not the 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, considering 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 accumulates 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. The different subjects for observation.—In conducting such investigations, several questions are to be considered, such as the transparency and specific gravity of the water, its phosphorescence; the face of the sky, 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 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. 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, 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
