The figure of the Earth and the amount of solidification (density) which it
has acquired are intimately connected with the forces by which it is
animated, in so far, at least, as they have been excited or awakened from
without, through its planetry position with reference to a luminous central
body. Compression, when considered as a consequence of centrifugal force
acting on a rotating mass, explains the earlier condition of fluidity of our
planet. During the solidification of this fluid, which is commonly
conjectured to have been gaseous and primordially heated to a very high
temperature, an enormous quantity of latent heat must have been liberated.
If the process of solidification began as Fourier conjectures, by radiation
from the cooling surface exposed to the atmosphere, the particles near the
center would have continued fluid and hot. As, after long emanation of heat
from the center toward the exterior, a stable condition of the temperature
of the Earth would at length be established, it has been assumed that with
increasing depth the subterranean heat likewise uninterruptedly increases.
The heat of the water which flows from deep borings (Artesian wells), direct
experiments regarding the temperature of rocks in mines, but, above all, the
volcanic activity of the Earth, shown by the flow of molten masses from open
fissures, afford unquestionable evidence of this increase for very
considerable depths from the upper strata. According to conclusions based
certainly upon mere analogies, this increase is probably much greater toward
the center.
That which has been learned by an ingenious analytic calculation, expressly
perfected for this class of investigations,*
p 173
regarding the motion of heat in homogeneous metallic spheroids, must be
applied with much caution to the actual character of our planet, considering
our present imperfect knowledge of the substances of which the Earth is
composed, the difference in the capacity of heat and in the conducting power
of different superimposed masses, and the chemical changes experienced by
solid and liquid masses from any enormous compression.
[footnote] *Here we must notice the admirable analytical labors of Fourier,
Biot, Laplace, Poisson, Duhamel, and Lame. In his 'Theorie Mathematique de
la Chaleur', 1835, p. 3, 428-430, 436, and 521-524 (see, also, De la Rive's
abstract in the 'Bibliotheque Universelle de Geneve', Poisson has developed
an hypothesis totally different from Fourier's view ('Theorie Analytique de
la Chaleur'.) He denies the present fluid state of the Earth's center; he
believes that "in cooling by radiation to the medium surrounding the Earth,
the parts which were first solidified sunk, and that by a double descending
and ascending current, the great inequality was lessened which would have
taken place in a solid body cooling from the surface." It seems more
probable to this great geometer that the solidification began in the parts
lying nearest to the center: "the phenomenon of the increase of heat with
the depth does not extend to the whole mass of the Earth, and is merely a
consequence of the motion of our planetary system in space, of which some
parts are of a very different temperature from others, in consequence of
stellar heat (chaleur stellaire)." Thus, according to Poisson, the warmth
of the water of our Artesian wells is merely that which has penetrated into
the Earth from without; and the Earth itself "might be regarded as in the
same circumstances as a mass of rock conveyed from the equator to the pole
in so short a time as not to have entirely cooled. The increase of
temperature in such a block would not extend to the central strata." The
physical doubts which have reasonably been entertained against this
extraordinary cosmical view (which attributes to the regions of space that
which probably is more dependent on the first transition of matter
condensing from the gaseo-fluid into the solid state) will be found
collected in Poggendorf's 'Annalen', bd. xxxix., s 93-100.
It is with the greatest difficulty that our powers of comprehension can
conceive the boundary line which divides the fluid mass of the interior from
the hardened mineral masses of the external surface, or the gradual increase
of the solid strata, and the condition of semi-fluidity of the earthy
substances, these being conditions to which known laws of hydraulics can
only apply under considerable modifications. The Sun and Moon, which cause
the sea to ebb and flow, most probably also affect these subterranean
depths. We may suppose that the periodic elevations and depressions of the
molten mass under the already solidified strata must have caused
inequalities in the vaulted surface from the force of pressure. The amount
and action of such oscillations must, however, be small; and if the relative
position of the attracting cosmical bodies may here also excite "spring
tides," it is certainly not to these, but to more powerful internal forces,
that we must ascribe the movements that shake the Earth's surface. There
are groups of phenomena to whose existence it is necessary to draw
attention, in order to indicate the universality of the influence of the
attraction of the Sun and Moon on the external and internal conditions of
the Earth, however little we may be able to determine the quantity of this
influence.
According to tolerably accordant experiments in Artesian wells, it has been
shown that the heat increases on an average about 1 degree for every 54.5
feet. If this increase can be reduced
p 174
to arithmetical relations, it will follow, as I have already observed,* that
a stratum of granite would be in a state of fusion at a depth of nearly
twenty-one geographical miles, or between four and five times the elevation
of the highest summit of the Hinalaya.
[footnote] *See the Introduction. This increase of temperature has been
found in the Puits de Grenelle, at Paris, at 58.3 feet; in the boring at the
new salt-works at Minden, almost 53.6; at Pregny, near Geneva, according to
Auguste de la Rive and Marcet, notwithstanding that the mouth of the boring
is 1609 feet above the level of the sea, it is also 53.6 feet. This
coincidence between the results of a method first proposed by Arago in the
year 1821 ('Annuaire du Bureau des Longitudes', 1835, p. 234), for three
different mines, of the absolute depths of 1794, 2231, and 725 feet
respectively, is remarkable. The two points on the Earth, lying at a small
vertical distance from each other, whose annual mean temperatures are most
accurately known, are probably at the spot on which the Paris Observatory
stands, and the Caves de l'Observatoire beneath it; the mean temperature of
the former is 51.5ºdegrees, and of the latter 53.3ºdegrees, the difference
being 1.8ºdegrees for 92 feet, or 1 degree for 51.77 feet. (Poisson,
'Theorie Math. de la Chaleur', p. 415 and 462.) In the course of the last
seventeen years, from causes not yet perfectly understood, but probably not
connected with the actual temperature of the caves, the thermometer standing
there has risen very nearly 0.4 degrees. Although in Artesian wells there
are sometimes slight errors from the lateral permeation of water, these
errors are less injurious to the accuracy of conclusions than those
resulting from currents of cold air, which are almost always present in
mines. The general result of Reich's great work on the temperature of the
mines in the Saxony mining districts gives a somewhat slower increase of the
terrestrial heat, or 1 degree to 76.3 feet. (Reich, 'Beob. uber die
Temperatur des Gesteins in verschielen en Tiefen', 1834, s. 134.) Phillips,
however, found (Pogg., 'Annalen', bd. xxxiv., s. 191), in a shaft of the
coal-mine of Monk-wearmouth, near Newcastle, in which, as I have already
remarked, excavations are going on at a depth of about 1500 feet below the
level of the sea, an increase of 1 degree to 59.06 feet, a result almost
identical with that found by Arago in the Puits de Grenell.
We must distinguish in our globe three different modes for the transmission
of heat. The first is periodic, and affects the temperature of the
terrestrial strata according as the heat penetrates from above downward or
from below upward, being influenced by the different positions of the Sun
and the seasons of the year. The second is likewise an effect of the Sun,
although extremely slow: a portion of the heat that has penetrated into the
equatorial regions moves in the interior of the globe toward the poles,
where it escapes into the atmosphere and the remoter regions of space. The
third mode of transmission is the slowest of all, and is derived from the
secular cooling of the globe, and from the small portion of the primitive
heat which is still being disengaged from the surface.
p 175
This loss experienced by the central heat must have been very considerable
in the earliest epochs of the Earth's revolutions, but within historical
periods it has hardly been appreciable by our instruments. The surface of
the Earth is therefore situated between the glowing heat of the inferior
strata and the universal regions of space, whose temperature is probably
below the freezing-point of mercury.
The periodic changes of temperature which have been occasioned on the
Earth's surface by the Sun's position and by meteorological processes, are
continued in its interior, although to a very inconsiderable depth. The
slow conducting power of the ground diminishes this loss of heat in the
winter, and is very favorable to deep-rooted trees. Points that lie at very
different depths on the same vertical line attain the maximum and minimum of
the imparted temperature at very different periods of time. The further
they are removed from the surface, the smaller is this difference between
the extremes. In the latitudes of our temperate zone (between 48 degrees
and 52 degrees), the stratum of invariable temperature is at a depth of from
59 to 64 feet, and at half that depth the oscillations of the thermometer,
from the influence of the seasons, scarcely amount to half a degree. In
tropical climates this invariable stratum is only one foot below the
surface, and this fact has been ingeniously made use of by Boussingault to
obtain a convenient, and as he believes, certain determination of the mean
temperature of the air of different places.*
[footnote] *Boussingault, 'Sur la Profondeus a laquelle se trouve la Couche
de Temperature invariable, entre les Tropiques', in the 'Annales de Chimie
et de Physique', t. liii., 1833, p. 225-247.
This mean temperature of the air at a fixed point, or at a group of
contiguous points on the surface, is to a certain degree the fundamental
element of the climate and agricultural relations of a district; but the
mean temperature of the whole surface is very different from that of the
globe itself. The questions so often agitated, whether the mean temperature
has experienced any considerable differences in the course of centuries,
whether the climate of a country has deteriorated, and whether the winters
have not become milder and the summers cooler, can only be answered by means
of the thermometer; this instrument has, however, scarcely been invented
more than two centuries and a half, and its scientific application hardly
dates back 120 years. The nature and novelty of the means interpose,
therefore, very narrow limits to our investigation regarding the temperature
p 176
of the air. It is quite otherwise, however, with the solution of the great
problem of the internal heat of the whole Earth. As we may judge of
uniformity of temperature from the unaltered time of vibration of a
pendulum, so we may also learn, from the unaltered rotatory velocity of the
Earth, the amount of stability in the mean temperature of our globe. This
insight into the relations between the 'length of the day' and the 'heat of
the Earth' is the result of one of the most brilliant applications of the
knowledge we had long possessed of the planet. The rotatory velocity of the
Earth depends on its volume; and since, by the gradual cooling of the mass
by radiation, the axis of rotation would become shorter, the rotatory
velocity would necessarily increase, and the length of the day diminish,
with a decrease of the temperature. From the comparison of the secular
inequalities in the motions of the Moon with the eclipses observed in
ancient times, it follows that, since the time of Hipparchus, that is, for
full 2000 years, the length of the day has certainly not diminished by the
hundredth part of a second. The decrease of the mean heat of the globe
during a period of 2000 years has not, therefore, taking the extremest
limits, diminished as much as 1/306th of a degree of Fahrenheit.*
[footnote] *Laplace, 'Exp. du Syst. du Monde', p. 229 and 263; 'Mecanique
Celeste', t. v., p. 18 and 72. It should be remarked that the fraction
1/306th of a degree of Fahrenheit of the mercurial thermometer, given in the
text as the limit of the stability of the Earth's temperature since the days
of Hipparchus, rests on the assumption that the dilation of the substances
of which the Earth is composed is equal to that of glass, that is to say,
1/18,000th for 1 degree. Regarding this hypothesis, see Arago in the
'Annuaire' for 1834, p. 177-190.
This invariability of form presupposes also a great invariability in the
distribution of relations of density in the interior of the globe. The
translatory movements, which occasion the eruptions of our present volcanoes
and of ferruginous lava, and the filling up of previously empty fissures and
cavities with dense masses of stone, are consequently only to be regarded as
slight superficial phenomena affecting merely one portion of the Earth's
crust, which, from their smallness when compared to the Earth's radius,
become wholly insignificant.
I have described the internal heat of our planet, both with reference to its
cause and distribution, almost solely from the results of Fourier's
admirable investigations. Poisson doubts the fact of the uninterrupted
increase of the Earth's heat
p 177
from the surface to the center, and is of opinion that all heat has
penetrated from without inward, and that the temperature of the globe
depends upon the very high or very low temperature of the regions of space
through which the solar temperature of the regions of space, through which
the solar system has moved. This hypothesis, imagined by one of the most
acute mathematicians of our time, has not satisfied physicists or
geologists, or scarcely indeed any one besides its author. But, whatever
may be the cause of the internal heat of our planet, and of its limited or
unlimited increase in deep strata, it leads us, in this general sketch of
nature, through the intimate connection of all primitive phenomena of
matter, and through the common bond by which molecular forces are united,
into the mysterious domain of magnetism. Changes of temperature call forth
magnetic and electric currents. Terrestrial magnetism, whose main
character, expressed in the three-fold manifestation of its forces, is
incessant periodic variability, is ascribed either to the heated mass of the
Earth itself,* or to those galvanic currents which we consider as
electricity in motion, that is, electricity moving in a closed circuit.**
[footnote] *William Gilbert, of Colchester, whom Galileo pronounced "great
to a degree that might be envied," said "magnus magnes ipse est globus
terrestris." He ridicules the magnetic mountains of Frascatori, the great
contemporary of Columbus, as being magnetic poles: "rejicienda est vulgaris
opinio de montibus magneticis, aut rupe aliqua magnetica, aut polo
phantastico a polo mundi distante." He assumes the declination of the
magnetic needle at any give point on the surface of the Earth to be
invariable (variatio uniuscujusque loci constans est), and refers the
curvatures of the isogonic lines to the configuration of continents and the
relative positions of sea basins, which possess a weaker magnetic force than
the solid masses rising above the ocean. (Gilbert, 'de Magnete', ed. 1633,
p. 42, 98, 152 and 155.)
[footnote] ** Gauss, 'Allgemcine Theorie des Erdmagnetismus', in the
'Resultate aux den Beob. des Magnet. Vereins', 1838, s. 41, p. 56.
The mysterious course of the magnetic needle is equally affected by time and
space, by the sun's course, and by changes of place on the Earth's surface.
Between the tropics, the hour of the day may be known by the direction of
the needle as well as by the oscillations of the barometer. It is affected
instantly, but only transiently, by the distant northern light as it shoots
from the pole, flashing in beams of colored light across the heavens. When
the uniform horary motion of the needle is disturbed by a magnetic storm,
the perturbation manifests itself 'simultaneously', in the strictest sense
of the word, over hundreds and thousands of miles of sea and land, or
propagates itself by degrees, in short intervals of time, in
p 178
every direction over the Earth's surface.*
[footnote] *There are also perturbations which are of a local character,
and do not extend themselves far, and are probably less deep-seated. Some
years ago I described a rare instance of this kind, in which an
extraordinary disturbance was felt in the mines at Freiberg, but was not
perceptible at Berlin. ('Lettre de M. de Humboldt a Son Altesse Royale le
Duc de Sussex sur les moyens propres a perfectionner la Connaissance du
Magnetisme Terrestre', in Becquerel's 'Traite Experimental de l'Electricite'
t. vii., p. 442.) Magnetic storms which were simultaneously felt from
Sicily to Upsala, did not extend from Upsala to Alten. (Gauss and Weber,
'Resultate des Magnet. Vereins', 1839, 128; Lloyd, in the 'Comptes Rendus de
l'Acad. des Sciences', t. xii., 1843, Sem. ii., p. 725 and 827.) Among the
numerous examples that have been recently observed, of perturbations
occurring simultaneously and extending over wide portions of the Earth's
surface, and which are collected in Sabine's important work ('Observ. on
Days of unusual Magnetic Disturbance', 1843), one of the most remarkable is
that of the 25th of September, 1841, which was observed at Toronto in
Canada, at the Cape of Good Hope, at Prague, and partially in Van Diemen's
Land. The English Sunday, on which it is deemed sinful, after midnight on
Saturday, to register an observation, and to follow out the great phenomena
of creation in their perfect development, interrupted the observations in
Van Diemen's Land, where in consequence of the difference of the longitude,
the magnetic storm fell on the Sunday. ('Observ.', p. xiv., 78, 85, and 87.)
In the former case, the simultaneous manifestation of the storm may serve,
within certain limitations, like Jupiter's satellites, fire-signals, and
well-observed falls of shooting stars, for the geographical determination of
degrees of longitude. We here recognize with astonishment that the
perturbations of two small magnetic needles, even if suspended at great
depths below the surface, can measure the distances apart at which they are
placed, teaching us, for instance, how far Kasan is situated east of
Gottingen or of the banks of the Seine. There are also districts in the
earth where the mariner, who has been enveloped for many days in mist,
without seeing either the sun or stars, and deprived of all means of
determining the time, may know with certainty, from the variations in the
inclination of the magnetic needle, whether he is at the north or the south
of the port he is desirous of entering.*
[footnote] *I have described, in Lametherie's 'Journal de Physique', 1804,
t. lix., p. 449, the application (alluded to in the text) of the magnetic
inclination to the determination of latitude along a coast running north and
south, and which, like that of Chili and Peru, is for a part of the year
enveloped in mist ('garua'). In the locality I have just mentioned, this
application is of the greater importance, because, in consequence of the
strong current running northward as far as to Cape Parena, navigators incur
a great loss of time if they approach the coast to the north of the haven
they are seeking. In the South Sea, from Callao de Lima harbor to Truxillo,
which differ from each other in latitude by 3 degrees 57' I have observed a
variation of the magnetic inclination amounting to 9 degrees (centesimal
division); and from Callao to Guayaquil, which differ in latitude by 9
degrees 50', a variation of 23.5 degrees. (See my 'Relat. Hist.', t. iii.,
p. 622.) At Guarmey (10 degrees 4' south lat.), Huaura (11 degrees 3' south
lat.), and Chancay (11 degrees 4' south lat.), Huaura (11 degrees 3' south
lat.), and Chancay (11 degrees 32' south lat.), the inclinations are 6.80
degrees, 9 degrees, and 10.35 degrees of the centesimal division. The
determination of position by means of the magnetic inclination has this
remarkable feature connected with it, that where the ship's course cuts the
isoclinalline almost perpendicularly, it is the only one that is independent
of all determination of time, and consequently, of observations of the sun
or stars. It is only lately that I discovered, for the first time, that as
early as at the close of the sixteenth century, and consequently hardly
twenty years after Robert Norman had invented the inclinatorium, William
Gilbert, in his great work, 'De Magnete', proposed to determine the latitude
by the inclination of the magnetic needle. Gilbert ('Physiologia Nova de
Magnete', lib. v., cap. 8, p. 200) commends the method as applicable "aëre
caliginoso." Edward Wright, in the introduction which he added to his
master's great work, describes this proposal as "worth much gold." As he
fell into the same error with Gilbert, of presuming that the isoclinal lines
coincided with the geographical parallel circles, and that the magnetic and
geographical equators were identical, he did not perceive that the proposed
method had only a local and very limited application.
p 179
When the needle, by its sudden disturbance in its horary course, indicates
the presence of a magnetic storm, we are still unfortunately ignorant
whether the seat of the disturbing cause is to be sought in the Earth itself
or in the upper regions of the atmosphere. If we regard the Earth as a true
magnet, we are obliged, according to the views entertained by Friedrich
Gauss (the acute propounder of a generaltheory of terrestrial magnetism), to
ascribe to every portion of the globe measuring one eighth of a cubic meter
(or 3 7/10ths of a French cubic foot) in volume, an average amount of
magnetism equal to that contained in a magnetic rod of 1 lb. weight.*
[footnote[ *Gauss and Weber, 'Resultate des Magnet. Vereins', 1838, 31, s.
146.
If iron and nickel, and probably, also, cobalt (but not chrome, as has long
been believed),* are the only substances which become permanently magnetic,
and retain polarity from a certain coerceive force, the phenomena of Arago's
magnetism of rotation and of Faraday's induced currents show, on the other
hand, that all telluric substances may possibly be made transitorily
magnetic.
According to Faraday ('London and Edinburgh Philosophical Magazine', 1836,
vol. viii., p. 178), pure cobalt is totally devoid of magnetic power. I
know, however, that other celebrated chemists (Heinrich Rose and Wohler) do
not admit this as absolutely certain. If out of two carefully-purified
masses of cobalt totally free from nickel, one appears altogether
non-magnetic (in a state of equilibrium), I think it probable that the other
owes its magnetic property to a want of purity; and this opinion coincides
with Faraday's view.
According to the experiments of the
p 180
first-mentioned of these great physicists, water, ice, glass, and carbon
affect the vibrations of the needle entirely in the same manner as mercury
in the rotation experiments.*
[footnote] *Arago, in the 'Annales de Chimie', t. xxxii., p. 214; Brewster,
'Treaties on Magnetism', 1837, p. 111; Baumgartner, in the 'Zeitschrift fur
Phys. und Mathem.', bd. ii., s. 419.
Almost all substances show themselves to be, in a certain degree, magnetic
when they are conductors, that is to say, when a current of electricity is
passing through them.