THE ACTINOMETRY OF THE SEA. 471 



not tlie bottom layer, because that receives a minimum, which, 

 though it cannot escape by evaporation, may nevertheless fail to 

 make any marked change in temperature — fail, not by reason of 

 no evaporation, but by the ever-changing movements which, con- 

 sidering the length of time required to heat the lower stratum 

 by such slow and gradual accumulation of heat, would alter its 

 place and vary its condition, and indeed removing it beyond the 

 reach of the observer. 



887. Its position. — The layer, therefore, which acciunulates 

 most heat and becomes warmest, should be neither at the bottom 

 nor at the top, but intermediate, the 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. 



888. T}ie different subjects for ohservation. — In conducting such 

 investigations, several questions are to be considered, such as the 

 transparency and specific gravity of the water, its pliospliorescence ; 

 the face of the sk}-, whether clear or cloudy ; the state of the sea, 

 whether rough or smooth ; 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 ascertain not only the maximum temperature and 

 average depth of the warmest stratum in the day, but the dif- 

 ference in its temperature 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 radiated thence again. They will reveal 

 to us knowledge concerning its actinometry in other aspects. 

 We shall learn how absorption by, as well as radiation from, the 

 under strata is affected b}^ a rough sea, as when the waves are 

 leaping and tossing their white caps, and how by its glassy 

 surface, as when the winds are hushed aud the sea smooth. 



889. Expected discoveries. — Here we are reminded, also, to 

 anticipate the discovery of new beauties and fresh charms among 

 the wonders of the sea. We have seen (§ 366) that while the 

 heat of the sun is impressed alike upon sea and land, never- 

 theless the solid part of the earth's crust radiates much more 

 freely than the fluid. On the land the direct heat of the sun 

 operates only upon a mere shell a few inches in thickness; at 

 sea it penetrates into the depths below, and operates upon a 

 layer of water many feet thick. The solid land-crust has its 



