HYDROGRAPHIC DISCOVERIES.

Globe, Volume I, Issue 19, 22 June 1874, Page 3

HYDROGRAPHIC DISCOVERIES.

THE THERMAL STRATIFICATION OP THE OCEAN. Among the splendid contributions to science made by the outstanding Challenger Expedition, we have now (writes the “ New York Herald”) to record the exposition of the thermal stratification of the Ocean, lb was said by the late Sir Roderick Murchison that the solving of this interesting problem, involving the oceanic circulation, would be as important in physical geography as was Harvey’s discovery of the circulation of the blood in physiology. If we may depend upon the data of deep-sea explorations taken by the Challenger, and published in one of the latest issues of the London “ Times,” the long-disputed question of marine temperature is practically set at rest. The data referred to arc the completeresultsof the Challenger’s thcrmometric survey of the North and South Atlantic, which have been recently aubmiited to the Hydrographic Department of the Admiralty. The first temperature section taken across the North Atlantic by the expeditionary surveyors ran from the Island of Tcneriffe to St, Thomas. In the eastern half of this district, at 900 fathoms depth, the cold water stratum was met, having a temperature of 40 degrees Fahrenheit; and from this to the bottom, which lay at 3150 fathoms, the mercury fell to 35.) degrees Fahrenheit. As the section crossed the tropic, about 100 leagues from St Thomas, at correspondingly extreme depths, the water grew still colder (34 degrees); and the cold bed of water also came nearer the surface,although the ship was nearer the Equator, This remarkable thermometric depression on the broad floor of the Atlantic was traced to the extension of a vast underflow of ice-cold water from the Antarctic regions, whore glacial discharges have an unobstructed gateway in the submarine channel between Cape Horn and the Cape of Good Hope. The instrumental readings reveal, therefore, the extraordinary phenomenon of a vertical difference of nearly 40 degrees in temperature between the surface and abyssal strata —the former being about 75 degrees, the latter as low as 35 degrees. From Madeira southward to St Vincent, as the survey proceeded, the nearer they approached the Equator, the hot surface stratum grew thinner and the cold Antarctic underflow surged higher upward from the ocean bottom. Just a little south of the Line, were found the highest surface temperature and the lowest bottom temperature encountered anywhere in the Atlantic, the former being 78 degrees and the latter 32 degrees—affording a contrast of 16 degrees in 2000 fathoms. In addition to these instructive and extended deep-sea observations, the Challenger, in her later cruise, discovers that, latitude for latitude, the South At'antic Ocean is much more glacial than the North Atlantic. This is owing, probably, to the wide and ever open communications it maintains with the South Polar basin. It would appear from the thermal contrast presented by the two Atlantics that the greater refrigeration in the Southern Sea demonstrates the continental (as opposed to marine) character of the Antarctic area. This has been before merely surmise, but it seems now corroborated by the excessive iciness of the Antarctic seas, as far as the Challenger has tested their depths with the mercurial thermometer.

The results obtained from this survey, invaluable to the science of the physical, geography of the sea, confirm the reasonings of several eminent American hydrographers, who gave that science its earliest impulses, Indeed, nearly all the deep-sea phenomena, now so reliably brought to light by these scientific researches—such as the underflow and reflow of the glacial and sun-heated strata respectively, their most marked contrast near the Equator, the movement of the superior Antarctic currents and the thermal contract between the North and South Atlantic, were matters of prediction among our physical geographers before the Challenger left England. The only great field that vessel is yet to furrow in the interests of ocean science is the Antarctic Ocean, which has long been invested with mysterious interest by the voyages of "Wilkes and Ross. Doubtless the of those waters will be rich in geographical revelations of the character of the South Polar territory, and may finally prove whether it is land or water. We shall look with the utmost interest for whatever sheds light on that interesting problem of physical geography. SURVEY OE THE PACIFIC OCEAN. The cruise of the Tusoarora, under the command of Captain Belknap, for the purpose of making geographical surveys of the Pacific Ocean, has thus far been successful, surveys having been made of the ocean bed for a distance of over one thousand miles from Cape Flattery, Washington territory. The soundings taken in this distance, thirty-four in number, indicate that the bottom of the Pacific Ocean is unlike that of the Atlantic Ocean between Newfoundland and Ireland. In the latter ocean the bottom is a comparatively level plateau,withgeutlc undulations thatforra high mountains, although the ascents are so gradual that the bottom may be considered, for all practical purposes, level. In the Pacific Ocean, however, there is a submarine mountain, 2400 feet in height, on which the grade of the eastern slope is 123 feet to the lineal mile. The greatest depth found in these surveys was 15,201 feet; the greatest depth in the Ncwfoundland-Ircland basin is 12,420 feet. From the mountain above described to the last sounding, where the greatest depth w£s found, the grade is only G feet to the lineal mile, which corresponds closely with the gradients of the At'antic Ocean, described by Professor Huxley as so gentle that a carriage might be driven from shore to shore without use of the skid or brake. The character of the bottom of the ocean is of almost as much importance as the depth of water and the strength of currents. That of the Newfoundland Ireland basin is a grey mud, consisting almost entirely of the skeletons of small marine animals, whose dried bodies also form the immense chalk cliffs of Dover and the chalk formations of the Continent of Europe. The bottom found in the Pacific surveys is described as “blue, black, and brown mud, with ooze and occasional mixture of gravel and shell.” An analysis of this mud may lead to important geological discoveries, and thus the cruise of the Tuscarora be made to subserve science, as well as to further the material interests to bo benefited by the laying of a Pacific cable,