The substitution of steamships for sailing vessels has brought about a diminution in the number of vessels belonging to the Italian mercantile marine, whether employed in the coasting trade, the fisheries or in traffic on the high seas.
In 367 Athens and Thebes sent rival ambassadors to Persia, with the result that Athens was actually ordered to abandon her claim to Amphipolis, and to remove her navy from the high seas.
The colour of ocean water far from land is an almost pure blue, and all the variations of tint towards green are the result of local disturbances, the usual cause being turbidity of some kind, and this in the high seas is almost always due to swarms of plankton.
Although observations on marine currents were made near land or between islands even in antiquity, accurate observations on the high seas have only been possible since chronometers furnished a practicable method of determining longitude, i.e.
The mercantile marine of the United States was almost driven off the high seas by the terror of these destructive cruisers.