§ 884-886. THE ACTINOMETRY OF THE SEA. 47 1 



How far below the sur- its Yight aiid lieat, it penetrates the sea to depths 



face does the heat of ° pi t 



the sun penetrate? morc or Icss profoiind, accordmg to the transpar- 

 ency of its waters. Let us, in imagination, divide these depths, 

 whatever they may be, into any number of stratifications or layers 

 of equal thickness. The direct heat of the sun is supposed to be 

 extinguished in the lowest layer ; the bottom layer, then, will re- 

 ceive and absorb the minimum amount of heat, the top the maxi- 

 mum ; consequently, each layer, as we go from the top to the bot- 

 tom, will receive less and less of the sun's heat. 



884. Kow, which will retain most heat and reach the highest 

 The stratum of warm- tcmperaturc ? Not the top layer, or that to which 

 est water. ^^^^^^ -^^^^ |g imparted, because by evaporation heat 

 is carried off from the surface of the sea almost as fast as by the 

 sun it is impressed upon the surface of the sea ; not the bottom 

 la3^er, because that receives a minimum, which, though it can not 

 escape by evaporation, may nevertheless fail to make any marked 

 change in temperature — fail, not by reason of no evaporation, but 

 by tho ever-changing movements which, considering the length of 

 time required to heat the lower stratum by such slow and grad- 

 ual accumulation of heat, would alter its place and vary its condi- 

 tion, and, indeed, remove it beyond the reach of the observer. 



885. The layer, therefore, which accumulates most heat and 

 Its position. bccomcs warmcst, should be neither at the bottom 



nor at the top, but intermediate, tlie exact temperature and depth 

 of which it is for observation to determine. To encourage such 

 determination and the investigations which it suggests is the main 

 object of this chapter. 



886. In conducting such investigations, several questions are 

 The different subjects ^^ be considcrcd, sucli as the transparency and 

 for observation. spccific gravity of the water, its i^^^osjphorescence ; 

 the face of the sky, whether clear or cloudy ; the state of the sea, 

 whether smooth or rough ; the condition of the weather, whether 

 calm or windy. Then the temperature should be tried, at various 

 depths and at various hours of the night and day, in order to as- 

 certain not only the maximum temperature and average depth of 

 the warmest stratum in the day, but the difference in its temper- 

 ature and position by day and by night. These observations will 

 afford the data, also, for computing the amount of solar heat that 

 penetrates the bosom of the sea, as well as the amount that is ra- 



