_The Unknown_.--I shall willingly communicate to you my views of the operation of time, philosophically considered. A great philosopher has said, man can in no other way command Nature but in obeying her laws; and, in these laws, the principle of change is a principle of life; without decay, there can be no reproduction; and everything belonging to the earth, whether in its primitive state, or modified by human hands, is submitted to certain and immutable laws of destruction, as permanent and universal as those which produce the planetary motions. The property which, as far as our experience extends, universally belongs to matter, gravitation, is the first and most general cause of change in our terrestrial system; and, whilst it preserves the great mass of the globe in a uniform state, its influence is continually producing alterations upon the surface. The water, raised in vapour by the solar heat, is precipitated by the cool air in the atmosphere; it is carried down by gravitation to the surface, and gains its mechanical force from this law. Whatever is elevated above the superfices by the powers of vegetation or animal life, or by the efforts of man, by gravitation constantly tends to the common centre of attraction; and the great reason of the duration of the pyramid above all other forms is, that it is most fitted to resist the force of gravitation. The arch, the pillar, and all perpendicular constructions, are liable to fall when a degradation from chemical or mechanical causes takes place in their inferior parts. The forms upon the surface of the globe are preserved from the influence of gravitation by the attraction of cohesion, or by chemical attraction; but if their parts had freedom of motion, they would all be levelled by this power, gravitation, and the globe would appear as a plane and smooth oblate spheroid, flattened at the poles. The attraction of cohesion or chemical attraction, in its most energetic state, is not liable to be destroyed by gravitation; this power only assists the agencies of other causes of degradation. Attraction, of whatever kind, tends, as it were, to produce rest--a sort of eternal sleep in Nature. The great antagonist power is heat. By the influence of the sun the globe is exposed to great varieties of temperature; an addition of heat expands bodies, and an abstraction of heat causes them to contract; by variation of heat, certain kinds of matter are rendered fluid, or elastic, and changes from fluids into solids, or from solids or fluids into elastic substances, and _vice versa_, are produced; and all these phenomena are connected with alterations tending to the decay or destruction of bodies. It is not probable that the mere contraction or expansion of a solid, from the subtraction or addition of heat, tends to loosen its parts; but if water exists in these parts, then its expansion, either in becoming vapour or ice, tends not only to diminish their cohesion, but to break them into