The American Quarterly Review, No. 17, March 1831

by Various

Art. IV.—Précis de la Geographie Universelle ou Description de toutes les parties du Monde, sur un plan Nouveau D'aprês les grandes divisions Naturelles du Globe, &c. Par Malte-Brun: Bruxelles, 1829.

We place at the head of our article, which we mean to devote to Physical Geography, the title of the latest edition that we have seen of the great work of Malte-Brun. This, which has already become well known to our American public in translation, has received some additions from its Belgian editors, but has not been fully brought up to the present state of Science, nor does it contain all the new discoveries which have been made in that part, namely, physical geography, to which our attention is more immediately directed. We shall, however, endeavour to supply these deficiencies so far as lies in our power.

Physical geography stands in immediate connexion with subjects which have already been presented to the readers of this journal, namely with Celestial Mechanics,[8] and with the Phenomena of our Atmosphere.[9] It shall be our endeavour to proceed from the facts laid down in the first of the two articles to which we have referred, to the more particular consideration of the state, the structure, and the condition of the globe we inhabit.

The earth is a planet of the solar system, the third in distance from the sun, revolving upon its own axis, and around that central body attended by a satellite; circumstances which affect in a most important manner the phenomena that are observed upon its surface. Composed of material substances that mutually attract each other, each particle of which has a greater or less centrifugal force in proportion to its distance from the axis of rotation, it has a figure that is consistent with a state of equilibrium under the joint action of these two forces, and which is such as would have been assumed by a fluid body actuated by them. The figure that fulfils these conditions is an oblate spheroid, the axis of the generating ellipse coinciding with the polar diameter of the body. Had the earth a figure absolutely spherical, or less flattened than is consistent with the conditions of equilibrium, the ocean, by which so large a part of its surface is covered, would have arranged itself in a meniscoid zone around its equatorial regions; were the figure, on the other hand, one of greater oblateness, the waters would have been divided and accumulated at either pole, leaving the equatorial regions dry. But did its figure fulfil the conditions of equilibrium, the fluid mass would tend to distribute itself equally over the whole surface, unless prevented by irregularities in the solid mass. The last is the actual state of things; the ocean occupies a bed formed of cavities, lying below the mean surface of the spheroid, and the land presents to us those asperities and elevations, which rise, although to a comparatively small height, above the general level.

Was then the earth originally in a fluid state, and has it assumed its present form under the strict action of mechanical laws, on a body of that class? are the bed of the ocean and the continents merely crusts formed upon the surface of a liquid globe? Does the interior still remain liquid, or has the induration proceeded until the whole internal mass has become solid? Nay, may not the interior be hollow, as we have recently seen gravely maintained, and heard sage legislatures recommend to the public attention?

Mathematical investigations of incontrovertible evidence, show us that were the earth of equal density throughout, the flattening at the poles would be 1/234 of the equatorial diameter; that in the hypothetical case of infinite density at the centre, and infinite rarity at the surface, the flattening would be no more than 1/578; while, were the surface more dense than the interior, or did a cavity exist within, the oblateness must be greater than 1/234. Actual measurements of portions of the surface, the variation in the length of the pendulum which beats seconds in different latitudes, and the effect of the earth's figure on the lunar motions, show us that the earth cannot be flattened more than 1/289, nor less than 1/312, or may, at a mean, be considered as a spheroid, whose polar and equatorial diameters are in the relation of 299 to 300.

Astronomers have ascertained the deflection of plumb lines from the vertical, by the action of mountains. The attraction of a projecting mass of known bulk and density, with one whose bulk alone is known, is thus determined, and hence the density of the latter may be calculated.

Even comparatively small masses of matter may be placed under such circumstances at the surface of the earth, that their mutual action can be observed uninfluenced by the preponderating attraction of the earth, and thus a new means of comparison obtained.

The pendulum whose vibrations ought to vary according to a definite law, as we recede from the surface of the earth, has that law affected by the elevated ground on which it is placed, and here again a comparison may be instituted between the general and local attractions.

All these modes of investigation concur in, and confirm the general result, that the mean density of the earth is about five times as great as that of water. Now as a great portion of the surface is composed of that fluid, and as the general density of the land is little more than twice as great as that of water, it follows incontestably that the interior of the earth is far more dense than its outer covering.