from the north-east trade-wind air; the latter, therefore, after rising up, is the cooler and the more compact; and as, by the theory of the crossings, it flows off to the south as an tipper current, it presses upon the barometer with more weight than the warmer and more moist air that feeds the current which is above and counter to the north-east trades. There is not in the whole range of marine meteorology a single well-established fact that is inconsistent with the theory of a crossing at the calm belts.
645. Cataclysms.—The geological record affords evidence that the climates of the earth were once very different from what they are now; that at one time intertropical climates extended far up towards the north; at another time polar climates reached, with their icebergs and their drift, far down towards the equator; that in remote ages most of what we now call dry land was covered with water, for we find on the mountains and far away in the interior of continents deposits many feet thick, consisting of sea-shells, marine animals, and organic productions of many sorts. These fossils, marks, and traces indicate that since their day, ages inconceivably great have elapsed. Not only so: the lines of drift, and boulders, and gashes with which the earth is scored and strewed, afford reason for the conjecture that there have been cataclysms, in which the waters have swept from north to south, and again from south to north, bearing with them icebergs, huge blocks of stone, rubble, drift, and sediment of various sorts. Lieutenant Julien, M. Le Hon, and M. Adhémar have, with much ingenuity, treated of this subject. They maintain that our earth has a " secular" as well as an annual summer and winter; that these "secular" seasons depend upon the precession of the equinoxes, and that the length of each is consequently 10,500 of our years; and that it is the melting of the polar ices in the " secular" season of one hemisphere, and their recongelation in the "secular" winter of the other, that causes a rush of the sea from one hemisphere into the other; and so cataclysms are produced at regular intervals of 10,500 years. In consequence of the inclination of the axis of the earth to the plane of its orbit, we have our change of seasons; and in consequence of the ellipticity of that orbit, the spring and summer of our hemisphere are at present longer than those of the southern. During the excess of time that the sun tarries on our side of the equator, the southern nights are prolonged, so that the night of
