The mean temperature of countries is found to be very stable, and but very small variations have been detected in modern times.  But that there have been important climatic changes, since the Christian era, cannot be doubted, unless we doubt history.  Not many centuries ago, it was a common thing for all the British rivers to freeze up during the winter, and to remain so for several months.  If space permitted, an interesting statement could he made of the changes which have taken place in vegetation in Greenland, and throughout certain northern parts of Europe,—­also in Palestine, Greece, and other southern countries,—­while we know that the earth’s inclination upon its axis has been unchanged.

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 deg. below zero, or possibly, says Humboldt, 1400 deg. 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 deg. 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.