DEEP-SEA EXPLORATION. The depths of the sea have been explored with precision only during comparatively recent years, Deep-sea investigations began in the necessity for accurate soundings for submarine cables, and this is still the main reason for sounding, but much more has been accomplished in this field by dredging for purely scientific purposes.
The most important part of our knowledge of deep-sea conditions has been gained since 1570. The Challenger expedition sent out by the British Government from 1S73 to 1870 (see CuALLEN. GER) engaged in pelagic investigations for nearly four years, sounding and dredging in the oceanic basins at more than 350 different places. This vessel was well equipped, and carried a scientific staff under the direction of Sir Wvville Thomson. A great amount of deep-sea work was accom plished. and the large series of Reports form the most important contribution ever made to the lit erature of this subject. Other extensive deep-sea investieations have been conducted at various times by most of the European governments, Addle vessels of the United States Coast Survey, or of the Fish Commission, have been engaged in them more or less regularly since ISIO. The Prince of Monaco has made very important con tributions to this department of knowledge. as he has for many years employed his yachts almost entirely in deep-sea work, and has devoted much time to the improvement of the appliances for invest igation.
The methods employed by the earlier investiga tors for measuring the depths were slow and un certain, for their soundings were made with hemp rope, which was greatly drifted by currents and gave inaccurate depths. The soundings and dredgings conducted on the Challenger were so made, and often under great difficulties. At the present time such investigations are made by means of wire, the first successful employment of which was by Sir William Thomson in 1S72.
Wire sinks rapidly, presents the least frictional surface, and is but little affected by currents: and the machinery is now so perfect that sound ings may be made with accuracy in the greatest depths. The improved methods show that the early soundings by the Herald, Congress. and other vessels with rope, supposed to have reached over 7000 fathoms, were erroneous, and that there are probably no such depths in the ocean.
The greatest depth known was discovered by the United States cable-survey ship Nero in 1900, near the island of Guam, where a sounding was made of 5269 fathoms, or nearly six statute miles, a depth sufficient to submerge the highest moun tains. It is probable that future soundings will reveal slightly greater depths. For four years prior to the voyage of the deepest water known was north of New Zealand, where the British ship Penguin sounded in 5155 fathoms. Off the coast of Japan, in 1874, the United States ship Tuscarora found a depth of 4655 fathoms; and in 1900 the United States Fish Commission steamship Albatross made a sounding in the western Pacific of 4813 fathoms. Many great depths have been discovered in the Atlantic, the deepest (4561 fathoms) off Porto Rico, by the United States Coast Survey steamer Blake. More than forty 'deeps? or depressions from 3000 to 5200 fathoms, are now known, some of them mere boles, others of vast extent. The deeps are well distributed over the seas, but none have been found north of the fifty-fifth degree of latitude. The average depth of the sea is probably not less than 2200 fathoms.