394 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



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 tempe- 

 rature 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-pro- 

 ducing agents of the sea. That the gentle trade-winds do it all ! 



759. .Do not the clouds, night and day, now present themselves 

 Day and night, to US in a uew 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 

 tlie harmonies of the ocean. 



760. The log-books of not less than 1843 vessels cruising on 

 Lo£?s overhauled for the polar sido of 35° S. havo, by the officers of the 

 k.^;p and ice. Observatory, been overhauled for kelp and ice. Of 

 tiiese, 367 (or one in five) mentioned kelp or sea-weed east of 

 Cape Horn ; 142 mention " rock-weed and drift matter" between 

 the previous meridian and lO'^ W., and chiefly between 35^ and 

 40^ S. '' Long kelp " is also found by Australian traders after 

 passing the Cape of Good Hope ; 146 logs make mention of it 

 between the meridians of 40^ and 120"^ E. It most abounds 

 along this line, however, between the meridians of 45"^ and 65^ 

 Ju,, and the parallels of 42^ and 48^ S. These sargassos are 

 t^ketched with a free hand on Plate IX. 



761. Sea-weed is frequently mentioned also by the homeward- 

 A sargasso in the bouud Australian traders on their way to Cape 

 Kouth Pacific. Horn: this collection has (§ 139) already been 

 alluded to. It now appears that instead of three, as stated in 

 former editions of this work, there are really five true sargassos, 

 as shown on Plate IX. 



