Mrs. Somerville remarks, that, though the temperature of any one place may be subject to very great variations, yet it never differs from the mean state more than a few degrees. Without this atmospheric covering of ours, it is considered that the temperature of the earth at its surface would be the same as that of the celestial spaces, supposed to be at least 76° below zero, or _possibly_, says Humboldt, 1400° below! Human life, without our atmosphere, could not exist for a single moment. It is computed, that, if the annual heat received by the earth on its surface could be equally distributed over it, it would melt, in the course of a year, a stratum of ice 46 feet thick, though it covered the whole globe, and as a consequence the amount of unradiated heat would render it uninhabitable. The relative position of the sun affects temperature, rather than its distance. In winter the earth is three millions of miles nearer the sun than in summer, but the oblique rays of the former season reach us in less quantity than the more direct The distribution of land and water, the nature of the soil, the indentation of bays, the elevation of land above the sea-level, insularity, etc., all, as we have already suggested, have a modifying influence on temperature. The atmosphere possesses also a reflecting and refracting power, arising from its varying density, and, perhaps, in the latter case, somewhat from its lenticular outline. But for this property we should have no twilight. The sun, instead of sending up his beams while 18° below the visible horizon, would come upon us out of an intense darkness, pass over our sky a brazen inglorious orb, and set in an instant amid unwelcome night. Reflection is the rebound of the rays of light or heat from an opposing surface at the same angle as that at which they fall upon it. These are called angles of incidence and reflection, and are equal. Refraction is the bending of a ray passing obliquely from a rarer into a denser medium. This may be observed when a rod is placed slantingly in a vessel of clear water; the part immersed will appear bent or broken. This is ordinary refraction. Terrestrial refraction is the same thing, occurring whenever there is a difference of density in the aerial strata. The atmosphere absorbs some portion of the light which it receives. It is not all reflected or refracted or even penetrative. Objects seen under various degrees of light, either convected or retarded by different media, appear near or distant, distinct or confused. Thus, we are often surprised at the apparent nearness and brightness of an opposite shore or neighboring island, in some conditions of the air, while at other times they seem distant and lie in shadowy obscurity. The looming up of a vessel on the water is another common instance of the principle of refraction.