Page:Popular Science Monthly Volume 9.djvu/656

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The Cruise of the "Challenger."Nature, for June 1st, gives an exceedingly interesting account of the voyage round the world recently completed by the Challenger. This voyage was undertaken chiefly for scientific purposes, the principal object being to "determine as far as possible the physical and biological conditions of the great ocean-basins of the Atlantic, the Southern Sea, and Pacific."

Important discoveries made during recent expeditions on the European border of the Atlantic and in the Mediterranean, by Dr. Carpenter, Mr. Gwyn Jeffries, and Prof. Wyville Thomson, stimulated a desire for further investigation, and this great voyage under direction of Prof. Thompson, as chief of the civilian staff, was inaugurated and carried through to a successful issue.

The ship left England on December 21, 1872, and returned to Spithead on May 24, 1876, having been absent a little less than three and a half years, and making a voyage of nearly 69,000 miles.

During this voyage 362 observing-stations were established, at each of which the depth and bottom temperature of the ocean were ascertained, and samples of the water, mud, and animals of the bottom, brought up for examination.

The direction and rate of currents were carefully studied, and "serial soundings" were made with special instruments to determine the temperatures at different depths. Upward of 50,000 meteorological observations were made during the first twelve mouths of the cruise.

The regular work of the expedition began at Teneriffe, from which point a line of soundings was carried across the Atlantic to the small island of Sombrero, a distance of 2,700 miles.

At 1,100 miles from Teneriffe, and 1,600 miles from Sombrero, bottom was found at 3,150 fathoms, which consisted of "perfectly smooth red clay, with scarcely a trace of organic matter," but at depths of only 2,200 fathoms the bottom was one mass of calcareous shells of foraminifera.

The red clay was found to be almost pure clay and a red oxide of iron with some manganese. This material is supposed by Prof. Thomson to be the residue or ash from decomposition of the shells. Experiments were made by Mr. Buchanan, of the staff of scientists, confirming this conclusion. He subjected globigerina ooze to the action of a weak acid, and found that after the carbonate of lime was removed there remained about one per cent, of a reddish mud, consisting of silica and alumina, and a red oxide of iron.

The globigerina shells were abundant at depths not exceeding 2,200 fathoms, but at greater depths a gray ooze occurred, the shells being in a state of decomposition; in deeper parts this disappeared, leaving the residuum of red clay.

This clay was found to be widely distributed in both the Atlantic and Pacific Oceans, and in many places contained concretions of the peroxide of manganese.

The specific gravity of ocean-water was carefully tested by Mr. Buchanan, and very unexpected results were obtained. The notion that the specific gravity increases with increase of depth seems unfounded, as it was ascertained to be greatest near the surface, diminishing to a depth of about 500 fathoms. From this downward it is nearly uniform.

Dredgings at great depths usually brought to the surface living organisms. At 3,150 fathoms (upward of three and a half miles) on the Atlantic cruise, only foraminifera were found, but other organisms were abundant at similar depths elsewhere.

By the serial temperatures taken in several places, it is evident that conditions exist which may greatly modify the distribution of the deep-sea fauna. Near Raine Island, not far from the entrance to Torres Straits, there was found at 2,650 fathoms, with bottom of red clay, a temperature of 35 Fahr. But it was also found that the same temperature occurred at a depth of only 1,300 fathoms. Here, then, the waters through 1,350 fathoms of depth, were of a uniform temperature. Over a wide area similar results were obtained, and the conclusion is, that this area, known as the Melanesian Sea, is so surrounded by a reef, rising to within 1,300 fathoms of the surface, that free communication of its waters with the outside ocean is prevented.

Animal life was found to be scarce in