TIDE-RIl'S AND SEA DRIFT. 409 



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 np at still 

 other places, we shall be the better enabled to appreciate the 

 force and eifect 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 sei*ve' to lower the tempe- 

 rature of the surface waters, by radiation, two degrees. Here, 

 then, is a difi'erence 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 temj)e- 

 rature 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 



* Vide Admiral Smyth's Memoir of tlie Mediterranean, p. 125. 



