Page:Collier's New Encyclopedia v. 01.djvu/416

LEFT ATLANTIS 330 ATMOSPKli-JtiE depth of 1,600 or 1,700 fathoms, while the valleys on either side sink to the depth of 3,000 or 4,000 fathoms. A ridge, called the Wyville-Thomson Ridge, with a depth of little more than 200 fathoms above it, runs from near the Butt of Lewis to Iceland, cutting off the colder water of the Arctic Ocean from the warmer water of the Atlantic. The South Atlantic, of which the greatest depth yet found is over 3,000 fathoms, resembles the North Atlantic in having an elevated plateau or ridge in the center, with a deep trough on either side. The saltiness and specific gravity of the Atlantic gradually diminish from the tropics to the poles, and also from within a short distance of the tropics to the equator. In the neighborhood of the British Isles the salt has been stated at one-thirty-eighth of the weight of the water. The North Atlantic is the great- est highway of ocean traffic in the world. ATLANTIS, or ATLANTICA, an is- land, said by Plato and others to have once existed in the ocean immediately beyond the Straits of Gades; that is, in what is now called the Atlantic Ocean, a short distance W. of the Straits of Gib- raltar. Homer, Horace, and some others made two Atlanticas, distinguished as the Hesperides and the Elysian Fields, and believed to be the abodes of the blessed. Atlantis is represented as hav- ing ultimately sunk beneath the waves, leaving only isolated rocks and shoals in its place. "The New Atlantis" is the title which Lord Bacon gives to a liter- ary fragment, in which he sketched out an ideal commonwealth. ATLANTOSAURTJS, a gigantic fossil reptile, order dinosauria, obtained in the upper Jurassic strata of the Rocky Mountains, attaining a length of 80 feet or more. ATLAS, an extensive mountain sys- tem in north Africa, starting near Cape Nun, on the Atlantic Ocean, traversing Morocco, Algiers and Tunis, and termi- nating on the coast of the Mediterranean; divided generally into two parallel ranges, running W. to E., the Greater Atlas lying toward the Sahara, and the Lesser Atlas toward the Mediterranean. The principal chain is about 1,500 miles long, and the principal peaks rise above or approach the line of perpetual con- gelation. The highest elevations are al- most 15,000 feet, many other peaks aver- aging 11,000 feet. Silver, antimony, lead, copper, iron, etc., are among the minerals. The vegetation is chiefly Eu- ropean in character, except on the low grounds and next the desert. ATLAS, in Greek mythology, the name of a Titan, whom Zeus condemned to bear the vault of heaven. The same namr^ is given to a collection of maps '^. ATLAS charts, and was first used by Gerard Mercator in the 16th century, the figure of Atlas bearing the globe being given on the title-pages of such works. ATLAS, in anatomy, is the name of the first vertebra of the neck, which sup- ports the head. It is connected with the occipital bone in such a way as to permit of the nodding movement of the head, and rests on the second vertebra, or axis, their union allowing the head to turn from side to side. ATMOSPHERE, literally, the air sur- rounding our planet, and which, as the etymology implies, is, speaking broadly, a "sphere" (not, of course, a solid, but a hollow one). With strict accuracy, it is a hollow spheroid. Its exact height is unknown. At 2.7 miles above the sur- face of the earth, half its density is gone, and the remainder is again halved for every further rise of 2.7 miles. Some small density would remain at 45 miles high. At 80 miles, this would have all but disappeared. But from sundry ob- servations, made at Rio Janeiro and elsewhere, on the twilight arc, M. Liais infers that the extreme limit of the at- mosphere is between 198 and 212 miles. i