• The relation of these and other limitations to the consequences of the modern doctrine referring the mighty operations which have formed and modified our continents to causes now in action, acting also under their present conditions, and with only their present forces, has not been adequately considered by subsequent geologists. Dr. Conybeare's views on this subject, we conceive, in common with those taken by him on other important points in the philosophy of geology, have not received from his successors the consideration they deserve : a circumstance of which the fact of his own withdrawal from the pur suits of science for many of the later years of his life, may afford some explanation.
The subject of the preceding portion of this article is one of physical geography and geology. But we may regard the surface of the globe in a wider philosophical sense, not as confined to that of the land and sea-bed, but as the limit of the extension of the matter constituting our planet ; and therefore, also, of a certain class of the actions which maintain it, though not of all the forces to which that matter is subject, some of them extending beyond that limit. In this sense the earth has two surfaces, that of the land and water taken together, and that of the thin spherical shell of air that rests upon and encompasses it. The latter is the termination in space of our planet ; the former, the termination of the solid and liquid matter of which the earth, or at least its superficial crust, consists, whatever may be the physical state of the more central interior, of which we know nothing. These surfaces are the final results of the equilibrium which the mutually acting and reacting forces of nature maintain in the globe.
Thus considered, the subject has an interest of its own, independent of the geographical and geological importance of the surface of the land or solid matter. Though the waters form only a residual film, as it were, of certain materials of the crust which are liquid at the mean temperature of the surface, a result of the &lel, equilibrium of the chemical and physical actions taking place below and above, yet their portion of the surface, so much greater than that of the exposed solid matter, or land, has a paramount influence on the constitution of the terminal shell of matter—the atmosphere—with respect to its aqueous constituent. The atmosphere, in like manner, is tho resultant residue of tho physical and chemical actions taking place upon the terraqueous surface, or below, within reach of the atmosphere, but consisting of materials which are a8riform at the mean temperature of the globe; some, indeed, at all known temperatures; others within a certain range of them.
The definite surface of the terraqueous globe, there is reason to conclude, is repeated in that of the atmosphere, the finite extent of which we conceive to be demonstrated by that of the column of mercury or water, or other gravitating fluid in the barometer. It
would appear, on first considering the subject, that the only changes this surface can undergo will be extensive perpetual undulation, of the nature of tides, and disturbances occasioned by the entrance and transit, and perhaps by the ignition, of the bodies which become meteors. It may, however, be true, agreeably to the views of Poisson, in particular respects anticipated by Graham and by Luke Howard, and with which also harmonise certain inferences regarding the struc ture of the higher regions of the atmosphere originally drawn by Mr. Brayley, and virtually adopted by the late Professor Daniell—that the atmosphere has a terminal film of solid matter—frozen air— forming upon a subterminal one of liquid matter—liquefied air—itself passing below into a stratum of air-vapour, and that into a still inferior one of dense air retaining its gaseous condition ; the rarest stratum of the air being thus beneath the summit of the atmospheric column, and not constituting the upper surface, as long tacitly assumed, and as indeed would follow from the law of elasticity of the air considered by itself. In this case, in addition to the movements alluded to, the surface of the atmosphere must he in a state of perpetual equilibrated motion and change, the ultimate physical resultant of the continual subversion and restoration of equilibrium, or at least, of the existing tendencies to those conditions, arising from the antagonism of heat and gravitation.
The complex superficial crust of our planet, of which the surfaces of the atmosphere and of the terraqueous globe are the superior limits, is the final result of the action of the sun upon those surfaces, reacted to by the interior forces of the earth, which the sun, as it were, governs, both by its direct action, gravitative, calorific, light-giving, and magnetic ; and also indirectly, through that of the waters and the air upon the laud, the solid, liquid, and shiriform elements of the crust being thus all subject to its action ; while a certain amount of action is also exerted upon them by the radiation of the stars, the suns of other systems. On the surface of the earth, therefore, hi its most compre hensive sense, are inscribed the indelible and enduring records of all the activities, terrestrial, solar, and celestial, which have produced, and are perpetually reproducing it ; and on that of the torraqueous globe, thus maintained iu equilibrium, organic nature and man exist, the well-being of the latter being the end, or final cause, of the whole.