Again, everybody knows that winter and summer, and the other phenomena which simulate or accompany them, such as wet and dry seasons, depend upon the fact that the earth's axis is not perpendicular to the plane in which the earth moves round the sun, but slightly inclined to it. Now, a year in itself, viewed as a measure of time, is merely the period which it takes the earth to perform one such complete revolution. During one half of each such revolution the north pole is turned at a considerable angle toward the sun, and during the other half, the south pole. When the north pole is so turned we call it summer in the northern hemisphere; when the south pole is being favored, and the north is receiving less light and heat, we call it winter. Let us suppose for a moment that the earth had not got this twist or kink in its axis; that the equator was always presented exactly toward the sun; what then would happen? Obviously, there would be no change of seasons. The day and night would have fixed lengths which never varied; climate would in each place be uniform and, barring accidents of elevation or distribution of land and water, the climate of each place would also depend entirely the whole year round on its distance from the equator. Roughly speaking, the temperature of a district would be the temperature it now possesses in March and September, only not quite so cold as March nor so warm as September, owing to the absence of accumulated heat from summer or of reserves of ice and snow from winter. In one word, under such conditions there would have been climates--marked belts of climate; but there would not have been seasons.