454 PHYSICAL GEOGRAPHY OP THE SEA, AND ITS METEOROLOGY. 



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

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

 determination and the investigations which it suggests is the main 

 object of this chapter. 



888. In conducting such investigations, several questions are 

 The diiferent subjects to bo considcrcd, such as the transparency and 

 for observation. spccific gTavitj of the Water, its i:>]iosioliorescence ; 

 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 mndy. Then the temperature should be tried, at various 

 depths and at various liom's of the night and day, in order to ascer- 

 tain not only the maximum temperature and average depth of the 

 warmest stratum in the day, but the difference 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 by 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 and the sea smooth. 



889. Here we are reminded, also, to anticipate the discovery- 

 Expected discoveries, of uew bcautics 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, nevertheless 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 temperature raised high by day and cooled low 

 doTMi by night ; but the most powerful sun, after beating down 

 all day with its fiercest intensity upon this liquid covering, has 

 not power to raise its temperatme more than three or four degrees. 

 This covering serves as a reservoir for the solar heat. In the depths 

 below it is concealed from the powers of intense radiation, and held 

 by the obedient ocean in readiness to be brought to the sm-face from 

 time to time, and as the winds and the clouds call for it. Here it 

 is rendered latent by the forces of evaporation, and in this form, 

 having fulfilled its office in the economy of the ocean, it passes off 

 into the an*, there to enter, in mysterious ways, upon the perform- 

 ance of its manifold tasks. 



