When the mind of man attempts to subject to itself the world of physical phænomena;—when in meditative contemplation of existing things he strives to penetrate the rich fulness of the life of nature, and the free or restricted operations of natural forces;—he feels himself raised to a height from whence, as he glances round the far horizon, details disappear, and groups or masses are alone beheld, in which the outlines of individual objects are rendered indistinct as by an effect of aerial perspective. This illustration is purposely selected in order to indicate the point of view from whence we design to consider the material universe, and to present it as the object of contemplation in both its divisions, celestial and terrestrial. I do not blind myself to the boldness of such an undertaking. Under all the forms of exposition to which these pages are devoted, the presentation of a general view of nature is the more difficult, because we must not permit ourselves to be overwhelmed by the development of the manifold and the multiform; but must dwell only on the consideration of masses, great either by actual magnitude, or by the place which they occupy in the subjective range of ideas.