GEOGRAPHY, is that science whiclk exhibits the results of our investigations respecting the planet we inhabit, whether we consider its figure and the disposition of the lands and water upon its surface, or the subdivisions which the different nations who inhabit it have made, by which it is considered as forming king doms and states.
The general curvature of the earth's surface is easily observable in the disap pearance of distant objects ; and, in par ticular, when the view is limited by the sea, the surface of which, from the com mon property of a fluid, becomes natu rally smooth and horizontal ; for it is well known that the sails and rigging of a ship come into view long before her hull, and that each part is the sooner seen as the eye is more elevated: On shore the frequent inequalities of the solid parts of the earth usually cause the prospect to be bounded by some ir regular prominence, as a hill, a tree, or a building, so that the general curvature is the less observable.
The surface of a lake, or sea, must be always perpendicular to the direction of a plumb line, which may be considered as the direction of the force of gravity ; and by means either of a plumb line, or of a spirit level, we may ascerain the angular situation of any part of the earth's sur face with respect to a fixed star passing the meridian : by going a little further north or south, and repeating the obser vation on the star, we may findthe differ ence of the inclination of the surfaces at both points; of course, supposing the earth a sphere, this difference in latitude will be the angle, subtended at its centre by the given portion of the surface, whence the whole circumference may be determined, and on these principles the earliest measurements of the earth were conducted. The first of these which can be considered as accurate, was executed by Picart, in France, towards the end of the seventeenth century.
But the spherical form is only an ap proximation to the truth. It was calcu lated by Newton, and ascertained expe rimentally by the French academicians, sent to the equator and to the polar cir cle, that, in order to represent the earth, the sphere must be flattened at the poles, and prominent at the equator. We may
therefore consider the earth as an oblate elliptic spheroid ; the curvature being greater, and consequently every degree shorter at the equator, than nearer the poles. if the density of the earth were uniform throughout, its ellipticity, or the difference of the length of its diameters, would be of the whole ; on the other hand, if it consisted of matter of incon siderable density, attracted by an infinite force in the centre, the ellipticity would be only ; and whatever may be the internal structure of the earth, its form must be between these limits, since its internal parts must necessarily`be denser than those parts which are nearer the surface. If, indeed, the earth consisted of water or ice, equally compressible with common water or ice, and following the same laws of compression with elastic fluids, its density would be several thou sand times greater at the centre than at the surface ; and even steel would be compressed into one-fourth of its bulk, and stone into one-eighth, if it were con tinued to the earth's centre : so that there can be no doubt but that the central parts of the earth must be much more dense than the superficial.
Whatever this difference may be, it has been demonstrated by Clairaut, that the fractions expressing the ellipticity, and the apparent diminution of gravity at the equator, must always make toge ther ; and it has been found, by-the- most accurate observations on the lengths of pendulums in different latitudes, that the force of gravity is less powerful by at the equator than at the pole, Tirtl whence the ellipticity is found to be of the equatorial diameter ; the form be ing the same as would be produced, if about three-eighths of the whole force of gravity were directed tow,irds a central particle, the density of the rest of the earth being uniform.