Page:Physical Geography of the Sea and its Meteorology.djvu/435

Rh or at the rate of once in twelve hours. Moreover, when it is recollected that these rains take place now here, now there; that the vapour of which they were formed was taken up at still other places, we shall be the better enabled to appreciate the force and effect of these irregular movements in the sea.

758. Ditto of cloud and sunshine.—Between the hottest hour of the day and the coldest hour of the night there is frequently a change of four degrees in the temperature of the sea. Let us, therefore, the more thoroughly to appreciate those agitations of the sea which take place in consequence of the diurnal changes in its temperature, call in the sunshine, the cloud without rain, with day and night, and their heating and radiating processes. And to make the case as strong as, with truth to nature, we may, let us again select one fifth of the Atlantic Ocean for the scene of operation. The day over it is clear, and the sun pours down his rays with their greatest intensity, and raises the temperature of the water two degrees. At night the clouds interpose, and prevent radiation from this fifth, whereas the remaining four fifths, which are supposed to have been screened by clouds, so as to cut off the heat of the sun during the day, are now looking up to the stars in a cloudless sky, and serve to lower the temperature of the surface waters, by radiation, two degrees. Here, then, is a difference of four degrees, which we will suppose extends only ten feet below the surface. The total and absolute change made in such a mass of sea water by altering its temperature two degrees is equivalent to a change in its volume of three hundred and ninety thousand millions of cubic feet. And yet there be philosophers who maintain (§ 123) that evaporation and precipitation, changes of temperature and saltness, and the secretions of insects, are not to be reckoned among the current-producing agents of the sea. That the gentle trade-winds do it all!

759. Day and night.—Do not the clouds, night and day, now present themselves to us in a new light? They are cogs, and pinions, and wheels in that grand and exquisite machinery which governs the sea, and which, amid all the jarring of the elements, preserves the harmonies of the ocean.

760. Logs overhauled for kelp arid ice.—The log-books of not less than 1843 vessels cruising on the polar side of 35° S. have, by the officers of the Observatory, been overhauled for kelp and