The Baltic Sea, being a simple depression in the great European plain, is but a few hundred feet deep. In the North Sea, the depth averages 300 feet, and rarely exceeds 600. The continent is here prolonged in the form of a submarine plain, whose highest portions form the British Isles.
The Border Seas of Asia, lying within the chain of continental islands, are only a few hundred feet in depth, while immediately without those islands, abrupt slopes descend to the great depths of the Pacific basin.
Smaller inlets are also of frequent occurrence, especially in districts where mountain ranges approach the borders of the ocean. Such are the lochs of Scotland, the voes of the Shetland Islands, and the fiords of Norway and Greenland. The term lagoon is usually applied to lake-like inlets.
Salt and Other Ingredients of Sea-water. The waters of the ocean are salt, holding in solution various saline matters. The saline ingredients amount to rather more than thirty-five grains in a thousand grains of sea-water. The most abundant of these is chloride of sodium or common salt, which in general forms about a third of the whole. Besides this, sea-water contains some magnesia, lime, potash, and traces of iodine and bromine.
The following table exhibits the exact percentage composition of sea-water.
One hundred parts by weight of sea-water contain:
| Water | 96.470 |
| Sodium Chloride | 2.700 |
| Magnesium Chloride | .360 |
| Potassium Chloride | .070 |
| Magnesium Sulphate | .230 |
| Calcium Sulphate | .140 |
| Calcium Carbonate | .003 |
| Magnesium Bromide | .002 |
| Traces of Iodides, Silica, etc., estimated | .025 |
| 100.000 |
How the Sea gets its Color. The color of sea-water is due to the character of the skies and clouds above, and to vegetable and animal objects growing and living in it. The luminosity or phosphorescence of the ocean is due to the decay of animal and vegetable substances, but in some cases it arises from the presence of myriads of living animals, which, like the glow-worm and fire-fly of the land and air, have the power of emitting light.
Ocean Temperature. The water of the ocean appears generally to agree with that of the climate in which it is situated. In warm latitudes the temperature of the deep sea diminishes with the depth below the surface until a certain depth is reached, below which it appears to retain an equable temperature, this being about 40 degrees Fahrenheit. In the Polar Seas, where the temperature of the surface is lower than 40 degrees the heat increases downward until it reaches that point. In latitude 70° the temperature of the ocean is considered to be the same at all depths.
