Geophysics

“ The material crust of the earth rests on this placid sphere of vigor, excepting in a few places, where, as in the crevice we have entered, gaps, or crevices, in matter exist, beginning from near the outer surface and extending diagonally through the medial and inner spheres into the intra-earth space beyond. This medial sphere is a form of pure force, a disturbance of motion, and although without weight it induces, or conserves, gravity. It is invisible to mortal eyes, and is frictionless, but really is the bone of the earth. ”

“ There is no conceivable situation in which gases could be expected to force subterranean water in a direction opposed to gravity in such quantities as to satisfy the conditions.
Is it possible, then, to point out any other force in nature which not only may, but which positively must, force waters out of springs at high elevations?
Let a, a, a, be a great circle of the earth attained by passing a plane through the earth's center C, perpendicular to its axis, and b, b, b, the circle cut by the same plane through the inner surface of the earth's supposed crust. ”

“ The laws of conservation may not be violated: the earth will respond.
How are the Spheres constructed?
The constitution of these two retro-acting spheres, and consequently of all the others of [Pg 26] the heavenly host, [3] at this point demands our attention. How are the spheres made up? How speaks the earth? The earth with which we are familiar—our sample—is formed of a slight crust, a core, to a greater or less extent and degree incandescent, and measuring 250,000 millions of cubic miles in dimensions, also an envelope which we call the atmosphere. ”

“ If we now assume that all rocks flow tinder pressure, that the critical point is variable and that the modulus of compression is also variable, being greater for the lighter rocks and less for the heavier, and that this modulus is greatly accelerated at the critical point, we have a law which will regiment the facts of geonomy as the facts of astronomy are marshaled by the law of gravity. ”

“ According to the field-concept, terrestrial manifestations of gravity are due to the earth's being the bearer of a gravitational field centred within the globe, and extending thence in all directions through space, across and beyond the earth's body. Every point in space, both inside and outside the earth, is characterized by a definite intensity of this field, the so-called gravitational potential. ”

“ Without entering into this difficult problem in physical geology, it may suffice if we carry with us the idea that our globe must once have had a greater diameter than it now possesses, and that the crumpling of its outer layers, whether due to mere contraction or, as has been suggested, to the escape also of subterranean vapors, affords evidence of this diminution. A little reflection suffices to show us that, even without any knowledge of the actual history of the contraction, we might anticipate that the effects would neither be continuous nor everywhere uniform. ”

“ Methods have been devised of measuring the attractive force of the whole globe, compared to that of some of its parts, certain mountains, for instance, and thus poising its mass against some known weight; and these methods, confirmed by astronomical inferences leave no doubt that the density and specific gravity of the globe is above five times as great as that of water at common temperatures and pressures. ”

“ Even the earth itself is composed, as geologists tell us, of various strata, composed of different substances, and varying from one degree of density to another. If every part of our globe be regulated according to the received laws of gravity, and the relative density of matter, why do we find almost all over the world, light alluvial soil in the vallies and [55] plains; and on the tops of the highest mountains, the more heavy granite, and some of the heaviest substances that nature knows? ”

“ The spheres, in many parts of the unfathomable ocean, is believed to be water quite through from the concave or convex surfaces to the great mid-plane-space, and probably the earthy or solid matter of the sphere, may in many places extend quite through from one surface to the [94] other, tending, like ribs or braces, to support the sphere in its proper form. Such a formation of spheres appears to be supported by various facts and phenomena; amongst the most prominent of which are Volcanoes and Earthquakes. ”

“ An argument for the Earth’s convexity is thought by many to be found in the following facts:—“Fluid or semi-fluid substances in a state of motion invariably assume the globular form, as rain, hail, dew, mercury, and melted lead, which, poured from a great height becomes divided into spherical masses, as in the manufacture of small shot, &c.” “There is abundant evidence from geology that the Earth has been a fluid or semi-fluid mass, and it could not, therefore, continue in a state of motion through space without becoming spherical.” ”

“ The remark is just: but although the dimensions of such a crust are, in truth, insignificant when compared to the entire globe, yet they are vast, and of magnificent extent in relation to man, and to the organic beings which people our globe. Referring to this standard of magnitude, the geologist may admire the ample limits of his domain, and admit, at the same time, that not only the exterior of the planet, but the entire earth, is but an atom in the midst of the countless worlds surveyed by the astronomer. ”

“ That the earth is not solid to the centre, nor is it possible that it could be, according to the law of attraction, but is a hollow sphere.
(2) That its greatest density must be at the region where the greatest mass of matter is to be found—as must have been always the case from the time it was a globe revolving on its axis, whether gasiform, liquid, or solid—which is now at 817 miles deep from the surface; and that the greatest density may not be much more than the mean of 5·66 times that of water ascribed to it by astronomers. ”

“ All the facts in any part of nature shall be tabulated, and the results shall indicate the same security and benefit; so slight as to be hardly observable, and yet it is there. The sphere is flattened at the poles, and swelled at the equator; a form flowing necessarily from the fluid state, yet the form, the mathematician assures us, required to prevent the protuberances of the continent, or even of lesser mountains cast up at any time by earthquakes, from continually deranging the axis of the earth. ”

“ The circumference of the earth at the equator, 40,000 kilometres. Surface of the earth, approximate estimate, 510,000,000 of square kilometers. Bulk of the earth, about 1,000 millards of cubic kilometres; that is, a cube having a metre in length, height, and thickness. Density of the earth, about five times that of the water. Time of the earth on the orbit around the sun, 365 days, 6 hours, 9 minutes, 10 seconds, 37 centimes. This gives the globe a speed of 30,400 miles travelled over by the rotation of the earth upon its axis. ”

“ The centre of gravity of the hemispherical base is, of course, in its axis; and therefore tends to approach the horizontal plane as much as possible, and this can never be accomplished, until the axis becomes perpendicular to the horizon. Whenever the curved surface is in any other position, the centre of gravity is not in the lowest place to which it can descend, as may be seen by the diagram which I have just sketched. ”

“ He also proved that the increase of gravity in proceeding from the equator to the poles is as the square of the sine of the latitude. This, taken with the former theorem, gives the means of determining the earth’s ellipticity from observation of the relative force of gravity at any two places. ”

“ Or rather, the portions of the labors of geologists which do merit this praise, belong to the two preceding divisions of the subject, and have been treated of there.
The history of Physical Geology, considered as the advance towards a science as real and stable as those which we have already treated of (and this is the form in which we ought to trace it) , hitherto consists of few steps. We hardly know whether the progress is begun. The history of Physical Astronomy almost commences with Newton, and few persons will venture to assert that the Newton of Geology has yet appeared. ”

“ Under dynamical geology are included important subdivisions, such as seismic geology, or the study of earthquakes, and vulcanology, or the study of volcanoes. Another large subject, glacial geology, belongs within the broad frontier common to both dynamical geology and physiography. A relatively new subdivision of geological science is orientational geology, which is concerned with the trend of earth features, and is closely related both to physiography and to dynamical and structural geology. ”

“ The surface in contact with the under rock is of very small extent, and the whole mass [104] is so nicely balanced, that, notwithstanding its magnitude, the strength of a single man applied to its under edge is sufficient to change its centre of gravity, and though at first in a degree scarcely perceptible, yet the repetition of such impulses, at each return of the stone, produces at length a very sensible oscillation! ”

“ The method assumes that gravity is measured on the earth’s surface at a sufficient number of points, and that it is conformably reduced. In order to secure the convergence of the expansions in spherical harmonics, it is necessary to assume all masses outside a surface parallel to the surface of the sea at a depth of 21 km. ”

“ The elevation of continents is still progressing slowly, and is being compensated for at some definite points by a perceptible sinking. All geognostic phenomena indicate a periodical alteration of activity in the interior of our planet. Probability of new elevations of ridges — p. 293-301 and notes.
g. The solid surface of the earth has two envelopes, one liquid, and the other aeriform. Contrasts and analogies which these envelopes — the sea and the atmosphere — present in their conditions of aggregation and electricity, and in their relations of currents and temperature. ”