Page:Popular Science Monthly Volume 17.djvu/305

Rh 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, 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 calculations, 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 Sun da 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