THE POPULAR SCIENCE MONTHLY. JULY, 1880. THE INTERIOR OF THE EARTH.* Br R. RADAU. HE additions that are being continually made to our knowledge of the composition and physical condition of the most distant heavenly bodies may well prompt one to ask why we are still so poorly informed concerning the constitution of the planet which the Creator has assigned to us for a dwelling-place. Mines and wells have barely scratched the solid crust that conceals the mysteries of the earth's depths. Our vague and uncertain ideas regarding the condition of the interior of the earth are based on analogies and inductions from facts observed on its surface or in the heavens. Very little light do we get on this subject from direct experiment. The bowels of the earth are not, indeed, easily accessible. Whatever the poet may say, the descensus Averni is not easily made; the domain of the stars is not thus hidden from us. For about two centuries large sums have been expended in the construction of gigantic telescopes with which to sound the depths of space; but no attempt, as a purely scientific undertaking, has been made to fathom the secrets of the underground world. The object of the numerous mines in different parts of the world has been simply the discovery of mineral riches, and the depths they have reached barely exceed, even in a few rare instances, a thousand metres; i. e., hardly the six-thousandth of the earth's radiuscorresponding, on a globe thirteen metres † (about forty-two feet) in diameter, to a puncture one millimetre (about four one-hundredths of an inch) in depth. Notwithstanding this paucity of positive data, it will not be unin * Translated from the "Revue des Deux Mondes," by Guy B. Seely. VOL. XVII.-19 teresting to review the state of our knowledge on this obscure subject, and to show on what sides the question is accessible to science. The form of the planets is itself an index, to a certain point, of the mode of their origin and their actual condition. These slightly flattened globes that wheel about the sun have been subject to the same laws that shape the drop of water and the grain of shot. It is impossible not to believe that they are specimens on a vast scale of the equilibrated form assumed by free fluid masses through the action of internal forces which assemble and unite their molecules. All these spheroids have been or still are liquid drops that have become flattened by reason of their rotary motion. Newton was led to infer the flattening of the poles from the idea that the earth had originally been in a liquid state, as the centrifugal force due to rotation tends to swell the equatorial at the expense of the polar regions. By the operation of the same force that impels a stone when swung in a sling to free itself, and that causes grindstones to burst when turned too rapidly, the particles of a revolving sphere tend to fly from the axis of rotation, and this centrifugal force, nil at the poles, increases as the equator is approached, and there attains its maximum intensity. The effect of this is to diminish weight, substances being a little less heavy at the equator than at the poles. Imagine the earth completely liquid: the equatorial portion, driven by centrifugal force, will be elevated while the poles will be depressed. To better comprehend this, let us imagine a siphon, the two arms of which, joined at the center, issue, one at one of the poles and the other at the equator. The two liquid columns therein can remain in equilibrium, as the globe revolves, only on condition that the equatorial column, which is exposed to the action of centrifugal force, be longer than the polar column, which has lost nothing of its weight from this cause. The sphere becomes a flattened spheroid. This change of form can be demonstrated by turning rapidly on its vertical axis a sphere of clay or of flexible steel circles, as used in illustration of physics. As the pliant mass solidifies more or less completely, this flattened form is preserved. That there is a discrepancy between weight at the equator and at the poles, more marked as we approach or recede from one or the other, may be shown by noting, by the tension of a spring, the weight of the same mass under different latitudes; but a more positive means of ascertaining this fact is furnished by the oscillations of the pendulum, which are retarded as the force of the earth's attraction diminishes. The astronomer Richer, having been sent to Cayenne in 1672 to observe the planet Mars, remarked that a timepiece regulated at Paris lost ten and a half minutes daily at Cayenne. It was this circumstance, at first inexplicable, that led Newton to suspect that the earth was a flattened spheroid. It will be evident that an exact knowledge of the figure of the earth has an important bearing on any hypothesis of the internal constitution of our planet. Geodesy, that science that may be called surveying on a grand scale, and which takes for its bases of measurement at once the earth and the heavens, has not yet completed its work. Since the labors of the Abbot Picard, to whom we owe the first measurement of a meridional degree, and the celebrated voyages of Bouguer and La Condamine to Peru, and of Maupertuis to Lapland, which confirmed the supposition of the flattening of the earth, there have been many other immense labors of a similar kind all over the world. The Société Géodetique Internationale, organized some years ago, is occupied in compiling and perfecting the results of these researches, and in deducing therefrom a provisionally definite result. We know with certainty that the form of the earth is not greatly different from that of a perfect sphere, for the flattening ascertained by geodetic measurements is, in round numbers, equal to 6, from which it follows that the equatorial radius does not exceed the polar by more than twenty-two kilometres* (a little less than fourteen miles). This number, which represents the amount of the equatorial swelling, is equal to four and a half times the height of Mont Blanc, but, on a ball thirteen metres in diameter, the twenty-two kilometres in question would make an inequality of only two centimetres (about three fourths of an inch), and this would be totally imperceptible to the eye. The natural inequalities of the earth's surface are comparatively insignificant; the Alps and Himalayas, on a ball thirteen metres in diameter, would be represented by projections of a few millimetres only, and the greatest ocean-depths would not exceed one centimetre. The question of the true figure of the earth is one of the most difficult of problems. From the time of Newton it had been held that the earth was a revolving ellipsoid-in other words, that the meridians were ellipses, and the equator and all the parallels true circles; and it only remained to determine the ellipticity of these meridians, all being supposed alike. It is now twenty years since Captain Clark's calculalations, based on the uniformity of the great triangulations made up to that period in various parts of the world, led to the conclusion that the equator itself has an elliptic form, and that, consequently, the meridians are ellipses unequally flattened. According to Clark, the equatorial flattening is, or about one tenth of the average flattening of the meridians. This depression, amounting to two kilometres, occurs under the meridian passing, in the east, through the Sunda Archipelago, and in the west through the Isthmus of Panama, while the enlargement occurs under the meridian of Vienna, crossing central Europe and Africa. Thus, according to the calculations, the world is an ellipsoid with three unequal axes. This supposition can be made to harmonize with the hypothesis of the primitive fluidity of the earth, the form in question being one of those assumed by free liquids in * The length of a kilometre is about five eighths of a mile. rotation. It was found, however, that Clark's calculations were considerably affected by certain anomalies probably existing in some of the geodetic calculations employed, and it seems that a majority of those competent to judge in these matters endorse the theory of a revolving ellipsoid. By the term "figure of the earth" is understood the geometrical form of an ideal surface coinciding with the mean level of the sea, and prolonged in thought beneath the continents. In fact, geodetic calculations are always reduced to the sea-level, the altitudes of the stations being first determined from levels based on the nearest coast-line. The great difficulty is to accurately determine this level for a given station. For a long time it was supposed that the surface of the open sea was a horizontal; in other words, that it was parallel to the surface of liquids in repose, and perpendicular to the direction of the plummetline. But this definition is insufficient, as may easily be shown. The apparent vertical indicated by the plummet-line or determined at the level of the sea, is simply the direction of weight, which may be materially affected by local attractions due to an irregular distribution of the masses composing the soil. The vicinity of a mountain will deflect the plummet to a considerable degree, and a subterranean cavity may cause a deflection in the opposite way. Let us now imagine the continents divided by a network of canals that connect all the seas, thus making of them one continuous sheet of water, as it were. Setting aside, for the purpose of the illustration, the oscillations caused by the tides, this sheet of water, assumed to be immovable, which represents the mean level of the sea, will exhibit elevations and depressions attributable to the local influences that deflect the plummet-line. The attraction of the continents causes a notable elevation of the sea-level along the coast, and a proportionate lowering of the mid-ocean. This influence of continents was described by M. Saigey in 1842, who gave as the probable height of the sea on the coasts of Europe thirty-six metres. Seven years later Mr. Stokes, the celebrated English physicist, attacked the question, bringing to bear upon it all the resources of mathematical analysis; and Philipp Fischer, in 1868, estimated that the disturbance of level due to the attraction of continents might amount to nine hundred metres. The mean level of the sea is, therefore, in all probability, an irregularly undulated surface, and the ideal or geometrical surface of the earth a regular spheroid, deviating but little from this average level, the accidental irreg ularity of which is in some way equalized. The triangulations by which the terrestrial arcs are measured define the dimensions and configuration of this spheroid by the comparison of distances measured on the earth with the corresponding angular amplitude ascertained from the astronomical latitudes and longitudes of the stations. The most delicate part of the operations consists in ascertaining the local attractions that cause the deviations of the plum |