Page:The American Cyclopædia (1879) Volume II.djvu/81

 ATLANTIC OCEAN ginning of Greek historical records. Some of their colonies on its coasts are said to have been founded as early as 1100 B. ^., and their commerce extended to the British islands and the Baltic. To the south they went equally far, and are believed to have even circumnavi- gated Africa six centuries before Christ, about the same time that the more timid Greeks re- corded the passage of the first navigator of their nation through the strait of Gibraltar. But the real importance of this ocean as the great high- way of modern civilization dates from the 14th and loth centuries, when the outlying groups of islands, the Canaries, Madeira, and the Azores, were first visited, and finally Columbus, cutting loose from coasting voyages, struck across its unknown waste to the discovery qf a new world. I. GEOGRAPHICAL DESCRIPTION. The limits of the Atlantic ocean have been taken rather arbitrarily, generally between the Arc- tic circle and a line drawn from Cape Horn to the Cape of Good Hope. In physical geog- raphy it is a brancli of the great southern ocean, forming a deep gulf of which the Arctic ocean is the blind end. Taken as a whole, the Atlantic has the shape of an irregular broad canal running north and south, with a deep bend to the west in the middle of its course. The projecting angles of the bordering conti- nents are said by Humboldt to correspond to the reentering ones on the opposite side. But in reality this correspondence is somewhat dis- torted, and thus narrows are formed by which the Atlantic is divided into three principal basins: the southern or Ethiopic, from the Antarctic ocean to the narrows between Cape San Roque and Senegambia ; the middle or At- lantic proper, from the same narrows to the range of islands formed by the British and Faroe islands and Iceland ; and the northern or Arctic. The Atlantic proper contrasts strongly with the Ethiopic by the great development of its shore line and the number of lateral arms or mediterranean seas in communication with it. Such are the Caribbean sea, the gulfs of Mexico and of St. Lawrence, Baffin and Hud- son bays, the Baltic, the North sea or German ocean, the Irish sea, and the Mediterranean with its dependencies the Adriatic and the Black sea. In the Ethiopic ocean, on the con- trary, the coasts are very uniform, with few indentations or bays, and no inland seas at all. The watershed of the continents bordering on the Atlantic basin is of remarkable extent, nil the other oceans of the earth put together re- ceiving but a fraction of the fresh-water drain- age in comparison. Several rivers of Asia and one or two in northwestern America can alone bear a comparison with those of the Atlantic basin. The number of islands in the Atlantic ocean is small when compared with those of the Pacific. Leaving aside those islands which are merely detached parts of the continents, we can count scarcely more than a dozen groups. Like most of that class, they are principally of volcanic origin. Of coral islands, so numer- ous in the Pacific, there are but two groups, the Bermudas and the Bahamas. II. DEPTH, "AND FIGUKE OF THE BOTTOM. The means em- ployed for ascertaining the depth are general- ly modifications of the old-fashioned lead and line. In moderate depths this method suffices in its simplest form. In great depths, however, its indications are apt to be untrustworthy, because the shock of the lead on the bottom ceases to be felt, and the line continues to run by its own weight or is carried off by currents without sensibly slackening. Sounding with a small line or twine, to be abandoned to- gether with the weight at each cast, was tried, but failed for want of means to determine when the bottom was reached. No sounding being now considered trustworthy unless a specimen of bottom is brought up as a proof that the lead has touched, it was found desirable to be relieved of the labor of hauling up the weight, and to bring up only the small apparatus and to collect the mud or sand. This was first accomplished by Lieut. Brooke's apparatus, a perforated cannon ball suspended in a sling which unhooks itself when the tension is re- lieved ; an iron rod passing through the hole in the ball is provided with a contrivance to bring up a specimen, and is the only weight remain- ing on the line. Lieut, (now Admiral) Sands substituted two hemispheres for the solid shot, falling off on each side of the central rod, thus allowing a larger specimen cup to be employed. An original method proposed by Prof. Trov- bridge consists in paying out the line (a small but strong twine) from a coil carried down with the weight, thus avoiding the friction of the line in passing through the water. The depth is registered by a screw similar to Mas- sey's. Propositions for sounding without line have been numerous, the weight carrying down a float which is released on the bottom and re- turns to the surface ; but none have been suc- cessful. In the United States coast survey deep-sea soundings are now usually made with a strong line and a heavy weight ; detaching the latter is not considered of great importance, since the hauling up is done by steam. The depth is registered by Massey's indicator, based on the principle of a propeller screw, free to revolve in passing downward, and com- municating its motion to a set of wheels regis- tering the number of revolutions. It is clamped loosely to a spindle so as to be free from the torsion of the line, and is carefully tested and its error determined in moderate depths. The Atlantic ocean in its northern basin is better known with regard to depth than any of the others ; nevertheless, there is need of more soundings before we can form a true idea of the figure of its bottom. Most of our knowl- j edge of it has been acquired during the last 30 years. Before that, a few soundings, now mostly considered untrustworthy, and some theoretical speculations, were the sum of our knowledge. Dr. Young deduced, chiefly from the theory of tides, a depth of about 15,000 ft.