one of them had to be lifted, had to be uncoupled, and had to be
laid aside. I need hardly say that such an operation was a very tedious
one. The collective weight of the working system of rods was about
twenty tons, and not less than ten hours’ hard work was required before
the tool was raised from the bottom to the surface. We may, I believe,
conclude that so much ingenuity and so much trouble was never before
expended on the act of boring a hole; but the results are full of
information on important problems of science.
I am not going to speak of the geological results of this exploration.
There is not the least doubt that the remarkable section of the earth’s
crust thus obtained is of much interest to geologists. Our object in now
alluding to this wonderful boring is, however, very different. Its
significance will be realised when we say that it gives us more full and
definite information about the internal heat of the earth than had ever
been obtained by any other experiment on the earth’s crust. No doubt
many previous observations of the internal heat of the globe were well
known to the investigators who feel an interest in these important
questions; but the exceptional depth of this boring, as well as the
exceptionally favourable conditions under which it was made, have
rendered the information derived from it of the utmost value to science.
We ought first to record our special obligation to the German engineer,
Captain Huyssen, who bored this wonderful hole. He was not only a highly
skilful mining engineer, diligent in the pursuit of his profession, but,
by the valuable scientific work he has done, he has shown himself to be
one of those cultivated and thoughtful students who love to avail
themselves of every opportunity of searching into Nature’s secrets. Our
thanks are due to him for the remarkable zeal with which he utilised the
exceptional opportunities for valuable scientific work that arose,
incidentally as it were, in connection with the work committed to him.
Of course, everybody knows that the temperature of the earth is found to
increase gradually as greater depths are reached. The rate at which the
increase takes place has been determined on many occasions. But when
opportunities have arisen for taking the temperature at considerable
depths below the earth’s surface, it has happened sometimes that the
observations have been complicated by circumstances which deprived them
of a good deal of their accuracy. If our object be to learn the law
connecting the earth’s temperature with the depth below the surface, it
is not sufficient to study the thermometric readings in different coal
pits. Throughout the workings in every pit there must be arrangements
for ventilation. The cool air has to be drawn down, and thus the
temperature indicated in the pit is forced below the temperature which
would really be found at that depth if external sources of change of
temperature were absent.
Captain Huyssen rightly deemed that the hole which he had pierced
presented exceptional opportunities for the study of the important
question of the earth’s internal temperature. Precautions had, of
course, to be observed. The hole, as might be expected, was filled with
water, and the water would tend, if its circulation were permitted, to
equalise the temperature at different depths. But the ingenious Captain
quickly found an efficient remedy for this source of inaccuracy. He
devised an arrangement, which I must not delay to describe, by which he
could place temporary plugs in the hole at any depths he might desire;
he then determined the temperature of the water in a short length, so
plugged above and below that the circulation was stopped, and
accordingly the water thus confined might be relied on to indicate the
temperatures of the strata which hold it.
[Illustration: Fig. 22.—SPECIAL THERMOMETER FOR USE IN DEEP BORINGS.]
The thermometer employed in an investigation of this sort is ingenious
though extremely simple. The ordinary maximum thermometer is not found
to be adapted for the purpose. The instrument (Fig. 22) employed in the
determination of underground temperatures is very much less complicated
and at the same time much more accurate. The contrivance is indeed so
worthy of notice that I do not like to pass it by without a few words.
The thermometer with which the temperature of the earth is ascertained
in such investigations is not like any ordinary thermometer. There is no
scale of degrees attached to it or engraved upon it, as we generally
find in such instruments. The instrument with which the temperature of
the deep hole was measured was merely a bulb of glass with a slender
capillary stem, the end of which was not closed. When it was about to be
lowered to test the temperature of the rocks at the lowest point to
which the drill had penetrated, the bulb and the tube were first filled
with mercury to the top, and brimming over. This simple apparatus was
attached to a long wire, by the aid of which it could be lowered down
this deep hole. Down it went till at last the thermometer reached the
bottom, which, as we have explained, it could not do until more than a
mile of wire had been paid out. The instrument was then left quietly
until it presently assumed the same temperature as the rocks about it.
There could be no interference by heat from other strata, as the
circulation of water was prevented by the plugging already referred to.
The temperature to which the thermometer had been exposed must,
therefore, have been precisely the temperature corresponding in that
particular locality to that particular depth below the earth’s surface.
As the thermometer descended, it passed through a succession of strata
of ever-increasing temperature. Consequently the mercury, which, it will
be remembered, had completely filled the instrument when it was at the
surface, began to expand according as it was exposed to greater
temperatures. As the mercury expanded, it must, of course, flow out of
the tube and be lost, because the tube had been already full. So long as
the mercury was gaining in temperature, more and more of it escaped from
the top of the tube, and the flow only ceased when the instrument was
resting at the bottom of the hole, and the mercury became as hot as the
surrounding rocks. No more mercury was then expelled, the tube, however,
remaining full to the brim. After allowing a sufficient time for the
temperature to settle definitely, the thermometer was raised to the
surface. As it ascended through the long bore the temperature
surrounding it steadily declined. With the fall in the temperature of
the mercury the volume of that liquid began to shrink; but the mercury
already expelled could not be recalled. When at last the instrument had
safely reached the surface, after its long journey down and up, and when
the mercury had regained the temperature of the air, the lessened
quantity that remained told the tale of the changes of temperature.
It is now easy to see how, even in the absence of an engraved scale on
the instrument, it is possible to determine, from the amount of mercury
remaining, the temperature to which the thermometer has been subjected
at the bottom of the boring. It is only necessary to place this
thermometer in a basin of cold water, and then gradually increase the
temperature by adding hot water. As the temperature increases the
mercury will, of course, rise, and the hotter the water the more nearly
will the mercury approach the top of the tube. At last, when the mercury
has just reached the top of the tube, and when it is just on the point
of overflowing, we may feel certain that the temperature of the water in
the basin has been raised to the same temperature as that to which the
instrument was subjected at the bottom of the boring. In each case the
temperature is just sufficient to expand the quantity of mercury
remaining in the instrument so as to make it fill precisely both bulb
and stem. When this critical condition is reached, it only remains to
dip a standard thermometer, furnished with the ordinary graduation, into
the hot water of the basin. Thus we learn the temperature of the basin,
thus we learn the temperature of the mercury in the thermometer, and
thus we determine the temperature at the bottom of the boring over a
mile deep.
I need not specify the details of the arrangements which enabled the
skilful engineer also to determine the temperature at various points of
the hole intermediate between the top and the bottom. In fact, taking
every precaution to secure accuracy, he made measurements of the
temperature at a succession of points about a hundred feet distant
throughout the whole depth. In each case he was careful, as I have
already indicated, to plug the hole above and below the thermometer, so
as to prevent the circulation of water in the vicinity of the
instrument. The thermometer, therefore, recorded the temperature of the
surrounding rocks without any disturbing element. Fifty-eight
measurements at equal distances from the surface to the greatest depths
were thus obtained.
We have now to discuss the instructive results to which we have been
conducted by this remarkable series of measurements. First let us notice
that there is much less variation in the subterranean temperatures than
in the temperatures on the earth’s surface. On the surface of the earth
we are accustomed to large fluctuations of temperature. We have, of
course, the diurnal fluctuations in temperature from day to night; we
have also the great seasonal fluctuations between summer and winter. But
below a certain depth in the ground the temperature becomes much more
equable. Whether the temperature on the surface be high or whether it be
low, the temperature of any particular point far beneath the surface
does not change to any appreciable extent. In Arctic regions the surface
of the earth may undergo violent seasonal changes of temperature, while
at a few feet below the surface the temperature, from one end of the
year to the other, may remain sensibly unaltered.
In deep and extensive caverns the temperature is sometimes found to
remain practically unaffected by the changes in the seasons. The Mammoth
Cave of Kentucky is a notable instance. The uniformity of the
temperature, as well as the mildness and dryness of the air, in those
wonderful subterranean vaults is such that many years ago a project was
formed to utilise the cavern as an abode for consumptive patients, for
whose cure, according to the belief then prevailing, an equable
temperature was above all things to be desired. Houses were indeed
actually built on the sandy floors of the cavern, and I believe they
were for some time tenanted by consumptive patients willing to try this
desperate remedy. The temperature may have been uniform and the air may
have been dry, but the intolerable gloom of such a residence entirely
neutralised any beneficial effects that might otherwise have accrued.
The ruins of the houses still remain to testify to the failure of the
experiment.
The heat received from the sun does not penetrate far into the earth’s
crust, and consequently the diurnal and even the seasonal changes of the
temperature at the surface produce less and less effect with every
increase of the depth. All such variations of temperature are confined
to within 100 feet of the surface. At the depth of about 100 feet a
fixed temperature of 52° Fahrenheit is reached, and this is true all
over the earth. It matters not whether the surface be hot or cold,
whether the latitude is tropical and the season is midsummer, whether
the latitude lie in the Arctic regions and the season be the awful
winter of iron-bound frost and total absence of sun—in all cases we find
that about 100 feet below the surface the temperature is 52°. With
sufficient accuracy we may say that this depth expresses the limit of
the penetration of the earth’s crust by sunbeams. The remarkable law
according to which the temperature changes below the depth of 100 feet
is wholly irrespective of the solar radiation.
The study of the internal heat of the earth may be said to begin below
the level of 100 feet, and the results that were obtained in the great
boring are extremely accordant. The deeper the hole, the hotter the
rocks; and Captain Huyssen found that for each sixty-six feet in descent
the temperature increased one degree Fahrenheit. To illustrate the
actual observations, let us take two particular cases. We have said that
the hole was one mile and 117 yards deep. Let us first suppose the
thermometer to be lowered 117 yards and then raised, after a due
observance of the precautions required to obtain an accurate result. The
temperature of the rocks at the depth of 117 yards is thus ascertained.
In the next observation let the thermometer be lowered from the surface
to the bottom of the hole, that is to say, exactly one mile below the
position which it occupied in the former experiment. The observations
indicate a temperature 80° Fahrenheit higher in the latter case than in
the former. We have thus ascertained a most important fact. We have
shown that the temperature of the crust of the earth at the depth of one
mile increases about 80°. This is at the rate of one degree every
sixty-six feet. I should just add, as a caution, that if we choose to
say the temperature increases one degree per sixty-six feet of descent,
we ought to suppose that we start from a point which is not higher than
that level of 100 feet above which as already explained, the temperature
of the rocks is more or less affected by solar heat.
We have described these particular observations in some detail because
they have been conducted under conditions far more favourable to
accuracy than have ever been available in any previous investigations of
the same kind. But now we shall omit further reference to this
particular undertaking near Leipzig. It is not alone in that particular
locality, not alone in Germany, not alone in Europe, not alone on the
surface of any continent, that this statement may be made. The statement
is one universally true so far as our whole earth is concerned. Wherever
we bore a hole through the earth’s crust, whether that hole be made in
the desert of Sahara or through the icebound coasts of Greenland, we
should find the general rule to obtain, that there is an increase of
temperature of about 80° for a mile of descent. This is true in every
continent, it is true in every island; and, though we cannot here go
into the evidence fully, there is not the least doubt that it is true
also under the floor of ocean. If beneath the bed of the Atlantic a hole
a mile deep were pierced, the temperature of the rocks at the bottom of
that hole would, it is believed, exceed by about 80° the temperature of
the rocks at the surface where the hole had its origin. We learn that at
the depth of a mile the temperature of the earth must generally be 80°
hotter than it is at the level of constant temperature near the surface.
It may perhaps help us to realise the significance of this statement if
we think of the following illustration. Let us imagine that the waters
of the ocean were removed from the earth. The ocean may in places be
five or six miles deep, but that is quite an inconsiderable quantity
when compared with the diameter of the earth. The change in the size of
the earth by the removal of all the water would not be greater,
proportionally, than the change produced in a wet football by simply
wiping it dry. Let us suppose that an outer layer of the earth’s
surface, a mile in thickness, was then to be peeled off. If we remember
that the diameter of the earth is 8,000 miles, we shall see that this
outer layer, whose removal we have supposed, does not bear to the whole
extent of the earth a ratio even as great as that which the skin of a
peach does to the fruit inside. But this much is certain, that if the
earth were so peeled there would be a wonderful difference in its
nature. For though practically of the same size as it is at present, it
would be so hot that it would be impossible to live upon it.
Next comes the very interesting question as to the temperature that
would be found at the bottom of a hole deeper still than that we have
been considering. Our curiosity as to the depths of the earth extends
much below the point to which Captain Huyssen drove down his diamond
drill. The trouble and the cost of still deeper exploration of the same
kind seem, however, to be actually prohibitive. To bore a hole two miles
deep would certainly cost a great deal more than twice the sum which
sufficed to bore a hole one mile deep. At a great depth each further
foot could only be won with not less difficulty and expense than a
dozen, or many dozen feet, at the surface. Mining enterprise does not at
present seem to contemplate actual workings at depths much over a mile,
so there does not seem much chance of any very much deeper boring being
attempted. We do not say that a hole two miles deep would be actually
impossible; it may well be wished that some millionaire could be induced
to try the experiment. We should greatly like to be able to lower a
thermometer down to a depth of two miles through the earth’s crust.
Seeing there is but little chance of our wish for such future
experiments being gratified, it is consolatory to find that actual
observations of this kind are not indispensable to the argument on which
we are to enter. Our argument can indeed be conducted a stage further,
even with our present information. The indications already obtained in
the hole one mile deep go a long way towards proving what the
temperature of a hole still deeper would be. We have already remarked
that it was part of Captain Huyssen’s scheme to obtain careful readings
of his thermometer at intervals of 100 feet from the surface to the
bottom of the hole. A study of these readings shows that the increase of
80° in a mile takes place uniformly at the rate of one degree for each
sixty-six feet of depth. As the temperature increases uniformly from the
surface down to the lowest point which our thermometers have reached, it
would be unreasonable to suppose that the rate of increase would be
found to suffer some abrupt change if it were possible to go a little
deeper. As the temperature rises 80° in the first mile, and as the rate
of increase is shown by the observations to be quite as large at the
bottom of the hole as it is at the top, we certainly shall not make any
very great mistake if we venture to assume that in the second mile the
temperature would also increase to an extent which will not be far from
80°. This inference from the observations leads to the remarkable
conclusion that at a depth of two miles the temperature of the earth
must be, we will not say exactly, but at all events not very far from,
160° higher than at the level of constant temperature about 100 feet
down.
As in the former case, we need not confine ourselves to any particular
locality in drawing this conclusion. The arguments apply not only to the
rocks underneath Leipzig, but to the rocks over every part of the globe,
whether on continents or islands, or even if forming the base of an
ocean. No one denies that the law of increase in temperature with the
depth must submit to some variation in accordance with local
circumstances. In essential features it may, however, be conceded that
the law is the same over all the earth. If we take 52° to be the
temperature of the level 100 feet down, which limits the seasonal
variations, and if we add that at two miles further down the temperature
is somewhere about 160° more, we come to the conclusion that at a depth
of a little over two miles the temperature of the rocks forming the
earth’s crust is about 212° Fahrenheit. Thus we draw the important
inference that if, the oceans having been removed, we were then to
remove from the earth’s surface a rind two miles thick—a thickness
which, it is to be observed, is only the two-thousandth part of the
earth’s radius—we should transform the earth into a globe which, while
it still retained appreciably the same size, would have such a
temperature that even the coolest spot would be as hot as boiling water.
This is indeed a remarkable result.
And now that we have gone so far, it is impossible for us to resist
making a further attempt to determine what the temperature of the
earth’s crust must be if we could send a thermometer still lower. A hole
one mile deep we have seen; I do not think we can hope to see a hole two
miles deep, but still it may not be absolutely impracticable; but a hole
of three or more miles deep we may safely regard as transcending present
possibilities in engineering enterprise. Are we therefore to be deprived
of all information as to the condition of our earth at depths exceeding
those already considered? Fortunately we can learn something. We are
assisted by certain laws of heat, and, though the evidence on which we
believe those laws is necessarily limited to the experience of Nature as
it comes within our observation, yet it is impossible to refuse assent
to the belief that the same laws will regulate the transmission of heat
in the crust of the earth two miles, three miles, or many miles beneath
our feet.
I represent, in the diagram shown in Fig. 23, three consecutive beds of
rock—A, B, and C—as they lie in the earth’s crust, a little more than a
mile beneath our feet. I shall suppose that the bed B is the very lowest
rock whose temperature was determined in the great boring. The drill has
passed completely through A, it has pierced to the middle of B, but it
has not entered C. The observations have shown that the temperature of
the stratum B exceeds that of the stratum A, and we further note that
this is a permanent condition—that is to say, B constantly remains
hotter than A. From this fact alone we can learn something as regards
the temperature of the stratum C which lies in contact with B. Of course
we are unable to observe the temperature of C directly, because by
hypothesis the boring tool has not entered that rock. We can, however,
prove, from the laws of the conduction of heat, that the temperature of
C must be greater than that of B; and this appears from the following
consideration.
[Illustration: Fig. 23.—AT THE BOTTOM OF THE GREAT BORE.]
It is plain that C must be either just the same temperature as B, or it
must be hotter than B, or it must be colder than B. If C were the same
temperature as B, then the law of conduction of heat tells us that no
heat would flow from one of these strata to the other. The laws of heat,
however, assure us that when two bodies at different temperatures are in
contact the heat will flow from the hotter of these bodies into the
colder, so long as the inequality of temperature is maintained. As B is
hotter than A, then heat must necessarily flow from B into A, and this
flow must tend to equalise the temperature in these strata, for B is
losing heat while none is flowing into it from C. Therefore B and A
could not continue to preserve indefinitely the different temperatures
which observation shows them to do. We are therefore forced to the
conclusion that B and C cannot be at the same temperature.
Next let us suppose that the temperature of the stratum B exceeded that
of C. Then, as A is colder than B, it appears that B would be lying
between two strata each having a temperature lower than itself. But
that, of course, cannot be a permanent arrangement, for the heat would
then escape from B on both sides. The laws of heat, therefore, tell us
that B could not possibly retain permanently a temperature above both A
and C. Observation, however, shows that the temperatures of A and B are
persistently unequal. We are therefore obliged to reject the supposition
that the temperature of C can be less than that of B.
We have thus demonstrated that the temperature of the stratum C cannot
be the same as that of B. We have also demonstrated that it cannot be
colder than B. We must therefore believe that C is hotter than B. This
proves that the stratum immediately beneath that stratum to which the
observations have extended must be hotter than it. Thus, though the
stratum below the bottom of the hole lies beyond the reach of our actual
observation, we have, nevertheless, been able to learn something with
regard to its temperature.
Having established this much, we can continue the same argument further;
indeed, it would seem that we can continue it indefinitely, so long as
there is a succession of such strata. Underneath the stratum C must lie
another stratum D. But we have shown that C must be hotter than B, and
precisely the same argument that has proved this will prove that D is
hotter than C. Underneath D comes the stratum E, and again the same
argument will apply. Inasmuch as D is hotter than C, it follows that E
must be hotter than D. These three strata, C, D, and E, are all beyond
the reach of the thermometer, we know nothing of their temperatures by
direct observation; but none the less is the argument, which we are
following strictly, applicable. Thus we obtain the important result that
in the crust of the earth the temperature must be always greater, the
greater the depth beneath the surface.
We have seen that the rate of increase of temperature with the depth is
about 80° for the first mile, and we deem it probable that the rate of
increase may be maintained at about the same for the second mile. But we
do not suppose that the rate of increase mile after mile will remain the
same at extremely great depths. It may perhaps be presumed that there
must be some increase of temperature all the way to the earth’s centre;
but the rate of increase per mile may change as the centre is
approached. The point of importance for our present argument is, that
the temperature of the earth must increase with the depth, though the
rate of increase is quite unknown to us at depths greatly beyond those
which the thermometer has reached. It is easy to see that the conditions
prevailing in the earth’s interior might greatly modify any conclusion
we should draw from observations near the surface. Our argument has been
based on the laws of heat, as we find them existing in matter on the
surface of the earth submitted to such ranges of different physical
conditions as can be dealt with in our laboratories; but at such
excessively high temperatures as may exist in the earth’s interior the
properties of matter may be widely different from the properties of
matter as known to us within the temperatures that we are able to
produce and control. The enormous pressure to which matter in the
interior of the earth must be subjected should also be mentioned in this
connection. It is wholly impossible to produce pressures by any
mechanical artifice which even distantly approach in intensity to that
awful force to which matter is subjected in the earth’s interior.
It may be instructive to consider a few facts with respect to this
question of pressure in the earth’s interior. A column of water 34½ feet
high gives, as everybody knows, a pressure of fifteen pounds on the
square inch. It will be quite accurate enough for our present purpose to
assume that the average density of rock is three times that of water:
the pressure of ten feet of rock would therefore produce the same
pressure as thirty feet of water, that is to say, fifteen pounds on the
square inch. The pressure due to the superincumbent weight of a mile of
rock would be more than three tons on the square inch. At the depth of
ten miles beneath the earth’s surface the pressure, amounting as it does
to over thirty tons on the square inch, would very nearly equal the
pressure produced on the inside of a 100-ton gun when the charge of
cordite has been exploded to drive the missile forth. This is indeed
about as large a pressure as can well be dealt with artificially, for we
know that the 100-ton gun has to be enormously strong if it is to resist
this pressure. But ten miles of rock is as nothing compared with the
thickness of rock that produces the pressures in the earth’s interior.
Even if a shell of rocks ten miles thick were removed from the surface
it would alter the diameter of our globe by no more than one
four-hundredth part. At the depth of about thirty miles from the surface
the pressure in the earth’s interior would amount to some 100 tons on
each square inch. With each increase in depth the pressure increases
enormously, though it may not be correct to say that the pressure is
proportional to the depth. A pressure of 1,000 tons on the square inch
must exist at a depth which is still quite small in comparison with the
radius of the earth.
We have not, and apparently cannot have, the least experimental
knowledge of the properties of matter at the moment when it is subjected
to pressure amounting to thousands of tons per square inch; still less
can we determine the behaviour of matter at that pressure of scores of
thousands of tons, to which much of the interior of the earth is at this
moment subjected. Professor Dewar, in his memorable researches, has
revealed to us the remarkable changes exhibited in the properties of
matter when that matter has been cooled to a temperature which lies in
the vicinity of absolute zero. We can, however, hardly hope that any
experiments will give us information as to the properties of matter when
heated to a temperature vastly transcending that which could ever be
produced in our most powerful electric furnaces, and at the same time
exposed to a pressure hundreds of times, or indeed we may say thousands
of times, greater than any pressure that has ever been produced
artificially by the action of the most violent explosive with which the
discoveries of chemistry have made us acquainted.
[Illustration: Fig. 24.—THREE CONSECUTIVE SHELLS OF THE EARTH’S CRUST.]
We really do not know how far the laws of heat, which have been employed
in showing that the temperature must increase as the depth increases,
can be considered as valid under the extreme condition to which matter
is subjected in the deep interior of our globe. The laws may be
profoundly modified. It suffices, fortunately for our present argument,
to say that, so far as observations have been possible, the temperature
does gradually increase with the depth, and that this increase must
still continue from stratum to stratum as greater depths are reached,
unless it should be found that by the excessive exaltation of
temperature and the vast intensity of pressure certain properties of
matter become so transformed as to render the laws of heat, as we know
them, inapplicable.
In subsequent chapters we shall have some further points to consider
with respect to the interior of the earth and its physical
characteristics, which are, however, not necessary for our present
argument. What we now desire to prove can be deduced from the
demonstrated fact that the earth’s temperature does steadily increase
from the level of constant temperature, 100 feet below the surface, down
to the greatest depth to which thermometers have ever been lowered. We
may presume that the same law holds at very much greater depths, even if
it does not hold all the way to the centre.
To make our argument clear, let us think of three different strata of
rock. This time, however, we shall suppose them to cover the whole
earth, and we shall consider them to lie within the first mile from the
surface; they will thus be well within the region explored by
observation (Fig. 24). We shall also regard them as shells of uniform
thickness, and it will be convenient to think of them as being so very
thin that we may consider any one of the shells called A to have
practically a uniform temperature. The next shell B immediately inside A
will have a slightly greater temperature, and be also regarded as
uniform, and the shell immediately inside that again will have a
temperature greater still. We shall call the innermost of the three
shells C, and C is hotter than the next outer shell B, while B is hotter
than A. The laws of heat tell us that as B and A are in contact, and
that as B is continually hotter than A, then B must be continuously
transmitting heat to A. In fact, B appears to be constantly endeavouring
to reduce itself to the temperature of A by sharing with A the excess of
temperature which it possesses. But if we consider the relation between
the shell B and the hotter shell C, immediately beneath it, we see that
precisely the same argument will show that B is constantly receiving
heat from C. We thus see that while B is continuously discharging heat
from its outside surface, it is as constantly receiving heat which
enters through its inside surface. Heat enters B from C, and heat passes
from B into A, so that B is in fact a channel through which heat passes
from C into A.
That which we have shown to take place in those three consecutive layers
in the earth’s crust must also take place in every three consecutive
layers. Each layer is continually receiving heat from the layer below,
and is as constantly communicating heat to the layer above. No doubt the
rocks are very bad conductors of heat, so that the transmission of heat
from layer to layer is a very slow process. But even if this flow of
heat be slow, it is incessant, so that in the course of ages large
quantities of heat are gradually transmitted from the earth’s interior,
and ultimately reach the level of constant temperature. There is
nothing, however, to impede their outward progress, so at last the heat
reaches the earth’s surface.
When the surface has been reached, then another law of heat declares
what must happen next. It is, of course, by conduction that the heat
passes from layer to layer in its outward progress, until it ultimately
gains the surface. At the surface the heat is then absolutely removed
from the solid earth either by the convection through the air or by
direct radiation into space.
I may here interrupt the argument for a moment to make quite clear a
point which might perhaps otherwise offer some difficulty to the reader.
When this outward flow of heat reaches the superficial layers it
becomes, of course, mixed up with the heat which has been absorbed by
the soil from the direct radiation of the sun, and this varies, of
course, with the hour of the day and with the season of the year. The
heat which steadily leaks from the interior has an effect on the rocks
near the surface, which is only infinitesimal in comparison with the
heat which they receive from periodic causes. We may, however, say that
whatever would be the temperature of the rock, so far as the periodic
causes are concerned, the actual temperature is always to some minute
extent increased by reason of the heat from the earth’s interior. The
argument is, perhaps, still clearer if, instead of attending to the
earth’s surface, we think only of that shell, some 100 feet down, which
marks the limit of the depth to which the seasonal and diurnal
variations of heat extend. The argument shows how the internal heat of
the earth, passing from shell to shell in the interior, reaches this
layer of constant temperature, and passing through it, enters into those
superficial strata of the earth which are exposed to the seasonal
variations. With what befalls that heat ultimately we need not now
concern ourselves; it suffices for our argument to show that there is a
current of heat outward across this level. It is a current which is
never reversed, and consequently must produce a never-ceasing drainage
from the heat with which it would seem that the interior of the earth is
so copiously provided.
Calculations have been made to ascertain how much heat passes annually
from the earth’s interior, across this surface of constant temperature,
out into the superficial regions from which in due course it becomes
lost by radiation. A convenient way of measuring a quantity of heat is
by the amount of ice it will melt, for of course a definite quantity of
heat is required to melt a definite quantity of ice. It has been
estimated by Professor J. D. Everett, F.R.S., that the amount of
internal heat escaping from our earth each year would be sufficient to
melt a shell of ice one-fifth of an inch thick over the whole surface of
the globe. We cannot indeed pretend that any determination of the actual
loss of heat which our earth experiences could be very precise.
Sufficient observations have not yet been obtained, for the operation is
so slow that an immense period would have to elapse before the total
quantity of heat lost would be sufficient to produce effects large
enough to be measured accurately. But now let us hasten to add that, for
the argument as to the nebular theory with which we are at present
concerned, it is not really material to know the precise rate at which
heat is lost. It is absolutely certain that a perennial leakage of heat
from the interior of the earth does take place. This fact, and not the
amount of that leakage, is the essential point.
And this loss, which is at present going on, has been going on
continually. Heat from the earth has been lost this year and last year;
it has been lost for hundreds of years and for thousands of years. Not
alone during the periods of human history has the earth’s heat been
declining. Even throughout those periods, those overwhelming periods
which geology has revealed to us, has this earth of ours been slowly
parting with its heat.
Let us pursue this reflection to its legitimate consequence. Whatever
may ultimately become of that heat, it is certain that once radiated
into space it is lost for ever so far as this globe is concerned. You
must not imagine that the warm beams of the sun possess any power of
replenishment by which they can restore to the earth the heat which it
has been squandering for unlimited ages; we have already explained that
the effect of the heat radiated to us from the sun is purely
superficial. Even amid the glories of the tropics, even in the burning
heat of the desert, the vertical sun produces no appreciable effects at
depths greater than this critical limit, which is about 100 feet below
the surface. The rigours of an Arctic winter have as little effect in
reducing the temperature of the rocks at that depth as the torrid heat
at the Equator has in raising it. The effect in each case is nothing.
The argument which we are here employing to deduce the nebulous origin
of our earth from the increase of temperature with increase in depth in
the earth’s crust must be cleared from an objection. It is necessary to
explain the matter fully, because it touches on a doctrine of very great
interest and importance.
That a rotating body should possess a quantity of energy in virtue of
its rotation will be familiar to anyone who has ever turned a grindstone
or watched the fly-wheel of an engine. A certain amount of work has to
be expended to set the heavy wheel into rotation, and when the machine
is called upon to do work it will yield up energy and its motion will
undergo a corresponding abatement. The heavy fly-wheel of the machine in
a rolling mill contains, in virtue of its motion, enough energy to
overcome the tremendous resistance of the materials submitted to it.
Once upon a time the earth revolved upon its axis in six hours, instead
of in the twenty-four hours which it now requires. At that time the
energy of the rotation must have been sixteenfold what it is at present.
This consideration shows that fifteen-sixteenths of the energy that the
earth originally possessed in its rotation has disappeared, and we want
to know what has become of it.
We are here entering upon a matter of some difficulty. It is connected
with that remarkable chapter in astronomy which describes the evolution
of the earth-moon system. The moon was originally a part of the earth,
for in very early times, when the earth was still in a plastic state, a
separation would seem to have taken place, by which a small piece broke
off to form the moon, which has been gradually revolving in an enlarging
orbit until it has attained the position it now occupies. A considerable
portion of the energy of the earth’s rotation has been applied to the
purpose of driving the moon out to its present path, but there is a
large remainder which cannot be so accounted for. It is well known that
the evolution of the moon has been a remarkable consequence of tidal
action. There are tides which sway to and fro in the waters on the
earth’s surface; there are tides in any molten or viscous matter that
the earth may contain, and there are even certain small tidal
displacements in the solid material of our globe. Tides of any kind will
generate friction, and friction produces heat, and the energy of the
earth’s rotation, which we have not been able to account for otherwise,
has been thus transformed into heat. Throughout the whole interior of
the earth heat has been produced by the tidal displacement of its parts.
The question therefore arises as to whether the internal heat of the
earth may not receive an adequate explanation from this tidal action,
which is certainly sufficient as to quantity. It is easy to calculate
what the total quantity of this tidal heat may have been. We know the
energy which the earth had when it rotated in six hours, and we know
that it now retains no more than a sixteenth of that amount. We know
also precisely how much was absorbed in the removal of the moon, and the
balance can be evaluated in heat. It can be shown, and the fact is a
very striking one, that the quantity of heat thus arising would be
sufficient to account many times over for the internal heat of the
earth. It might therefore be urged plausibly that the internal heat
which we actually find has had its origin in this way. And if this were
the case the argument which we are using in favour of the nebular origin
of the earth, would be, of course, invalidated.
We may state the issue in a slightly different manner, as follows. Heat
there is undoubtedly in the earth; that heat might have come from the
primæval nebula as we have supposed, and as in actual fact it did come.
But apparently it might have come from the tidal friction. Why then are
we entitled to reject the latter view, and say that the tidal friction
will not explain the internal heat, and why are we compelled to fall
back on the only other explanation?
Lord Kelvin suggested a test for deciding to which of these two sources
the earth’s internal heat was to be attributed. Professor G. H. Darwin
applied the test and decided the issue. We have dwelt upon the rate at
which the heat increases with the descent, this rate being about one
degree every sixty-six feet. Now the distribution of the heat, if it had
come from the tidal action, would be quite different from the
distribution which would result from the gradual efflux of heat from the
centre in the process of cooling. And, speaking quite generally, we may
surmise that the heat produced by tidal friction would be distributed
rather more towards the exterior of the earth than at its centre. We
might therefore reasonably expect that if the internal heat of the earth
arose from tidal friction it would be more uniformly distributed
throughout the globe, and there would not be so great a contrast between
the high temperature of the interior and the lesser temperatures near
the surface as there is when the heat distribution is merely the result
of cooling. It has been proved that if the internal heat had its origin
from the tidal friction, the rate of increase with the depth would be
totally different from what it is actually found to be. It would be
necessary to go down 2,000 feet to obtain an increase of one degree,
instead of only sixty-six feet, as is actually the case.
Hence we conclude that the increasing heat met with in descending
through the earth’s crust is not accounted for by tidal friction; it has
its origin in the other alternative, namely, from the cooling of the
primæval nebula. The heat which was undoubtedly produced by the tidal
friction has gradually become blended with the heat from the other, and,
as we must now say, the principal source. The facts with regard to the
rate of increase with depth thus show that, whatever the tides may have
done in producing internal heat, there has been another and a still more
potent cause in operation. The important conclusion for our present
purpose is that our argument may justly proceed without taking account
of the effect of tidal friction.
We are led by these considerations to a knowledge of a great
transformation in the nature of our globe which must have occurred in
the course of ages. We have seen that this earth is gradually losing
heat from its interior, and we have seen that this loss of heat is
incessant. From the fountains of heat, still so copious, in the interior
the supply is gradually dissipating. Now heat is only a form of energy,
and energy, like matter, cannot itself be created out of nothing. There
can be no creation of heat in our earth without a corresponding
expenditure of energy. If, therefore, the earth is radiating heat, then,
as there is no known or, indeed, conceivable source of energy by which
an equivalent can be restored, it follows that the earth must have less
internal heat now than it had at any earlier period. No doubt the
process of cooling is excessively slow. The earth has less internal heat
at present than it had a hundred years ago, but I do not suppose that
even in a thousand years, or perhaps in ten thousand years, there would
be any appreciable decline in the quantity of heat, so far as any
obvious manifestations of that heat are concerned. It is, however,
certain that the earth must have been hotter, even though there are not
any observations to which we can appeal to verify the statement; and as
our retrospect extends further and still further through the ages we see
that the globe must have been ever hotter and ever still hotter.
Whatever be the heat contained in our earth now, it must have contained
vastly more heat ten million years ago; how otherwise could the daily
leakage of heat for all those ten million years have been supplied? It
follows that there must have been much more heat somewhere in our earth
ten million years ago than there is at present, and the further our
retrospect extends the hotter do we find the earth to have been. There
was a time when the temperature of the earth’s surface must have been
warmed not alone by such sunbeams as fell upon it, but by the passage of
the heat from the interior.
No matter how early be the period which we consider, we find the same
causes to be in operation. There was a time when, owing to the internal
heat, the surface of the earth must have been as hot as boiling water.
The loss of heat by radiation must then have taken place much more
copiously than it does at present. The argument we are pursuing must
therefore have applied with even greater force in those early days.
There was a time when the materials at the surface of the earth must
have been intensely heated, when they must have even been red-hot. There
was a time when the earth’s surface must have had a temperature like
that of the lava as it issues from a volcano. There must have been a
time when the surface of the earth was not even solid, when indeed it
was a viscid liquid, and earlier still the liquid must have been more
and more incandescent. From that brilliant surface heat was vehemently
radiated. Each day the globe was hotter than on the succeeding day.
There is no break in the argument. We have to think of this glowing
globe passing through those phases through which we know that all matter
will pass if only we apply to it sufficient heat. The globe assumed the
liquid state from that state which is demanded by a temperature still
higher, the state in which the matter is actually in the form of vapour.
Even the most refractory substances will take the form of vapour at a
very high temperature.
Thus we are conducted to a remarkable conception of the condition in
which the materials now forming our solid earth must have been in the
exceedingly remote past. What is now our earth must once have been a
great quantity of heated vapour. It need hardly be said that in that
form the volume of the earth was much larger than the volume which the
earth has at present, while no doubt the mass of the earth then was even
less than the mass of the earth now, by reason of the meteoric matter
which has been drawn in by our globe.
But even when our earth was in this inflated state of vapour our
argument can be still maintained. Thus we see that the earth, or rather
the cloud of vapour which was ultimately to form the earth, is ever
growing larger and larger in our retrospect, ever becoming more and more
rarefied; and it may well have been that there was a time when the
materials of this earth occupied a volume thousands of times greater
than they do at present.
In a previous chapter we have seen how the sun was at one time in the
nebulous state, and now we have been led to a similar conclusion with
regard to the earth. At that time, of course, the sun was greatly in
excess of its present dimensions, and the earth was also greatly
swollen. The nebula which formed our sun, and the nebula which formed
our earth, were both so vast as to be confluent; they were indeed both
part of the same vast nebula.
Such has been the Earth’s Beginning so far as modern science can make it
clear to us. We have at least indicated the course which events must
have taken according to the laws of nature as we understand them. Many
of the details of the great evolution are no doubt unknown at present,
and perhaps must ever remain so. That the events which we have
endeavoured to describe do substantially represent the actual evolution
of our system is the famous Nebular Theory.
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