Arabia, animals of, xii, 249, 327, 342, 344, 359; Danish scientific expedition, xvi, 123; plains of, xiv, 217; source of coffee, xiii, 231, 283 Arabian Horses, xii, 307 Arabian Language, xv, 162; words from, in English, 161 Arabic Numerals, xv, 184, xvi, 62, 103 Arabs astronomy of, ii, 11, 36-9, 302, xvi, 100; bananas known to, xiii, 216; mathematical advances, ii, 12, xvi, 54, 103; medical science of, x, 31-3, 36, 37-8, 39, 40, 100; sciences of, xvi, 54, 100; sugar introduced by, xiii, 215 Arago, discovery of magnetism of rotation, vi, 21; lightning studies, i, 146 Aral, Sea of, depression of, xiv, 203; shallowness and salinity, 206-7; size of, 204 Arapaima Fish, xii, 154 Arara Cockatoo, v, 9-10 Arcadian Range, xiv, 227 Arcathagus, x, 25 Arc Furnaces, vii, 303 Arc Generators, vii, 291 Archæopteryx, xii, 239-41 Archegonia, xiii, 158, 159, 161 Archeozoic Era, iii, 164-75; life in, 262, 263, 265, xv, 71 Archeozoic Rocks, iii, 164-74; graphite found in, 249-50; iron ores in, 358 Arches, false and true, xv, 268-9; weak in earthquakes, xiv, 342 Archimedes, iv, 25, 26; mathematical and other work, xvi, 89, 90; principle of, iv, 30, 102-5, 107; screw of, 26-7 (fig.) Arc Lamp, Arc Light, iv, 310-11, 352, vi, 279, 280-3; Bunsen's carbon rods, xvi, 189; compared with sun, ii, 169; Davy's experiment with, vi, 19; direct and alternating currents on, vi, 332, vii, 208-9; graphite electrodes, vii, 300, 308 Arc Process, i, 36, vii, 323-4 Arcs of Lowitz, i, 366 Arctic Archipelago, xiv, 20 Arctic Current, xiv, 304-5 Arctic Ocean, copepods of, xii, 84; depths, xiv, 22; enclosed character, 22, 290, 299 Arctic Plants, in mountains, xiii, 321, xiv, 365-6, 376-7 Arctic Regions, conditions of life in, xv, 123-4; forests of Coal Age, xiii, 307; frozen soil of, xiv, 75; growing season in, 375; plant conditions in, 365; seals of, xii, 335; skin canoes of, xv, 264; snow line in, 72-3; wolves of, xii, 340-1; winds, i, 127, 128 (see also Polar Regions) Arcturus, angular diameter, ii, 151; decreasing distance, 120; displacement lines in spectrum, 119; gaseous state, 382; origin of name, 302; parallax, 316; "solar" star, 115 Arequipa Observatory, ii, 145-6 Arethusa (plant), xiii, 186 (fig.) Argentina, ancient sloths of, xii, 283; hail rods, i, 343; pampas of, (see Pampas); stock-raising, xiv, 384; weather service, i, 228-9 (note), ii, 186-7; wheat cultivation, xiii, 211 Argon, discovery and character, i, 10, 11, 12, viii, 67, 181, 309; electric lamp filler, i, 33; periodic classification, viii, 182-3; symbol and atomic weight, 383 Argonauts, (shellfish), xii, 77-8 Argonne Forest, xiv, 91