INTRODUCTION TO SOCIOLOGY 613
actual earth is a globe, having a radius of about 1,600 leagues, situated 37,000,000 of leagues from the sun, around which it revolves in 365 days. Its axis is inclined 67 to the plane of the ecliptic.
These form-limits of the earth result from relationships, past and present, between the internal mass in the course of its evolution and the external conditions, among which, and under the influence of which, these forms have been realized. Amo.ig the external forces which have contributed to limit the earth, and therefore to give it its general form, it is necessary to point out especially the other planets of the solar system. The limitation of the structure and movements of the solar system itself depends upon a vaster system. Moreover, since the discovery of Neptune, the frontiers of the solar system, becom- ing better known, extend more than one thousand millions of leagues. The movements, like the forms of the earth and the planets, are limited movements. All the planets move around the solar mass, executing at the same time rotary movements upon their own axes. All travel their eliptical orbits according to a general law of proportionality of the areas of portions of the elipse traveled over successively by the straight line which joins a planet to the sun, to the time employed in the traveling. The earth is a spheroid surrounded by a mass of air. This atmosphere is its real external covering, a covering which like- wise is limited by the nature of its own internal composition, in relationship with the terrestrial mass and by the nature of bodies exterior to the atmosphere. Each of the many geological strata, in its turn, is limited by its internal constitution in rela- tionship with the external strata. The earth and the celestial bodies in general are, then, formations limited in their structure and their movement in consequence of the reciprocal action of external forces upon central masses ; their equilibration results from this reciprocal action.
Upon the structure and the equilibrated movement of astro- nomical bodies there are dependent other special structures and movements which are likewise equilibrated. For example, the structure and the movement of the earth determine the distribu-
